desalination process engineering part i · desalination process engineering part i mark wilf ph. d....

Mark Wilf Ph. D.

Phone: +1 858 444 7334

RO Technology

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1

Desalination Process Engineering Part I

Mark Wilf Ph. D. 2018

Mark Wilf Ph. D.

Phone: +1 858 444 7334

RO Technology



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1. RO feed water quality requirements

1.1. Introduction

1.2. Feed water types

1.3. Sparingly soluble constituents

1.4. Particulate matter

1.5. Organic matter

1.6. Biological activity

1.7. Feed water temperature

1.8 Feed water salinity

1.8.1. Feed water pH.

2. Feed water supply alternatives

2.1. Introduction

2.2. Brackish wells

2.3. River bank filtration wells

2.4. Beach wells

2.5.Horizontal wells

2.6.Slant wells

2.7.Seawater infiltration galleries.

2.8. Open surface intake

2.8.1. Configuration of open intakes.

2.9. Estimation of intake cost

2.10. Collocation with power plant

2.11. Design criteria of feed water supply systems and configurations

2.12. Equipment and hydraulic profile

3. Configuration of feed water pretreatment process

3.1. Conventional pretreatment

3.1.1. Configurations and components of conventional pretreatment system

3.2. Membrane pretreatment

3.3. Raw water quality and pretreatment requirements.

4. Pretreatment equipment

4.1. Screening

4.2. Dissolved air flotation (DAF)

4.2.1. Example of calculation of operating cost of the DAF unit.

4.3. Coagulation and flocculation

4.4. Granular media filtration

4.4.1. Pressure filter

4.4.2. Gravity filters

4.4.3. Solids management system

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4.4.4. Pretreatment system design method

4.5. Cartridge filtration

4.6.Membrane pretreatment

4.6.1. Configurations and components of membrane pretreatment system

4.6.2. Settling and screening

4.7. Filtration membranes and membrane unit configuration

4.7.1. Fundamentals of the membrane filtration water transport process

4.7.2. Membrane material and membrane configurations

4.7.3. Membrane filtration process

4.7.3.1. Pressure driven technology

4.7.3.1.1. Configuration of pressure driven membrane filtration unit

4.7.3.1.2. Sizing of pressure driven membrane filtration unit

4.7.3.1.3. Operating cost of pressurized membrane filtration unit

4.7.3.2. Configuration of immersed, vacuum driven, membrane filtration system

4.7.3.2.1. Sizing of immersed, vacuum driven, membrane filtration unit.

4.7.3.2.2. Operating cost of immersed membrane filtration system

4.7.4. Comparison of conventional and membrane filtration technologies as pretreatment

for seawater RO desalination systems

4.7.5. Offering of commercial membrane filtration technology

5. RO System

5.1.Membrane elements and pressure vessels

5.2.Membrane unit configuration

5.2.1. Single stage and multistage

5.2.2. Sideport, multiport and center port configuration

5.2.3. Two pass, partial two pass, split partial

5.3.Membrane cleaning

5.3.1. Configuration of membrane cleaning unit

5.3.2. Sequence of operation of cleaning unit

5.4.Membrane flushing unit configuration

5.5.RO membranes unit design criteria in accordance with feed water quality.

5.5.1. Feedwater quality parameters

5.5.2. Membrane fouling

5.5.2.1.Oxidative degradation of membrane performance

5.5.2.2.Colloidal fouling

5.5.2.3.Fouling by organic matter

5.5.2.4.Biofouling

5.5.2.5.Inorganic scale and determination of permeate recovery rate

5.6.Average permeate flux

5.7.Membrane unit design procedure

5.7.1.1.Permeate capacity and permeate quality limits

5.7.1.2.Selection of average permeate flux, recovery rate and array

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5.7.1.3.Selection of membrane type

5.7.1.4.Membrane train size and configuration

5.7.2. Utilizing computer programs in membrane unit design

5.7.2.1.Performance safety margins

5.7.3. Configuration of RO membrane unit for high feed salinity operation – 100,000

m3/day product water capacity.

6.0.Design for RO high pressure pump and ERD

6.1.Raw water supply and transfer pumps

6.2.High pressure pumps

6.3.Optimized control methods for high pressure pump discharge head and capacity

6.3.1. Application of energy recovery devices (ERD) in RO systems

6.3.1.1.Selection of ERD

6.3.1.2.Pelton wheel

6.3.1.3.Turbocharger

6.3.1.4.Pressure exchangers (isobaric devices)

6.3.1.5.Cost and economic benefits of ERD

7.0.Chemical dosing equipment design

7.1.Selection criteria for chemicals used in the RO process

7.2.Procedure for determination of chemicals dosing rate

7.3.Criteria for sizing of chemicals storage equipment

7.4.Selection of chemical dosing pumps capacity and materials of construction

7.5.Example of sizing of chemical dosing system for SWRO plant of permeate ca-

pacity of 100,000 m3/day operating at recovery rate of 45%.

8.0.Instrument and control system

8.1.Process control strategy

8.2.Control loops in RO system

8.3.Process control and performance normalization software

8.4.Instrument selection criteria and their location in the RO system

8.5.Frequency of data collection and representative range of operating parameters.

8.6.Methods of control of operation of chemical dosing systems

8.7.Pumps process control in brackish and nanofiltration applications

8.8.Pumps process control in seawater applications

8.8.1. “Train dedicated” configuration

8.8.2. “Pressure center” configuration

9.0.Selection of materials of construction of equipment and components

10.0. Example of design of 100,000 m3/day brackish RO desalination system

10.1. Raw water source

10.2. Product water quality

10.3. Pretreatment system

10.4. Equipment description

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10.5. Cleaning in place (CIP) unit

10.6. Post treatment

10.7. Equipment list

10.8. Computer projections for RO membrane unit without blending

10.9. Computer projections for RO unit with blending

11.0. Example of design of 100,000 m3/day seawater RO desalination system

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Desalination Process Engineering Manual – List of Figures

Figure 1.1. Concentration factor of concentrate stream in RO applications

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Figure 1.2. Schematics of spiral wound RO membrane element

Figure 1.3. Schematics of configuration feed – concentrate channel in RO membrane

element.

Figure 1.4. Schematic configuration of Silt Density Index (SDI) apparatus.

Figure 1.5. SEM picture of clean filter pad, before SDI memasurement.

Figure 1.6. SEM picture of clean filter pad, after SDI measurement. SDI = 2.2


Figure 1.8 Temperature correction factor for polyamide composite membranes.

Figure 1.9 Temperature effect on permeate salinity in brackish RO systems

Figure 1.10. Temperature effect on permeate salinity in seawater RO systems. Feed

pressure values are marked as red bars and permeate salinity as gray

bars

Figure 2.1. Schematic configuration of veridical beach well.

Figure 2.2. Schematic diagram of collector well

Figure 2.3. Schematics of horizontal directional drilling (HDD) intake

Figure 2.4. Model of HDD intake system at 82,000 m3/day seawater RO desalination

plant, Alicante, Spain. Courtesy Neodren.

Figure 2.5. Infiltration gallery at sweater RO desalination plant, Fukuoka, Japan.

Figure 2.6. Schematic configuration of open surface off shore seawater intake.

Figure 2.7. Example of off shore intake structure (courtesy Ian Larsen)

Figure 2.8. Alternative configuration of intake structure (courtesy Ian Larsen)

Figure 2.9. Installation of concrete anchors on intake pipe (courtesy Ian Larsen).

Figure 2.10. Delivery of intake pipe with concrete anchors installed to the plant site

(Courtesy Oceana)

Figure 2.11. Intake and discharge system at SWRO desalination plant, Fukuoka, Japan

Figure 2.12. Conveyance of the feed intake and concentrate discharge lines at Fukuoka,

Japan

Figure 2.13. Concentrate discharge structure at the SWRO desalination plant, Fukuoka,

Japan.

Figure 2.14. Flow diagram of SWRO desalination plant collocated with power plant at

Carlsbad, CA.

Figure 3.1. Configuration of RO unit operating with well water.

Figure 3.2. Configuration of pretreatment unit in desalination plant treating well water.

Figure 3.3. RO system treating surface water.

Figure 3.4. Schemat-

ic configuration of seawater desalination system with membrane pretreatme

nt.

Figure 4.1.1. Dual flow intake band screen (web page Horim Industries Inc., Korea)

Figure 4.1.2. Automatic disc strainers. (Arkal Filtration Systems)

Figure 4.2.1. Schemat-

ic diagram of DAF system integrated with multi media filtration unit (courte

sy UnitedKG)

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Figure 4.2.2. Configuration of DAF air saturator

Figure 4.2. 3. Relation between air pressure and concentration of dissolved air in water.

Figure 4.2.4. Relation between turbidity of DAF effluent and concentration of dissolved

air in water.

Figure 4.2.5. Picture of DAF installation at the Tuas, Singapore, SWRO desalination

plant.

Picture 4.3.1. Transi-

tion of Turbidity and Zeta Potential with increasing dose of coagulant (court

esy Peter Hillis).

Figure 4.3.2. Schematic diagram of configuration of coagulation – flocculation unit

Figure 4.4.1. Drawing of a vertical pressure filter (courtesy of Tonka Company)

Figure 4.4.2. Drawing of horizontal filter (courtesy of Tonka Company)

Figure 4.4.3. Examples of filter nozzles (web page of FTR, Istanbul)

Figure 4.4.4. Configuration of effluent flow control in a pressure filter

Figure 4.4.5. Configuration of vertical pressure filters with valves required for utilization

of internal source backwash water.

Figure 4.4.6. Configuration of horizontal four chambers pressure filter with valves re-

quired for utilization of internal source backwash water.

Figure 4.4.7. Alternative configuration for backwash of pressure filters from the high ser-

vice line.

Figure 4.4.8. Configuration of SWRO desalination plant at Carboneras, Spain

Figure 4.4.9. Schematic configuration of a gravity media filter.

Figure 4.4.10. Configuration of filtration layers in a gravity filter

Figure 4.4.11. Schematics of filtration driving head in the gravity filter.

Figure 4.4.11. Aerial picture of sweater RO desalination plant showing layout of gravity

filters (courtesy GES Engineering).

Figure 4.4.13. Schematic diagram of filtration system including solids management unit.

Figure 4.4.14. Jar test equipment utilized in estimation of the required dosing rate of alum

based coagulant. (Courtesy Peter Hillis).

Figure 4.4.15. Schematic diagram of a pilot unit for testing of a gravity filtration process.

Figure 4.5.1. Schematic configuration of cartridge filtration unit.

Figure 4.5.2. Picture of horizontal housing of cartridge filter.

Figure 4.5.3. Horizontal cartridge filter housing in open position.

Figure 4.6.1. Block diagram of submersible membrane filtration system

Figure 4.6.2. Block diagram of pressure driven membrane filtration system

Figure 4.6.3. Microstrainer configuration offered by Arkal.

Figure 4.6.4. Side view of microstrainer assembly. System width 13.9 m

Figure 4.6.5. Top view of microstrainer assembly. System length 10.1 m

Figure 4.6.6. Wedged screen strainer configuration

Figure 4.7.1. Temperature correction factor vs. water temperature

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Figure 4.7.2. Separation size range of filtration technologies.

Figure 4.7.3. Filtrate flow direction in pressure drive capillary membranes: a – PDI, b -

PDO.

Figure 4.7.4. . Filtration step – direct flow mode of operation

Figure 4.7.5. Backwash step.

Figure 4.7.6. Integrity test sequence.

Figure 4.7.7. Configuration of pressure driven membrane filtration unit

Figure 4.7.8. Pressure driven membrane filtration system (courtesy Simens Water Tech-

nologies)

Figure 4.7.9. a. Valves position during filtration step

Figure 4.7.9. b. Valves position during backwash step

Figure 4.7.9. c. Valves position during backwash step

Figure 4.7.9. d. Valves position during chemical enhanced backwash step

Figure 4.7.9. e. Valves position during system draining step

Figure 4.7.9. f. Valves position during air pressurizing step

Figure 4.7.10. Configuration of immersed, vacuum driven, membrane filtration unit

Figure 4.7.11. Layout of immersed membrane filtration system (courtesy Siemens Water

Technologies)

Figure 4.7.12. Configuration of conventional pretreatment system for SWRO

Figure 4.7.13. Configuration of immersed membrane pretreatment system for SWRO

Figure 4.7.14. Membrane filtration train with Hydracap modules – Hydranautics

Figure 4.7.15. Membrane filtration train with Xiga modules – Norit

Figure 4.7.16. Membrane products offered by Inge

Figure 4.7.17. Membrane products offered by Pall – Asahi

Figure 4.7.18. Pressurized membrane filtration products (CP) offered by Siemens

Figure 4.7.19. Immersed membrane filtration products (CS) offered by Siemens

Figure 4.7.20. Immersed membrane filtration products (ZW-1000) offered by GE (Zenon)

Figure 5.1.1. 200mm by 1000 mm spiral wound element

Figure 5.1.2. 200mm by 4000 mm spiral wound element

Figure 5.1.3. Configuration of pressure vessel with membrane elements

Figure 5.1.4. Flux distribution along the length of pressure vessel

Figure 5.2.1. Single stage membrane unit configuration

Figure 5.2.2. Schematic diagram of two stage membrane unit

Figure 5.2.3. Mechanical drawings of RO membrane train

Figure 5.2.4. Configuration of a side port pressure vessel

Figure 5.2.5. Single stage membrane unit with side port pressure vessels

Figure 5.2.6. Single stage membrane unit with multiport port pressure vessels

Figure 5.2.7. Two stage membrane unit with multiport port pressure vessels - configura-

tion 1

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Figure 5.2.8. Two stage membrane unit with multiport port pressure vessels - configura-

tion 2

Figure 5.2.10 Schematic configuration of membrane unit utilizing center port pressure

vessels

Figure 5.2.11 Schematic configuration of a two pass unit

Figure 5.2.12 Two pass system with second pass concentrate recirculation

Figure 5.2.13 Schematic configuration of partial two pass processing

Figure 5.2.14 Permeate salinity distribution along the pressure vessel

Figure 5.2.15 Split partial two pass configuration

Figure 5.2.16. Two pass split partial processing

Figure 5.3.1. Configuration of membrane cleaning unit

Figure 5.71. Alternative configurations of membrane trains in large capacity RO plants

Figure 5.7.2 Computer projections program – water analysis data entry screen

Figure 5.7.3. Computer projections program – process parameters and membrane array

entry screen

Figure 5.7.4. Computer projections program – membrane elements look up table

Figure 5.7.5. Computer projections program – screen display of calculation results

Figure 5.7.6. Printout of calculation results

Figure 5.7.7. Split partial configuration of a 12,500 m3/day SWRO train

Figure 6.1. Energy usage in RO desalination systems

Figure 6.2 Pressure centers configuration of a large capacity SWRO plant.

Figure 6.3. Pelton Wheel

Figure 6.4. Pelton Wheel – electric motor – high pressure pump unit

Figure 6.5 Concentrate foaming at the Pelton Wheel outlet

Figure 6.6 Configuration of Hydraulic Turbocharger

Figure 6.7. Brackish RO train with Hydraulic Turbocharger in the interstage position

Figure 6.8 Hydraulic Turbocharger positioned after high pressure pump in seawater

RO unit

Figure 6.9. Examples of configurations of seawater (left) and brackish (right) RO units

with Hydraulic Turbocharger

Figure 6.10. Example of calculations of pressure boost provided by Hydraulic Turbo-

charger in seawater and brackish RO membrane unit

Figure 6.11. Hydraulic Turbocharger equipped with electric motor.

Figure 6.12. Schematic configuration of RO membrane unit with isobaric energy recov-

ery device.

Figure 6.13 Configuration of DWEER energy recovery device

Figure 6.14 DWEER isobaric EDR assembly operating in 330,000 m3/day SWRO plant,

Ashkelon, Israel.

Figure 6.15 Configuration of PX energy recovery device (ERI)

Figure 6.16 Large assembly of PX ERD’s.

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Figure 6.17. ISave ERD introduced by Danfoss

Figure 8.1. Basic process control of RO membrane unit

Figure 8.2. Schematic configuration of control system in RO desalination plant

Figure 8.3 Control system configuration – no backup control equipment

Figure 8.4. Control system configuration – ‘hot” backup (PLC only)

Figure 8.5 Control system configuration – complete backup (PLC and IO’s)

Figure 8.6. Pumping units and RO membrane trains in “train dedicated” configuration

Figure 8.7. Pumping units and RO membrane trains in “pressure centers” configuration

Figure 8.8 Pumping unit with Pelton Wheel EFD

Figure 8.9 High pressure pump and hydraulic turbocharger in feed entry position

Figure 8.10 10 Hydraulic turbocharger in the interstage position

Figure 8.11 High pressure pumping unit utilizing isobaric ERD

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Desalination Process Engineering Manual – List of Tables

Table 1.1. Examples of representative compositions of brackish, sea water and

secondary wastewater effluent.

Table 2.1. Listing of large SWRO desalination facilities that utilize beach wells

Table 2.2. Economic weight of various alternatives of sea water supply to RO de-

salination plants

Table 2.3. Intake – outfall system cost parameters

Table 2.4. Cost components of intake – outfall system. 300,000 m3/day inflow,

150,000 m3/day outfall

Table 2.5 Example of values of constants used in Haze – Williams and Darcy

equations

Table 2.6. Example of pipe friction loses according to pipe diameter.

Table 3.1. Representative raw water quality associated with major types of water

supply sources

Table 3.2. Recommended configuration of pretreatment system according to raw

water quality

Table 4.2.1. Representative design parameters of DAF system

Table 4.2.2. Example of energy usage of air saturation unit for a DAF system for the

raw water flow of 1000 m3/hr.

Table 4.2.3. Design parameters of a DAF unit, 100,000 m3/day effluent capacity

Table 4.3.1. Preliminary specifications of coagulation – flocculation unit. Nominal

flow capacity 100,000 m3/day.

Table 4.4.1. Range of specifications parameters of filtration media.

Table 4.4.2. Recommended values of filtration media parameters

Table 4.4.3. Design parameters of media filtration system utilizing horizontal filters.

System effluent capacity 100,000 m3/day.

Table 4.4.4. Design parameters of media filtration system utilizing gravity filters.

System effluent capacity 100,000 m3/day.

Table 4.4.5. Design parameters of solids management unit for a filtration system.

Filtration system capacity 100,000 m3/day

Table 4.7.1. Example of the permeability results at ex-factory test and during field

operation

Table 4.7.2. Attributes of PDI and PDO membranes configuration

Table 4.7.3. Summary of process parameters of pressure driven and submersible

technologies.

Table 4.7.4. Air assisted backwash of the pressurized (PDI) membrane filtration

system

Table 4.7.5. Sequence of operation of pressure driven membrane filtration unit.

Table 4.7.6. Water quality parameters of surface water source

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Table 4.7.7. Example of operational parameters and schedule of pressurized mem-

brane filtration system

Table 4.7.8. Example of sizing calculations of 200,000 m3/day pressurized mem-


Table 4.7.9. Example of sizing of chemicals dosing equipment in pressurized mem-


Table 4.7.10. Listing of subunits and major equipment in pressurized membrane fil-

tration system

Table 4.7.11. Breakdown of energy usage in pressurized membrane filtration system.

Filtrate capacity 200,000 m3/day

Table 4.7.12. Summary of volumes required for CEB and CIP in pressurized mem-


Table 4.7.13. Summary of chemicals usage in pressurized membrane filtration sys-

tem. Filtrate capacity 200,000 m3/day.

Table 4.7.14. Summary of operating cost of pressurized membrane filtration system

Table 4.7.15. Sequence of operation of immerse, vacuum driven, membrane filtration

unit.

Table 4.7.16. Example of operational parameters and schedule of immersed mem-


Table 4.7.17. Example of sizing calculations of 200,000 m3/day immersed membrane

filtration system

Table 4.7.18. Listing of subunits and major equipment, immersed membrane filtra-

tion system

Table 4.7.19. Breakdown of energy usage in immersed membrane filtration system.

Filtrate capacity 200,000 m3/day

Table 4.7.20. Summary of volumes required for CEB and CIP in immersed mem-


Table 4.7.21. Summary of chemicals usage in immersed membrane filtration system.

Filtrate capacity 200,000 m3/day.

Table 4.7.22. Summary of operating cost in immersed membrane filtration system

Table 4.7.23. Summary of comparison of relative advantages of multimedia and

membrane filtration systems for SWRO applications

Table 4.7.24. Estimated capital cost of multimedia filtration and membrane filtration

units. Filtrate capacity 200,000 m3/day

Table 4.7.25. Listing of established suppliers of commercial membrane filtration

products

Table 5.1.1. Recovery rates of individual elements in pressure vessel according to

number of elements.

Table 5.1.2. Example of representative dimensions of commercial pressure vessels

for RO applications

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Table 5.2.1. Comparison of side port and center port configurations

Table 5.2.2. Comparison of conventional two pass and split partial two pass config-

uration

Table 5.3.1. General specifications of cleaning equipment

Table 5.5.1. Summary of membrane fouling categories and their symptoms

Table 5.5.2. Summary of membrane fouling processes for various desalination ap-

plications and possible cause of fouling

Table 5.5.3. Controlling phosphate scaling through pH adjustment

Table 5.5.4. Quantity of sulfuric acid required to maintain given pH of the concen-

trate

Table 5.5.5. Ksp and concentration limits for scale forming compounds common to

RO

Table 5.5.6. Practical limits of saturation values in RO applications

Table 5.5.7. Common range of permeate recovery rate in RO applications

Table 5.5.8. Process parameters affected by permeate recovery rate

Table 5.6.1. Common range of average permeate flux in RO applications

Table 5.6.2. Process parameters affected by permeate flux

Table 5.7.1 Range of RO design parameters according to application and feed wa-

ter source

Table 5.7.2. Examples of representative membrane elements models according to

applications

Table 5.7.3. Representative offering of nanofiltration membrane elements

Table 5.7.4. Representative offering of brackish membrane elements

Table 5.7.5. Representative offering of seawater membrane elements

Table 5.7.3.1. Basic process parameters of a 100,000 m3/day SWRO system

Table 6.1. Calculation of energy total energy usage in RO systems of permeate

capacity of 40,000 m3/day

Table 6.2. Comparison of operating parameters of pumping unit in SWRO

100,000 m3/day plant.

Table 6.3. Comparison of cost of pumping – energy recovery equipment alterna-

tives for SWRO 100,000 m3/day plant.

Table 7.1. . Listing of chemicals used in RO and NF applications.

Table 7.2. Procedures for determination of chemicals dosing rate

Table 7.3. Representative materials of construction for chemical dosing systems

Table 7.4. System process information for 100,000 m3/day SWRO system

Table 7.5. Usage of treatment chemicals in 100,000 m3/day SWRO system

Table 7.6. Sizing of chemical dosing pumps

Table 9.1. Selection of piping material according to application

Table 9.2. Relevant composition and PREN values of alloy steels

Table 9.3. Recommended flow velocity range in RO applications

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Table 10.1. Well water quality

Table 10.2 Permeate and product water quality

Table 10.3. Pretreatment system design data

Table 10.4. System design data

Table 10.5 CIP unit design data

Table 10.6. Post treatment unit design data

Table 10.7. Major equipment list

Guidelines for process development and design of brackish water and seawater membrane desalina-

tion systems. The guidelines will be based on design approach for a 100,000 m3/day seawater and

brackish water desalination units.

3. RO feed water quality requirements

1.9. Introduction

Feed water quality requirements for RO and NF applications are results of unique conditions of the

RO/NF process and configuration and properties of membrane elements.

During the membrane desalination process fraction of feed water is converted to permeate and vol-

ume of feed water is reduced. Concentration of all constituents is increased. The concentration of

constituents in concentrate stream, leaving the membrane unit is higher than concentration in the

feed water according to the concentration factor (CF), which is related to recovery rate (R):

CF = 1/(1 – R) (1.1)

The recovery rate in RO units ranges from 40% - 60% for seawater systems and 75% - 90

% in RO/NF units. As illustrated in Figure 1.1, concentration of all constituents, entering th

e membrane unit with the feed stream will be increased significantly in the concentrate stre

am.

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Figure 1.1. Concentration factor of concentrate stream in RO applications.

Accordingly, concentration of some soluble constituents of the feed water could exceed their satura-

tion concentration during the desalination process, precipitate from solution and deposit on the

membrane surface.

The other concern is related to presence of suspended and colloidal solids in the feed water. In the

spiral configuration of membrane element, shown schematically on Figure 1.2, feed water flows

through narrow feed channels, shown schematically on Figure 1.3.

0.001.002.003.004.005.006.007.008.009.00

10.00

0% 20% 40% 60% 80% 100%

Co

nce

ntr

atio

n f

acto

r

Recovery rate

Concentration Factor in RO System

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Feed

Concentrate

Product

Membrane

Permeate

Carrier

Brine Spacer

Figure 1.2. Schematics of spiral wound RO membrane element

Feed channel

Feed spacer

configuration

0.7 mm (0.031”)

Configurations of feed channel and feed spacer net

Figure 1.3. Schematics of configuration feed – concentrate channel in RO membrane element.

The nominal height of the feed channel is about 0.7 mm and the channel is filled with feed spacer

that promotes turbulence. Therefore, the effective channel opening for feed flow is even smaller

than 0.7 mm. Any particulate matter, present in feed water, could be trapped in the feed spacer and

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block water flow in the feed channels. For this reason, particulate matter has to be removed from

RO feed water in the feed water pretreatment system.

Feed water quality parameters are defined mainly in terms of concentration of constituents that

could exceed saturation limits and precipitate on the membrane surface and particulate matter that

could block passage of the membrane elements feed channels and also deposit on the membrane

surface, increasing resistance to water flow.

1.10. Feed water types

The composition and quality of water, considered for processing by reverse osmosis is influenced

by its origin. For reverse osmosis application water of interest is the one with ions composition that

exceeds potable water limits. In brackish water RO applications the ions that commonly exceed po-

table water limits are calcium, magnesium, sulfates and chlorides. Less common dissolved constitu-

ents of brackish water, that may require reduction of concentration, are fluoride and nitrate. Some

waters may contain also excessive concentrations of iron, manganese, organic matter, color, hydro-

gen sulfite and sometimes even radioactive isotopes.

Potable water limits are specified by World Health Organization (1) and regional Health Authorities.

Accordingly, one may define brackish water as any water of composition of soluble species exceed-

ing potable water limits. The potable water limits, or acceptance of water composition for potable

application can vary from country to country, according to local affordability of treatment methods.

However, it is commonly accepted that water of salinity exceeding 1000 ppm is considered as

brackish and requires treatment for salinity reduction.

The upper limit of water salinity that can be effectively treated with brackish RO membranes, in a

single pass configuration, is about 10,000 ppm. On the low end of salinity spectrum there are some

water sources that have salinity in the potable range but still require membrane treatment. This is usually due to presence of excessive concentration of hardness, iron, organics and/or color. The low

salinity water sources are usually treated with loose RO membranes, commonly called as softening

or nanofiltration membranes.

The composition of brackish water can vary widely. The composition is usually specific for the aq-

uifer it originates from. If brackish water aquifer is very large and/or water is pumped at the rate it

is being replenish by natural infiltration, then the composition remains stable. In case of excessive

pumping the composition may change. In case of utilization of coastal aquifer there is possibility of

seawater intrusion and salinity increase. For other locations there will be influence on composition

from adjacent underground bodies of water due to hydrostatic pressure difference.

Seawater sources are characterized by high salinity, in the range of 30,000 ppm TDS to 47,000 ppm

TDS. The ion composition includes mainly sodium and chloride, about 85% combined. The remain-

ing fraction consists of sulfate (~ 8%), magnesium (~ 4%), calcium, potassium (~1.2% each) and

bicarbonate (~0.6%). Boron is one of low concentration constituents. It is present in seawater at

concentration of about 3 – 5.5 ppm. However, boron concentration is becoming increasingly im-

portant parameter of the process design as its concentration is being specified in RO permeate. Due

to relatively low rejection by RO membranes of boron species existing in seawater, stringent boron

specifications have significant effect on process design and product water cost. At majority of loca-

tions the ions composition of seawater is quite consistent and fluctuates in narrow range. Tempera-

ture of seawater usually reflects the seasonal fluctuations of ambient temperatures but could be af-

fected by temperature of local water currents. At some locations, where rivers discharge or rain sur-

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face run off is present, the fluctuations of salinity and concentration of suspended matter could vary

in a wide range and require careful consideration during the process design stage.

As a part of the design process of RO plant the feed water sources should be evaluated to determine

the following:

1. How feed water ions composition and temperature will affect quality of permeate and the

required feed pressure?

2. Does water source contain sparingly dissolved species at concentration that could result in

membrane scaling at the design recovery rate?

3. Does water source contain particulate matter that could plug feed channels of membrane el-

ements?

4. Does water source contain organic matter at concentration that could adsorb on membrane

surfaces and result in significant permeability decline?

5. What is the level of biological activity? Can it result in biofouling of membranes?

Preliminary determination of the suitability of given water source for RO processing and require-

ments of the pretreatment process, is conducted based on results of analysis of water samples from

the water source under consideration. During the initial evaluation of water analysis it is important

to check if the water analysis report contains values of important water quality parameters and con-

centration of major ions. The primary group of water composition data includes: pH, temperature,

turbidity, electric conductivity and concentrations of Ca, Mg, Na, K, HCO3, SO4, Cl SiO2, Fe, Mn

and TOC. Additionally, concentration of any species that their maximum concentration had been

defined in permeate, should be also determined in the feed water source, for example concentration

of NO3 or B. The analysis should be balanced, i.e. sum of miliequivalents of positively charged

ions (cations) should be similar to the sum of equivalents of negatively charged ions (anions).

In addition to feed water composition, suitability of raw water for RO processing is defined by the water source. The common sources of feed water for RO/NF applications are:

1. Deep wells. The water originating from deep wells is usually brackish of low or high sa-

linity. In most cases well water from deep wells has very low concentration of suspended

and colloidal particles.

2. Shallow wells. This category includes low salinity brackish wells, drilled in the shallow aquifer, river bank filtration wells and wells supplying seawater: beach or collector wells.

Water from these wells has also low concentration of particulate matter but sometimes

could contain elevated concentration of organics. .

3. Surface water sources usually represent seawater intakes. In majority of cases, concentra-tion of suspended solids fluctuates in a wide range and extensive pretreatment is required

for suspended solids reduction.

4. Wastewater, either secondary or tertiary effluent. This water source is characterized by el-

evated concentration of suspended solids and organics. In cases when wastewater source represents effluent of membrane bioreactor (MBR), then it contains elevated concentra-

tion of organics but concentration of suspended solids is quite low.

Table 1.1. Examples of representative compositions of brackish, sea water and secondary

wastewater effluent.

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Feed constituent Low salinity brackish High salinity brack-

ish

Medium salinity

seawater

Secondary effluent

Ca 83 254 449 50

Mg 66 120 1387 10

Na 350 1214 12179 350

K 4 12 418 10

HCO3 340 100 158 159

SO4 74 778 3200 100

Cl 600 2034 21555 454

F 0.8 2.4 1 2

NO3 46 1.0 10

B 5 0.6

SiO2 29 16 10

TOC 0.5 0.5 1.0 15

1563.8 4514.4 39347 1215

1.11. Sparingly soluble constituents

During the RO process concentration of all constituents increases due to reduction of the feed water

volume. This increase of concentration is function of permeate recovery (Equation 1.1). Some of

the constituents presented in natural waters can precipitate if the concentration product of salt form-

ing ions exceeds its solubility product: Ksp. For a given salt of composition CmAn in equilibrium of

solid phase salt (S) with dissolved ions, the Ksp is defined as:

CmAn(S) = mC+n + nA-m (1.2)

Ksp = [C+n ]m [A-m]n (1.3)

SI = [C]m [A]n / Ksp (1.4)

Where C stands for cation and A for anion, m and n are valency coefficients. Brackets [ ] indicate

molar concentration of a given ion in solution. SI is the saturation index, indicating excess concen-

tration of a given salt in comparison to its saturation value.

The Ksp is determined through measurement of ions concentrations in solution at saturation condi-

tions (in equilibrium with solid phase). Ksp value is specific for a given salt and it is function of

temperature and ionic strength of the solution. In brackish systems, treating natural waters, the salt

of concern is mainly calcium carbonate. Less frequently calcium sulfate and silica are at concentra-

tions that my result in scale formation. In very rare cases barium sulfate, ferrous sulfide, and ferrous

carbonate could be present at concentrations that may form scale at high recoveries. In RO sys-

tems treating municipal effluents calcium phosphate sometimes forms in the tail elements. Calcium

carbonate is the most common scaling constituent in brackish waters but also the easiest to control

either with pH adjustment or use of scale inhibitor.

Discussion on scaling potential of sparingly soluble salts is included in Chapter 5.4.2

1.12. Particulate matter

RO feed water entering membrane elements, should have low concentration of particulate matter.

The commonly accepted quality indicators of RO feed water in this respect include:

1. Turbidity

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2. Suspended solids concentration

3. Silt density index (SDI)

Turbidity determination, usually expressed as nephelometric turbidity units (NTU) is determined

through measurements of intensity of light scattered by suspended particles in water samples.

Suspended solids concentration is determined by filtration of measured volume of water sample and

weighting of dry residue on the filter.

The SDI is determined through measuring the rate of filtration of water sample through the filter

(Figure 1.4).

Determination of all three above indicators is described in ASTM procedures (3, 4, 5). Out of above

three indicators only turbidity can be measured continuously on line. The other two are conducted

as discrete measurements on water samples taken periodically.

Silt Density index

SDI = 100*(1 – t0/t15)/15

t0 t15 SDI (15 min)

18 22 1.2

18 32 2.9

18 50 4.3

46 50 0.5 (?)

15 min

500 ml

Feed

pipe

Figure 1.4. Schematic configuration of Silt Density Index (SDI) apparatus.

Another water quality parameter that is being used to monitor operation of pretreatment system is

number of particles measured with particle counters. This could be applied as on line, semi-

continuous measurement. Application of particle counters for RO application is still at very early

stages. So far no relations had been reported as being established between particle counter meas-

urements and fouling rate or performance stability of RO membranes.

The feed water indicator, most relied on in RO applications, is SDI. It is based on measurement of

rate of declining flow, at a constant pressure, of a water sample through a porous filter membrane of

nominal 0.45 micron porosity. The filter is placed in a simple circular holder (Figure 4) and con-

nected to feed water line at applied pressure of 2 bar (30 psi). The time measured for filtration of a

constant volume (500 ml) at the beginning of the test (t1) and after 15 minutes (t2) is used to calcu-

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late the SDI according to the following equation:

SDI = 100% (1-t1/t2)/15 (1.5)

If the filter plugs to fast for meaningful determination of the filtration time, the volume of filtrate

being collected or time between measurements can be decreased. However, the SDI values deter-

mined for shorter test time or smaller filtrate volume are only indicative of poor water quality and

not very useful for the pretreatment system design process. As shown in Figure 1.4 (last entry in the

table) it is possible to have long filtration times and still calculate low SDI values. If the reading for

filtration time t1 significant exceeds 30 seconds then most likely there is a problem with equipment

or test conditions. The SDI method is very sensitive to concentration of foulants but it is not very

accurate. No meaningful correlation has been established between values of SDI and turbidity.

Attempts to improve accuracy of this method led to introduction of Modified Fouling Index (MFI).

The test for MFI is based on measurement of pressure increase required for maintaining of constant

filtration rate through well defined membrane filter (6). The MFI results are more reproducible than

SDI but the test is difficult to perform manually and automatic equipment is necessary (7) for a rou-

tine determination in the field conditions.

According to the equation 1.5, the maximum value of SDI (for 15 min measuring interval) can be

only 6.67. The majority of membrane manufacturers specify upper limit for feed water SDI as 5.

However, field results show that for stable, long term performance of RO elements, the SDI of feed

water should be consistently below the value of 4. Some limited research works (X) demonstrated

that in respect of solids concentration the SDI scale is a geometric one. Therefore, for water having

SDI = 3 and another water having SDI = 5 the corresponding suspended solids concentration differ-

ence is about four times higher.

The following figures shows the SEM pictures of clean filter (Figure 1.5) and two filters (Figure 1.6

and 1.7) after being used for SDI determination of seawater feed. Figure 1.6 shows SEM picture of filter pad after SDI determination that resulted in SDI value of 2.2. Figure 1.7 shows correspond-

ing SEM picture for SDI value of 4.8. The tick deposit on the filter, which was used to filtrate water

sample with higher SDI, is clearly visible.

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Figure 1.5. SEM picture of clean filter pad, before SDI memasurement.


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Field results have demonstrated that in majority of cases water from deep wells has very low SDI,

usually less than 1. RO systems, operating with good quality well water feed, practically do not

show any pressure drop increase across the membranes or flux decline. Surface water, after a con-

ventional pretreatment, usually has SDI in the 2 – 4 range. RO system processing feed water with

SDI in the 2 – 3 range shows stable membrane performance. Membrane cleaning frequency for

such feed water does not exceed 1 -2 per year. RO systems processing feed water of higher SDI, in

the 3 - 4 range, usually suffer from some degree of membrane fouling and somewhat higher mem-

brane cleaning frequency may be required. Long term operation of RO system with feed water hav-

ing SDI above 4 is not recommended. As mentioned already, past attempts to correlate turbidity

with SDI values were not successful. These two feed water quality indicators correlates to the num-

ber and size of suspended particles in a different way. However, usually the feed water with SDI in

the 2 – 3 range has corresponding turbidity below 0.1 NTU, usually at 0.05 NTU range.

1.13. Organic matter

Organic matter in RO feed is customary expressed as a total organic carbon (TOC). Surface water,

water from shallow wells and municipal secondary effluent usually contains some concentration of

dissolved organics. In surface waters the organic matter originates from decomposition of humic

substances. In shallow well water the presence of organics could be result of water infiltration

through strata containing natural organic matter. In both cases the TOC concentration is usually in

the range of 0.5 – 3 ppm, most of the time below 1 ppm. This low concentration of TOC in the fed

water has little effect on membrane permeability.

In RO feed originated from secondary effluent, the TOC concentration fluctuates in the wide range

between 2 – 10 ppm. The presence of high concentration of organic matter results in flux decline

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due to adsorption on the membrane surface. The initial rate of flux decline, due to organic adsorp-

tion is rapid, about 10 – 30% decrease from the initial permeability value. Afterwards, the permea-

bility usually stabilizes, the decline levels off. However, if colloidal matter is also present in the

feed water, the permeability decline is usually more severe, due to formation of thick, mixed fou-

lants layer of low water permeability.

Some potential water sources for RO processing are contaminated by oil and grease. Such condi-

tions could exist in industrial wastewater streams or in seawater in the areas of heavy maritime traf-

fic. Oil and grease have to be completely removed from the feed water prior entering the RO mem-

brane elements. Low concentration of oil and grease will not result in structural damage of the

membrane element but will cause severe decline of water permeability. Conventional method of

removing low concentration of oil and grease includes air flotation and/or media filtration with

flocculation using metal based flocculants.

1.14. Biological activity

Majority of well water sources have very low biological activity due to low concentration of nutri-

ents. The surface water sources could have at times very high level of biological activity. The in-

creased biological activity shows up during periods of intensive algae blooms, also called “red tide”.

During these periods, turbidity and suspended solids concentration could increase significantly, up

to the levels of 30 – 50 NTU and 10 – 30 ppm of suspended solids. In parallel, level of TOC could

increase to the range of 10 ppm TOC and above. During the cycle of algae growing and dying off,

the supply of nutrients increases, creating conditions for accelerated grow of bacteria.

During the conditions of algae bloom it is very difficult to maintain stable operation of pretreatment

and RO unit, unless pretreatment system has been design according to expected high level of bio-

logical activity.

The secondary effluent, originating from municipal wastewater, also has high level of biological activity, mainly high values of bacteria counts. However, this activity can be effectively controlled

prior to RO unit by maintaining 2 – 4 ppm of chloramines in the feed water. Wastewater reclama-

tion is the only RO application that biological activity can be controlled by applying chloramines.

Past attempts of applying this method to control biological activity in brackish or seawater systems

always resulted in accelerated membrane damage.

1.15. Feed water temperature

Feed water temperature affects both permeate salinity and the required operating pressure. Both

water and salt transport follow a similar trend (Equation 1.6 and Figure 1.8). The commercial RO

systems are designed to operate at constant permeate capacity (constant permeate flux). Therefore,

increase of feed water temperature will result in increased permeate salinity (higher quantity of salt

will pass the membrane and will be dissolved in a constant volume of permeate). This increase is

about 3% per degree C. The effect is similar for brackish and seawater systems (Figures 1.9 and

1.10). The changes of water permeability with temperature affects the net driving pressure required

(Equation 1.7). However, in parallel, temperature also affects osmotic pressure of the feed water

(Equation 1.8). With temperature changes net driving pressure and osmotic pressure have opposite

effect on feed pressure (Equation 8). An increase of feed water temperature at low temperature

range (~ up to about 30 C) enables production of a given permeate flow at reduced feed pressure

both in seawater and brackish water systems. At higher temperatures the reduction of feed pressure

in seawater systems levels off, mainly due to increase of osmotic pressure of the average feed (Fig-

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ure 1.10).

TCF = 1/exp(C (1/(273+t)-1/298)) (1.6)

Where TFC is temperature correction factor, t is temperature Centigrades, C is constant, characteris-

tic of membrane barrier material. For polyamide membranes a C values of 2500 – 3000 are being

used.

1.8 Feed water salinity

Feed water salinity affect the feed pressure required for production of a given quantity of p

roduced water per unit membrane area through value of the osmotic pressure. Osmotic pres

sure is related to water salinity according to equation 1.7.

Posm = R (T + 273) (mi) (1.7)

Where Posm is osmotic pressure (in bar), R is universal gas constant (0.082 l atm/mol K), T is the

temperature (in C), and (mi) is the sum of molar concentration of all constituents in a solution.

1.9 Net driving pressure

The feed pressure is function of osmotic pressure of feed – concentrate solution according t

o equation 1.8.

Pf = NDP + Pos + Pp + 0.5* Pd (- Posp) (1.8)

Where: Pf is feed pressure, Pos is average feed osmotic pressure, Pp is permeate pressure, Pd is pres-

sure drop across RO elements and Posp is osmotic pressure of permeate.

The NDP is related to specific permeability of the membrane and the design flux rate.

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Figure 1.8 Temperature correction factor for polyamide composite membranes.

Figure 1.9. Temperature effect on permeate salinity in brackish RO systems

0.00

0.20

0.40

0.60

0.80

1.00

1.20

1.40

1.60

1.80

2.00

0.0 10.0 20.0 30.0 40.0 50.0

RE

LA

TII

VE

FL

UX

VA

LU

E

FEED WATER TEMPERATURE, C

PERMEATE FLUX CHANGE WITH TEMPERATURE

0

100

200

300

400

500

0 5 10 15 20 25 30 35 40 45 50

Pe

rme

ate

sa

lin

ity

, p

pm

TD

S

Feed water temperature, C

Brackish RO, flux 28 l/m2-h

Feed 6,000 ppm TDS

Feed 1500 ppm TDS

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Figure 1.10. Temperature effect on permeate salinity in seawater RO systems. Feed pressure

values are marked as red bars and permeate salinity as gray bars.

1.16. Feed water pH.

Natural sources of brackish and seawater have relatively narrow range of water pH. The brackish

waters have pH in the range of 7.0 – 8.0. The pH of seawater is usually in the range of 7.5 – 8.3.

The pH is result of equilibrium of concentration of dissolved carbon dioxide (CO2) and bicarbonate

(HCO3). The concentration of bicarbonate in brackish water vary in a wide range. However, its con-

centration is usually above level of 250 ppm. Bicarbonate concentration in seawater is much lower,

almost always below 200 ppm, in most cases in the range of 140 – 160 ppm.

Knowledge of correct value of raw water pH is important in brackish and seawater applications.

In brackish water applications, prediction of pH of the concentrate stream enables determination of

potential for calcium carbonate scaling conditions.

In seawater applications, scaling is less of a problem. However, water pH could affect effectiveness

of the coagulation – flocculation process and also rate of boron rejection.

4. Feed water supply alternatives

5.8. Introduction

Seawater RO system

0

200

400

600

800

1000

1200

5 10 15 20 25 30 35 40 45

Feed water temperature, C

Fe

ed

pre

ss

ure

an

d

sa

lin

ity

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The selection of water source for the desalination system is usually made by the end user according

to professional recommendation provided by a hired consultant. In majority of cases, EPC Contrac-

tor has to adopt system configuration to raw water quality specified in the project specifications. In

isolated cases the EPC contractor has some degree of freedom in selecting location of raw water

extraction point. Therefore, it is responsibility of EPC contractor to collect information about raw

water quality and its seasonal and diurnal fluctuations.

Whenever possible, location of the water source for RO application should be selected to assure

good and stable water quality. Once the raw water source is selected and water quality specified, the

feed water treatment system has to be configured to produce adequate feed water quality also at the

periods of most adverse raw quality.

5.9. Brackish wells

Brackish wells are constructed to pump water from underground aquifers. In most cases, salinity of

water increases with depth of the water producing aquifer. If the aquifer is confined or semiconfined,

then composition of water being pumped will remain stable over time. A confined aquifer is a pre-

ferred water source for desalination applications, due to stability of water composition that trans-

lates to stable performance of the desalination unit.

In majority of cases, water extracted from brackish wells is characterized by low concentration of

suspended solids and organic matter. Water turbidity is below 0.1 NTU and SDI below 1.

However, if aquifer has been contaminated by infiltration of water from agricultural of industrial

activity, it may contain elevated concentrations of fertilizers (high nitrate content), organic matter or

even some concentration of toxic constituents.

The chemistry of water in an aquifer is controlled by chemical composition of formation where aq-

uifer exists and water movements that occurred over time.

The dissolved constituents present in ground water are fairly predictable however, their concentra-

tion is highly variable, between different locations. In most cases, wells, located in the same aquifer

and pumping water from the same levels, will produce water of very similar composition. Therefore,

information of water composition from existing well is a good indicator about water composition

from new wells that will be constructed in the same aquifer.

When new brackish wells are considered for an RO desalination systems, their configuration and

logistic of operation should stability of water quality and supply.

Important issues are:

1. Prevent formation of corrosion products. Wells casing, that will be in contact with brack-

ish water should be made of materials that will not corrode. For shallow wells PVC and

FRP casing could be used. For deep wells, where structural strength is important, stain-

less steel, 316 type, could be used. The maximum depth for use of PVC and FRP is cas-

ing limited by pipe diameter and wall thickness. For PVC casing, the maximum suitable

depth is in the range of 100 – 200 m. FRP can be used for deeper wells if FRP piping has

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adequate mechanical strength (collapse pressure), defined by pipe manufacturer.

2. Prevent mixing of water from different production zones. Pumping should be conducted

only from the designated layer. Other water bearing zones that well pipe is crossing

should be isolated by proper grouting. Mixing of water from different production zones

could results in precipitation of some constituents. It is specially critical in cases of

pumping water from anaerobic formation.

3. Prevent entrainment of sediments with pumped water. Measures taken to reduce carryo-

ver of solids with pumped water includes installation of proper screen, maintaining low

flow velocity through the screen pump well column. The screen opening should be in the

range of 0.5 – 1.0 mm. The flow rate through the screen should not exceed 0.03 m3/sec.

The flow rate in the well pipe should be below 1.5 m/sec. Wells located in limestone for-

mation may operate without screen with good results. Well pumps motors should have a

soft start to avoid creation of sudden water movements and carryover of solids.

4. Prevent environmental effect on water quality. To maintain consistent quality of well wa-

ter, it should not be exposed to the environment (air). Whenever possible, storage tanks

prior to RO processing should be avoided. Well pumps should have foot valves to main-

tain water level in the pump during system shut down. Decrease of water level will pull in

air, that may introduce air born contaminates or change ORP values (in case of anaerobic

water).

5. Avoid mixing water from different sources. For RO applications only water from the

same aquifer should be combined for use in the given desalination unit. Waters of dissim-

ilar aeration levels, i.e. water from anaerobic and aerobic sources should not be mixed to-

gether. Mixing of such sources could result in oxidation of soluble iron or H2S and pre-

cipitation.

6. Minimize addition of chemicals to well water. Well water should not be treated with

chemicals, prior to RO. Especially, well water of dedicated use for RO unit should not be

chlorinated. In the RO unit acid and/or scale inhibitor is used to prevent formation of

mineral scale during the RO process. Except for these two chemicals, no other chemicals

should be used when treating well water..

5.10. River bank filtration wells

River bank filtration (RBF) is applied for extraction of low salinity brackish water from a surface

source. The shallow RBF wells are located in aquifer that is hydraulically connected to the surface

water body, either river or lake. Because the water flows through aquifer formation it is filtered and

contains much lower concentration of colloidal and suspended solids than the surface source. How-

ever, in majority of cases, the quality of water obtained from RBF is not as good as water pumped

from brackish wells.

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The major issues with RBF water supply are:

1. Hydraulic conductivity of the aquifer. It applies to body of the water bearing formation and

also to permeability of the bottom of the surface water source. The water from the surface

water source infiltrates through the bottom surface, depositing suspended solids. The RBF

acts as a slow filtration rate filter without backwash. At high concentration of suspended sol-

ids, deposition and clogging of the infiltration surface could be quite rapid. Rivers flow ve-

locity of over 1m/s is usually sufficient to disperse deposits and maintain filtration rate.

2. Fluctuation of water salinity and water temperature. RBF system is hydraulically connected

to the water in the river. Changes of water salinity and water temperature are conveyed to

the underlying aquifer and reflected in the water conditions delivered to the desalination

plant.

3. Fluctuation of water supply. During a periods of severe draught, flow in the river could de-

crease affecting quantity infiltrating to the underlying aquifer and also reducing rate of dis-

persion of deposits on the infiltration surface.

4. Presence of organic matter. The concentration of organic matter in water pumped through

RBF system could be at times much higher than water pumped from brackish aquifers. Sea-

sonally, surface water could develop biological activity (algae bloom) leading to high con-

centration of organics. Also, shallow surface layer could contain high concentration of or-

ganic matter form by vegetation deposits.

The quality of RBF water is much higher than the hydraulically connected surface water source.

However, frequently it is not sufficiently good to send water directly to the RO unit without addi-

tional treatment.

5.11. Beach wells

Vertical beach wells are considered as effective alternative for supply of good quality seawater

to seawater RO desalination plants. The concept and configuration are very similar to the river

bank filtration discussed above and shown schematically on Figure 2.1. The differences be-

tween RBF and beach wells are related to water salinity and environmental conditions of the

body of water connected to the well. Due to high concentration of chlorides in seawater, mate-

rials of construction of the beach well column and the well pump have to be corrosion resistant

in seawater environment. Because vertical beach wells are shallow wells, PVC and FRP can be

used in beach wells casing and screens construction. Pumps have to be made of corrosion re-

sistant alloys (duplexs or equivalent).

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Figure 2.1. Schematic configuration of veridical beach well.

In cases that beach wells are located close to the water line, the beach well structure could be

exposed to waves and beach erosion. Therefore, a protective containment may be required.

In most cases chemical composition of water pumped from a beach well is very similar to com-

position of seawater water in coastal area. However, at some sites, significant differences have

been found.

It is more common for beach well to be under influence of low salinity water flowing under-

ground to the ocean. In this case, water from beach well will have lower salinity than sweater.

However, opposite conditions, i.e. water from beach well having much higher salinity than

seawater has been also found at some locations where beach wells were considered.

Another issues of concern are presence in beach well water of constituents that are absent in

open seawater but could have harmful effect membrane operation. These include presence of

elevated concentration of iron, manganese and/or hydrogen sulfide.

For example, seawater pumped from beach wells, at Morro Bay seawater RO desalination facil-

ity, contains high concentration of iron (5 – 15 ppm). Initial attempts of operation of RO unit

directly with this water source, resulted in unsustained frequency of replacement of cartridge

filters. A green sand iron removal system had to be installed to enable reliable operation of the

RO unit. Similar conditions of high concentration of iron and manganese in beach well water

are experienced in the 14,500 m3/day seawater desalination plant at Santa Cruz, Mexico. Also

here, green sand system is being use as a feed water pretreatment step.

Presence of hydrogen sulfite was discovered in beach well water at the seawater RO desalina-

tion unit at Avalon, Catalina Island, CA. The hydrogen sulfite was result of anaerobic condi-

tions in formation where beach well has been located.

At some locations, beach wells are very effective solution for supply of good quality feed water

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to the RO unit. If the costal formation has good water permeability and do not release excessive

quantity of particles during pumping (usual conditions with limestone rock), utilization of

beach wells as feed water supply source will result in very simplified pretreatment and reliable

operation of the RO unit. Large seawater RO plants that successfully utilize beach wells are lo-

cated in Malta, Caribbean and Oman. Some of the facilities utilizing beach wells are listed in

Table 2.1.

Table 2.1. Listing of large SWRO desalination facilities that utilize beach wells

Location Number of wells Capacity, m3/day

Sur, Oman 33 4,000 – 10,000 each

Pembroke, Malta 54,000

Malorca, Spain 16 5,700 each

Ghar Lapsi, Malta 24,000

Pemex, Mexico 3 14,500 each, horizontal wells

Fukuoka, Japan 82,900 infiltration gallery

Morro Bay, California 5 5,000

5.12. Horizontal wells

Horizontal wells, also called Ranney type intake wells, are another approach to provide seawater

that is filtrated already by water bearing formation. The collector wells are located on shore, in

coastal area.

\The Ranney wells are constructed by placing large concrete ring segment on shore. The ring seg-

ment has a cutting shoe on the bottom rim. The ring is sunk in place by excavating soil inside the

ring. Once the firs ring is in place, the next one is mounted on top of it forming well caisson. When

the designed depth is reached a concrete plug is placed in the bottom and horizontal laterals are

driven through openings in the bottom segment ring. The laterals have perforations for seawater to

infiltrate to the caisson.

The caisson could have diameter of 3 – 6 m and depth of up to 50 m. The laterals could be 20 – 60

m long. Schematic diagram of horizontal well is shown on Figure2.2.

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Caisson

Laterals

Pump

Collector well configuration

Diameter 3 – 6 m (10 – 20 ft)

Depth 10 – 50 m (30 – 150 ft)

Yield 80 – 4,000 m3/hr

(0.5 – 25 MGD)

Figure 2.2. Schematic diagram of collector well

After completion of wet well construction it is topped with structure that contains pump(s) and elec-

tric gears.

The collector wells have larger water infiltration area than vertical beach wells. Therefore, on the

average, the collector wells will have higher water yields that it would be possible with vertical

beach well, constructed at the same location.

Seawater produced by collector wells is of a similar quality as would be produced by vertical beach

wells at the same location.

5.13. Slant wells

The slant wells, also called horizontally directional drilled wells (HDDW), evolved from commer-

cial technology used to lay underground cables and oil and gas pipelines lines. Schematic diagram

of slant well is shown on Figure 2.3..

Probably the best known company, providing this technology for seawater RO application, is Ne-

odren, located in Spain. Within the last 10 years Neodren installed number of HDDW systems,

mainly in Spain.

The slant well consist of high density pipe, entering the soil, below the seabed at 15 – 20 degree an-

gle. The pipe diameter is around 450 mm and subsurface length could be 200 – 400 m. Seawater

enters the HDD pips through wall perforations, sized at about 120 micron size.

Configuration of HDD pipe is shown on Figure 2.3.

HDD intake structure for a large desalination plant would usually consist of multiple pipes as

shown on Figure 2.4, depicting intake structure used at 82,000 m3/day sweater RO plant at Alicante,

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Spain. Seawater obtained from HDD intake is of better quality than produced from an open intake.

However, additional pretreatment of feed water is still required to reduce the SDI to the level re-

quired in RO plants.

Figure 2.3. Schematics of horizontal directional drilling (HDD) intake

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Figure 2.4. Model of HDD intake system at 82,000 m3/day seawater RO desalination plant, Alican-

te, Spain. Courtesy Neodren.

5.14. Seawater infiltration galleries.

Infiltration galleries are similar in operation to HDD intakes. The difference is mainly in method of

construction and use of man made filtration layer rather than existing seabed, as it is in case of

HDD structures. The infiltration galleries are constructed by removing layer of designated seabed

area, placing perforated collector pipes and cover them with a layer of granular filter pack. Sche-

matic diagram of 102,000 m3/day infiltration gallery at the seawater desalination plant at Fukuoka,

Japan, is shown on Figure 2.5.

The infiltration galleries operate as a slow filters with filtration rate in the range of 0.12 – 0.25

m3/m2/hr. Like other slow sand filters, backwash is not used for filterability recovery. The filtera-

bility is restored by periodically scarping few cm of a lop layer. After few years of such operation

the filter bed has to be replaced with new filtration pack.

Fukuoka Infiltration Gallery: 102,000 m3/day (27 mgd)

64X313 m (210’X1027’)

Excavation thickness 3 m (10’)

Figure 2.5. Infiltration gallery at sweater RO desalination plant, Fukuoka, Japan.

At this point the only commercial seawater plant that utilizes infiltration gallery is 50,000 m3/day

RO plant at Fukuoka, Japan.

5.15. Open surface intake

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The open surface intake is the most common method of supplying seawater to RO desalination

plants. This configuration is usually the lowest cost water supply alternative, as indicated in Table

2.2.

Seawater supplied from an open intake always requires filtration pretreatment prior to entry to

membrane unit. The extend of pretreatment will depend on expected seasonal fluctuations of sea-

water quality. The quality of surface seawater could be affected both by costal discharge, weather

conditions and seasonal biological activity (algae bloom). Once the expected range of seawater

quality has been determined, a proper pretreatment method could be designed.

Table 2.2. Economic weight of various alternatives of sea water supply to RO desalination plants

Well type Capacity Development cost

Vertical well 400 – 2,000 m3/day (0.1 – 0.5 MGD)

$200K - $500K

Slant well 800 – 8,000 m3/day (0.2 – 2.0 MGD)

$1M - $1.5 M

Collector well 2,000 – 100,000 m3/day (0.5 –

25 MGD)

$3M - $5M

Beach filtration galleries Very high output Very high cost

Conventional intakes Very high output Moderate cost

5.15.1. Configuration of open intakes.

Open intakes configuration will depend to some extend on system size. Intakes for small

capacity desalination systems may consist of pump attached to a dock or suspended from a

floating platform and connected by short pipe segment to the on shore raw water storage

tank.

Large desalination system could utilize on shore intake located at the end of a channel pro-

truding into shore. Another, more common configuration is off shore velocity cap structure

connected by pipe to on shore wet well (Figure 2.6)

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Figure 2.6. Schematic configuration of open surface off shore seawater intake.

The off shore intake configuration, usually more expensive than the channel intake, enables system

designer some freedom in selection of intake location with somewhat improved seawater quality.

If possible, the location of off shore intake should be selected at depth that is below the depth of

storm base waves according to the annual lowest tide. The intake should be located at the depth that

will not interfere with marine traffic and location of low biological activity.

From the aspect of sweater quality it is desirable to collect seawater at a depth of over 10 m. How-

ever, such a depth may not be available at a reasonable distance from shore. If a required seabed

depth is located at distance larger than 500 m off shore, it is more cost effective to select location at

smaller immersion depth and shorter distance.

An example of intake structure is shown on Figure 2.7. The top of intake structure is 4 m below low

tide level. The water intake opening is 1 m above seabed level.

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Figure 2.7. Example of off shore intake structure (courtesy Ian Larsen)

As shown on the above drawing, the intake structure has provision for chlorination. The chlorina-

tion is only applied intermittently, as shock chlorination, to control grow of marine organism inside

the intake structure. Another measure to reduce biological activity is concrete cover on the top of

the intake structure that prevents direct light to reach inside the intake.

Another configuration of intake structure is shown on Figure 2.8. This structure is being used in

54,000 m3/day RO seawater desalination plant in Mediterranean area. The structure has inlet open-

ings in the lower part of the intake head. In this configuration also there is no direct access of light

to the internal surfaces of the intake structure. The grow of marine life inside the intake is controlled

by application of intermittent shock chlorination (5 – 10 ppm of free chlorine) and mechanical re-

moval of incrustations from intake walls. The mechanical cleaning procedure involves applying a

hydraulic trust to plastic ball(s) and sending them through intake pipe to the off shore structure. The

intake structure has to have built in an access (man hole) for recovery of cleaning balls. This pipe

cleaning procedure is called “pigging”. During application of pigging, Ro plant is out of operation.

The pigging procedure could take 4 – 8 hr.

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Figure 2.8. Alternative configuration of intake structure (courtesy Ian Larsen)

The important issues in designing of intake structures are reduction of impingement and entrain-

ment marine life during operation of desalination plant. Low rate of removal of marine life with

seawater is achieved by maintain low inflow velocity of seawater into the intake structure The open

inlet area should be designed to maintain inlet flow below 0.2 m/sec.

The intake structure is connected to the on shore located wet well by a single or multiple pipes. The

connecting pipes are laid in a trench or anchored to the seabed utilizing concrete blocks (shown on

Figure 2.9 and 2.10).

To assure longevity of intake structure and connecting piping, it has to be constructed from materi-

als resistant to seawater corrosion. Usually the intake structure is made of concrete and connecting

piping from high density polyethylene (HDPE) or FRP.

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Figure 2.9. Installation of concrete anchors on intake pipe (courtesy Ian Larsen).

RO sweater desalination plant converts fraction of seawater into product water and discharges con-

centrate, corresponding to a flow of 40 – 50% of the feed water flow, back to the sea. The concen-

trate outfall line, starting at the discharge of energy recovery devices, extends to the ocean, to the

discharge point. The terminal point of outfall line has to be at location that satisfies the following

conditions:

1. Provides rapid dispersion and salinity reduction.

2. Will not results in short circuit of discharged concentrate with the intake inlet.

3. Will be located at a depth that protect discharge structure from storm waves, marine traffic,

etc.. (similar to considerations for intake structure).

Figures 2.11 and 2.12. show configurations of intake and concentrate discharge arrangements at the

Fukuoka desalination plant.

The drawing shows that the top if intake structure is about 7 m and concentrate structure about 11 m

below the seawater level. Both lines are extending about 200 m into the ocean, with the terminal

points about 60 m apart.

As indicated on Figure 2.12, the diameter of the Intake line is 1.2 m and of concentrate line 0.7 m.

These differences of pipes diameter reflects the difference of flow rates of feed and concentrate dur-

ing normal operation of the plant. Some plants design both feed and concentrate lines of the same

diameter. This is to enable operation of pretreatment system at full capacity, also during the time

when the RO membrane units are not in operation (commissioning period) or when they are operat-

ing at partial capacity.

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Figure 2.10. Delivery of intake pipe with concrete anchors installed to the plant site (Courtesy

Oceana)

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Figure 2.11. Intake and discharge system at SWRO desalination plant, Fukuoka, Japan

0.7m dia. x 230 m in length Maintenance man-

hole

Discharging pipe system

Diffuser Equip-ment

1.2m dia. x 220 m in length Maintenance man-hole

Intake pipe system

Intake tower

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Figure 2.12. Conveyance of the feed intake and concentrate discharge lines at Fukuoka, Japan

Effective dispersion of the concentrate is an important issue in seawater desalination plants. Usually,

plant permitting process includes submittal evaluation of concentrate dispersion based on modeling.

The method of disposal and dispersion of concentrate applied at the Fukuoka desalination plant is

demonstrated on Figure 2.13.

According to this model of dispersion process, the background salinity of the ocean is reached al-

ready at the distance of 12 from the discharge point. This is archived by configuring the concentrate

discharge structure with set of nozzles that increase concentrate discharge velocity and create turbu-

lence. Pressure drop losses on discharge nozzles have to be included in development of hydraulic

profile for the outfall line.

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Effluent volume: 67,000m3/d

Discharging velocity: 6.0m/sec

Concentrated seawater: 5.8%

Raw seawater: 3.5%

l l l

0 8 12

l l l5.8 3.6 3.54 Concentration(%)

Distance (m)

Diffuser Equipment for Discharged of Concentrate,

Figure 2.13. Concentrate discharge structure at the SWRO desalination plant, Fukuoka, Japan.

5.16. Estimation of intake cost

The estimation of cost of intake and outfall system is based on system configuration, local condi-

tions and equipment and material used in intake construction.

It includes the following cost items:

1. Cost of connecting pipe, based on pie weight, material cost and pipe length.

2. Cost of pipe components

3. Cost of pipe placement

4. Other components and equipment, derived based on reference cost and scale up fator.

5. Contingency factor

Example of estimation of intake – outfall system cost

Table 2.3. Intake – outfall system cost parameters

Cost component Parameter and cost

Intake flow 300,000 m3/day

Intake pipe length 1,500 m

Outfall flow 150,000 m3/day

Intake outfall length 1,000 m

Pipe material HDPE

Pipe diameter 1,600 mm

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Pipe weight 300 kg/m

Pipe and assembly cost $3/kg ($900/m)

Pipe placement cost $1/kg ($300/m)

Table 2.4. Cost components of intake – outfall system. 300,000 m3/day inflow, 150,000 m3/day

outfall

Intake pipe ($1,950/m) $2,925,000

Outfall pipe (1, 950/m) $1,950,000

Subsurface structure $1,000,000

Beach crossing $2,000,000

Siphon structure $200,000

Clearwell pumps $2000,000

Total $10,675,000

Contingency (30%) $3,200,000

Total $13,875,000

5.17. Collocation with power plant

The intake cost could contribute 5 – 20% of the overall cost of the desalination system. At some lo-

cations permitting of intake and concentrate discharge is a very lengthy process, creating significant

delay of implementation of desalination projects.

Collocating of desalination plant at the site of power plant that utilizes significant volume of sea-

water for cooling is economically attractive alternative to stand alone desalination system.

Electric power plants in coastal areas, utilize large volume of seawater for cooling of steam conden-

sers. After the cooling process, seawater is discharged back to the ocean. The temperature of dis-

charged seawater is 5 – 10 C higher than the temperature of inlet water.

RO seawater plant can be incorporated into water circuit of the power plant by taping to the sea-

water line on condenser discharge. After RO processing, the concentrate is return to the same dis-

charge line, downstream of the feed connection point. Schematic diagram of the seawater flow in

RO plant collocated with power plant is shown on Figure 2.14.

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Power plant co-sitting configuration

Figure 2.14. Flow diagram of SWRO desalination plant collocated with power plant at Carlsbad,

CA.

According to Figure 2.13, the power plant pumps 2.3 Mm3/day (600 MGD) of seawater for conden-

ser cooling. After passing through condenser, 380,000 m3/day of the 2.3 Mm3/day flow would be

diverted to the SWRO facility. The SWRO plant will operate at 50%recovery rate. Therefore,

190,000 m3/day of concentrate would be returned to the power plant discharge stream. The salinity

of SWRO concentrate will be about 68,000 ppm TDS. After mixing with 1.9 Mm3/day of condenser

discharge, the resulting salinity will be about 37,000 ppm TDS, only 9% above normal seawater

salinity.

The collocation alternative seems is beneficial, both improving process economics and addressing

issues of permitting of seawater withdrawal and discharge.

However, evaluation of feasibility of collocation for a specific project should consider the following

issues:

1. Quality of seawater source. Power plant quality requirements of cooling water are signifi-

cantly less stringent than required in RO applications. Therefore, concentration of suspended

solids, including biological debris, in discharged cooling water could be significantly higher

than in seawater produced by well designed SW RO dedicated intake.

2. Some power plant condensers are constructed from metals that undergo some level of corro-

sion and add metal contaminants to the discharged stream. Concentration of metals should

be carefully evaluated in respect of membrane fouling.

3. It is common for power plant to conduct continuous or intermittent chlorination of seawater

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inflow. Usually, heat treatment procedure is applied periodically to remove incrustation from

the condenser surfaces. During heat treatment procedure RO plant will have to be shut down

during that time to avoid presence of sharp edged shell fragments in the feed stream.

4. The increase of seawater temperature during the cooling cycle could be beneficial if the

seawater original temperature is low, below 20C. At such conditions, increased seawater

temperature will result in lower feed pressure and lower energy requirement of the SWRO

system. However, during the periods of high seawater temperature (> ~ 25 - 30 C), the addi-

tional temperature increase contributed by the condenser of the power plant, could result in

less beneficial operating conditions of the SWRO system. In the higher temperature range,

above 30 C, the reduction of feed pressure with temperature is marginal. On the other hand

there will be significant increase of permeate salinity at higher temperature. Therefore, to

compensate for permeate salinity increase a larger second pass processing will have to be

included in system configuration. At such conditions, the additional equipment cost and

higher operating cost could be higher that savings realized by not including dedicated intake

and concentrate discharge in scope of the desalination project. The present commercial RO

membrane have maximum temperature limit of 40C. Therefore, the design of using conden-

ser discharge as RO feed water, should have provision not to exceed this maximum feed wa-

ter temperature, otherwise, plant operation will be outside membrane warranty terms set up

by membrane manufacturer.

5.18. Design criteria of feed water supply systems and configurations

The design criteria of feed water supply for RO units have to follow sound engineering principle

and experience gained by the desalination industry in design of similar systems.

Some of the design considerations are listed below:

1. Saline waters and seawater are highly corrosive, therefore, raw water supply systems com-

ponents should be constructed from corrosion resistant materials.

2. Raw water supply system should not introduce additional constituents to the water that may

adversely affect RO membranes or other equipment.

- Exposure of raw water to light should be minimized as light exposure my provide

energy for biological growth.

- Chlorination of raw water should be minimized as presence of oxidants could result

in development of nutrients for biological activity in the RO system.

- Stagnant area in the water supply system should be avoided as stagnant water sup-

ports biological activity.

- Raw water should not be aerated beyond what is required by the treatment process

(DAF, biological filters). Excessive concentration of oxygen could result in elevated

corrosion rate of metal components.

5. Raw water supply lines should be short as possible to reduce system cost and holdup volume

of water in the water supply system.

6. Number of pumping steps in the water delivery and concentrate outfall systems should be

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reduced to an absolute minimum. Each pumping step increases system cost and energy re-

quirement of the process.

5.19. Equipment and hydraulic profile

As mentioned already above, all components of water supply system should be made from corro-

sion resistant materials. For piping, the materials of choice are high density polyethylene HDPE and

FRP. The HDPE is manufactured according to standardized sizes (diameter and wall thickness).

Dimensions of FRP piping and components could vary between different manufacturers.

For seawater applications, wetted pars of valves should be made of duplex steels (disk) and cast

iron with EPDM liner (valve body).

Seawater storage tanks could be made of concrete with protective coating, FRP or glass lined stain-

less steel.

Transfer pumps should be constructed from duplex steel. Due to high cost of this construction mate-

rial, whenever practical, a horizontal split case pumps, should be specified in place of vertical

pumps.

In the seawater applications the recovery rate is about 50%. Accordingly, any energy expanded in

the water supply system contributes twice its value to the final energy usage per unit of product wa-

ter produced. Therefore, it is important to select intake pumps with high hydraulic efficiency. The

design of raw water supply and outfall system should follow a hydraulic profile developed for spe-

cific site conditions and elevation differences.

As shown schematically in Figure 2.6, the water level in the wet well, located on shore, will dpend

on the level of seawater: the high and low tide. The depth of the wet well and immersion of the

pump has to be designed according to the lowest tide (specified in the project documents or listed in

reference information) and the friction loses in the connecting piping. The friction loses of the pip-

ing connecting off shore intake structure with the wet well are calculated at conditions of maximum

flow.

The friction loses are combined loses of friction head in the pipe and form loses. The pipe friction

head is the pressure loss in straight segments of the connecting pipe. The form loses are friction los-

es in valve and fittings. The form loses are calculated for individual components and are combined

together in one value.

The pipe loses can be calculated using Haze – Williams formula (Equation 2.1)

(𝐻

𝐿) = 10.643 (

𝑄

𝐶)1.852 𝐷−4.87 (2.1)

Where: H is head loss over length L, Q is flow rate, D is pipe diameter and C is a roughness con-

stant. The roughens constant depends on the condition of the internal pipe walls. The rougher the

wall surface, the lower the value of C, resulting in higher head loses.

The form loses, H, can be calculated according to Darcy Formula, listed as equations 2.2 and 2.3.

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(𝐻

𝐿) =

𝐾

𝐷ℎ 𝑉2/2g (2.2)

𝐻 = 𝐾 𝑉2/2g (2.3)

In the above equations, K is a Darcy constant, V flow velocity and g is the gravity constant.

Examples of C and K values that are applied in the Hazen – Williams and Darcy equations are listed

in Table 2.5

Table 2.5 Example of values of constants used in Haze – Williams and Darcy equations

Component C K

Plastic pipe 140 – 150

Steel pipe 145 – 150

Rounded inlet

0.25

Rounded outlet

1.00

Gate valve

0.20

Butterfly valve

0.20

Globe valve

10.00

Elbow 45 deg

0.30

Elbow 90 deg

1.10

Example of calculation result of friction loses in straight HDPE pipe, applying Hazen Williams

equation, is provided in Table 2.6. It is evident that with small diameter pipe, large friction loses

could be created if pumping over long distances is necessary.

Table 2.6. Example of pipe friction loses according to pipe diameter.

Flow rate: 200,000 m3/day, Q = 2.36 m3/sec. Roughness constant, C = 140

Pipe diameter, m Flow velocity, m/sec Hydraulic gradient, m/100m

2.5 0.5 0.006

1.7 1.0 0.040

1.4 1.5 0.110

1.2 2.0 0.230

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The hydraulic profile is the basis for selection of proper equipment and process optimization. Based

on hydraulic profile, the designer is able to specify required discharge head of pumping equipment.

Process optimization is conducted to reduce overall energy usage. This is achieved through selec-

tion of pipe diameter, system configuration and elevations for location of equipment and tanks. As

mentioned before, the objective is to reduce number of pumping steps and exposure of water to out-

side environment (air and light).

6. Configuration of feed water pretreatment process

6.1. Conventional pretreatment

The term “conventional pretreatment” usually refers to use of granular media filtration as op-

posed to membrane filtration. However, this differentiation is being blurred today as it is com-

mon to design pretreatment systems that integrate granular media separation or clarification

through settling with the membrane filtration step.

6.1.1. Configurations and components of conventional pretreatment system

The configuration of desalination unit and the pretreatment system depends on source and

quality of the raw water.

In case of desalination system operating with raw water pumped from dedicated brackish

wells or seawater beach wells, the pretreatment is very minimal, as shown on Figure 3.1.

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Figure 3.1. Configuration of RO unit operating with well water.

The configuration of pretreatment unit is limited to sand separator, addition of acid and/or

scale inhibitor and cartridge filtration, as shown on Figure 3.2.

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Figure 3.2. Configuration of pretreatment unit in desalination plant treating well water.

Some systems, that operate with shallow wells, that occasionally produce water of elevated

turbidity, utilize high filtration rate pressure filters prior to the cartridge filters. In case of

presence of media filtration, sand separation equipment is not required.

In isolated cases additional pretreatment is applied to the well water to adsorb and remove

organic industrial microcontaminants utilizing activated carbon filters. Some systems uti-

lize green sand filters for removal iron and/or manganese from the feed water if present in

high concentrations.

Desalination systems that treat surface water require some form of filtration prior to car-

tridge filtration.

If the concentration of suspended solids is moderate, than utilizing granular media filtration

could be sufficient.

Granular media filter could be configured as pressure filters, ether vertical or horizontal, as

shown schematically on figure 3.3.

Figure 3.3. RO system treating surface water.

Pretreatment configuration shown on Figure 3.3 includes chlorination, acidification, fol-

lowed by addition of ferric coagulant. The coagulated water is filtrated with vertical pres-

sure filters. The assumption is that effluent from the media filters will have quality suffi-

cient for introduction to membrane unit. The cartridge filter located after the media filtra-

tion step is to protect pumps and membrane elements from sudden influx of particulate mat-

ter, for example due to granular filter media break through.

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Chlorination of raw water should be only applied as a intermittent measure. Continuous

chlorination could result in conversion of TOC to assimilable organic carbon (AOC). In-

creased AOC could be utilized by bacteria that survived chlorination and promote bacterial

grow in the membrane unit.

Whenever, chlorine is added to the feed water, it has to be reduced, preferably using sodium

bisulfate according to the following reaction:

NaHSO3 + HOCl = NaHSO4 + HCl (3.1)

Three parts of sodium bisulfate are required for one part of free chlorine. The reaction is

very rapid, proceeding to a complete dechlorination in seconds.

ORP analyzer is used to monitor dechlorination reaction. The target ORP value for dechlo-

rination is below 200 mV.

The granular pressure filters, either in vertical or horizontal configurations, are usually used

in small and medium size systems. In very large desalination systems gravity media filters

are being used.

However, in Spain, pressure filters are being used in majority of seawater desalination sys-

tems of all sizes.

Majority of pretreatment systems that utilize granular media filtration, incorporate coagula-

tion and flocculation to improve removal of colloidal particles. Coagulats of choice are iron

based salts. The usual dosing rate of metal coagulants is in the range of 1 – 20 ppm. Some-

times organic polymers are also added in the coagulation process to increase flock strength.

The usual dosing range of filtration polymers is in the range of 0.1 – 1 ppm. Utilization of

coagulants requires incorporation of solids management system to treat backwash stream

produced by granular filters. The objective of solids management system is to concentrate

and dewater backwash effluent to a solids concentration above 20%. At this level of solids

concentration, the backwash residual can be transport off site and dispose to land fill. Gran-

ular media filtration systems that do not utilize coagulants usually are able to discharge

backwash effluent together with concentrate stream back to the ocean.

At some locations surface seawater may seasonally experience high concentration of sus-

pended and colloidal solids. The rough indicator of raw water quality is turbidity. At turbid-

ity levels above 20 – 30 NTU a clarification step could be required prior to media filtration.

The selection of clarification technology will depend on nature of suspended solids. If sol-

ids are manly of inorganic nature (silt) than most likely settling type clarifiers would be

more effective.

However, if suspended solids are of organic nature (algae), which are buoyant and may not

settle well, than dissolved air flotation (DAF), would be more effective technology to apply.


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Membrane filtration, both microfiltration and ultrafiltration, is pretreatment technology of

choice for wastewater reclamation systems. In these applications, membrane filtration re-

placed almost completely conventional pretreatment methods used previously.

Similar trend is observed in seawater desalination systems treating seawater from open in-

takes.

The configuration of seawater desalination system that utilizes membrane pretreatment is

similar to system with conventional pretreatment, with media filtration replaced with mem-

brane unit, as shown on Figure 3.4. The immersed (vacuum driven) membrane filtration

units are immersed in membrane tanks, similarly to granular media filters. In pressure driv-

en units the membrane modules are connected in parallel and configured as membrane

trains.

The membrane backwash unit plays similar role as filtration media backwash unit used in

granular media filters. One of the differences is use chemicals, mainly NaOCl, during

chemical enhanced (CEB) backwash

Number of seawater desalination systems that utilize membrane filtration is growing. Still

the majority of seawater RO plants in operation and new systems being built utilize the

conventional filtration technology.

Figure 3.4. Schematic configuration of seawater desalination system with membrane pre-

treatment.

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Compared to the membrane filtration systems, the pretreatment systems based on granular

media filtration are in most cases less expensive to construct and operate. Evaluation of

configuration and economics of pretreatment systems based on membrane filtration and

granual media filtration is provided in Chapter 4.6.

Another disadvantage of membrane filtration pretreatment technology is extended use of

chlorination for membrane performance recovery. As mentioned before, use of chlorine in

the pretreatment process could lead to biofouling of RO membranes. In addition, the hy-

draulic conditions of operation of membrane filtration systems lead to breaking of microor-

ganism present in the seawater and release of cell fluids. This matter could be metabolize

by other bacteria for cell growth and formation of biofilms in membrane unit.

Utilization of coagulation improves stability of operation of membrane filtration systems,

improving stability of membrane permeability. Addition of metal coagulant at the level of

0.1 – 0.5 ppm could result in significant improvement of performance.

However, utilization of coagulat will require inclusion of solids management system as a

part of pretreatment unit. Therefore, the tendency is not to include coagulation in configura-

tion of membrane pretreatment systems and relay of backwash procedures to maintain a

sufficient level of membrane permeability.

The membrane filtration technology consists of wide range of configurations and some

membrane modules configurations are more robust in respect of treatment of streams con-

taining high concentration of suspended solid, for example membrane elements utilized in

membrane bioreactor (MBR) applications.

Usually the MBR membrane modules have low membrane area packing density and would

be quite expensive for use as pretreatment in RO desalination systems.

In most cases, the membrane modules utilized in RO applications are the same as being

used in potable applications.

The tolerance of membrane filtration technology to high concentration of suspended solids

in raw water is similar to granular media filtration. Also here if the raw water turbidity ex-

ceeds 20 – 30 NTU range, an initial clarification step may be necessary to maintain stable

performance of membrane filtration system.

The addition of initial clarification step is necessary if raw water experiences high turbidity

levels for extended periods of time. If the increase of turbidity is only during small fraction

of the annual operating time, maintaining lower output capacity of the pretreatment system

or even temporary discontinue of its operation could be considered as more cost effective

solution rather than addition of clarification equipment that will be idle most of the time.

The schematic diagram on Figure 3.4 shows cartridge filtration as part of the pretreatment

system. The usual approach is to include cartridge filtration if membrane filtrate is store in

an intermittent tank prior to membrane unit. If membrane filtrate flows directly from filtra-

tion system to the suction of RO feed pumps, cartridge filtration equipment could be omit-

ted from the feed water pretreatment process.

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6.3. Raw water quality and pretreatment requirements.

The quality of raw water is closely related to water source. The configuration of pretreatment sys-

tems for RO applications has evolved around specific water sources. For each RO application:

brackish, seawater and wastewater reclamation pretreatment system configurations have been de-

veloped that can effectively produce feed water of adequate quality.

Table 3.1 includes water quality parameters for the three major water types of water supply sources.

The last row of the table lists configuration of pretreatment systems that in majority of circumstanc-

es will be sufficient to produce feed water of adequate quality.

Table 3.1. Representative raw water quality associated with major types of water supply sources

Quality parameter Well water Surface water Secondary effluent

Turbidity < 1 NTU < 5 NTU 2 - 10 NTU

SDI < 1 < 5 not measurable

Suspended solids < 1 ppm < 5 ppm < 20

TOC < 1 ppm < 5 ppm < 20

SiO2 < 25 ppm n. a. n. a.

Common configura-tion of pretreatment system

Acidification

Scale inhibitor

Cartridge filtration

Acidification

Flocculation

Media filtration


Membrane filtration

Acidification

Scale inhibitor


However, at some locations, quality of raw water could fluctuate outside range listed in Table 3.1.

In such cases the pretreatment system has to be augmented by additional treatment steps. Table 3.2

provides recommended configurations of pretreatment system according to water source and range

of water quality parameters.

Table 3.2. Recommended configuration of pretreatment system according to raw water quality

Water source Water quality parame-

ters

Configuration of pre-

treatment system

Comments

Brackish well water Turbidity < 0.2 NTU

TSS < 2

SDI < 1.0

Acidification

Scale inhibitor


Brackish well water Turbidity > 0.2 NTU

TSS > 2

SDI >1.0

Sand filtration

Acidification

Scale inhibitor

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Brackish well water Turbidity < 0.2 NTU

TSS < 2

SDI < 1.0

Presence of dissolved

Fe & Mn

Acidification

Scale inhibitor


Maintain feed water at

anaerobic conditions

Seawater beach well Turbidity < 0.2 NTU

TSS < 2

SDI < 1.0

Cartridge filtration If seawater is under

influence of brackish

water, acidification

and scale inhibitor may

be required

Seawater beach well Turbidity >0.2 NTU

TSS >2

SDI >1.0

Sand filtration


If seawater is under

influence of brackish

water, acidification

and scale inhibitor may

be required

Seawater open intake Turbidity < 5 NTU

TSS < 5

TOC < 2

Acidification

Coagulation + floccu-

lation

Single stage granular

dual media filtration

Short excursion of tur-

bidity up to 20 NTU is

possible for few days

in year

Seawater open intake Turbidity < 5 NTU

TSS < 5

TOC < 2

Membrane filtration Short excursion of tur-



in year

Seawater open intake Turbidity 5 -20 NTU

TSS > 5

TOC > 2

Acidification


lation

Two stage granular





in year

Seawater open intake Turbidity 5 -20 NTU

TSS > 5

TOC > 2

Acidification


lation

Membrane filtration


bidity up to 30 NTU

are possible for few

days in year

Seawater open intake Turbidity > 20 - 30

NTU

TSS > 5

TOC > 2

Settling clarification


lation



Suspended solids

mainly inorganic parti-

cles(silt)

Seawater open intake Turbidity > 20 - 30 Settling clarification Suspended solids

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NTU

TSS > 5

TOC > 2


lation

Membrane filtration

mainly inorganic parti-

cles(silt)


NTU

TSS > 5

TOC > 2

DAF


lation



Suspended solids

mainly organic and

biological parti-

cles(algae)


NTU

TSS > 5

TOC > 2

DAF


lation

Membrane filtration

Suspended solids

mainly organic and

biological parti-

cles(algae)

Wastewater secondary

effluent

Turbidity 2 – 20 NTU

TSS < 20

TOC < 20

Membrane filtration

Acidification

Scale inhibitor


Chloramine concentra-

tion of 2 – 4 ppm

maintained in the RO

unit

Wastewater MBR ef-

fluent

Turbidity <1 NTU

TSS < 2

TOC < 20

Acidification

Scale inhibitor


Chloramine concentra-

tion of 2 – 4 ppm

maintained in the RO

unit

7. Pretreatment equipment

7.1. Screening

The extend of screening of raw water will depend on application and type of the pretreatment pro-

cess.

For brackish RO system, operating with well water, usually no screening is applied in the pretreat-

ment system. Only in case that well releases sand particles, sand trap screen should be applied. In

majority of cases, the quantity if sand being released from well is very small and manually cleaned

sand screen will be adequate. Usually, the sand screen is configured as a wedge wire type cylindri-

cal shape screen installed in flow through vessel that is part of the raw water supply piping (sche-

matically shown on Figure 3.2).

Raw water coming from an open intake structure could contain large objects that could damage

pumping equipment. The initial screening is conducted by the bar screen installed at the entry open-

ings to the intake structure. In submersed structures the opening bar spacing will be in the range of

3 – 10 mm. This is to maintain entrance form velocity not to exceed 20 cm/sec. In channel configu-

ration intakes the opening bar spacing if the “trash rack” would usually be larger in the range of 50

– 100 mm. The large spacing trash racks would be followed by finer screens.

Large capacity installations could utilize automatically washed traveling band screens as shown on

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Figure 3.5.

Figure 4.1.1. Dual flow intake band screen (web page Horim Industries Inc., Korea)

RO seawater systems that utilize multimedia filtration in the pretreatment unit would not require

any additional screening beyond 3 – 10 mm range screens. Any debris that will enter the pretreat-

ment system will be stopped on the surface of filtration media layer and will be discharged from the

system during the filter backwash step.

RO systems that utilize membrane filtration in the pretreatment would require micron range screens

ahead of membrane filtration system. The rating of screens is specified by membrane manufacturers,

usually in the range of 80 – 120 microns.

Variety of screening equipment is available on the market. The important issues in selection of

screening equipment are:

1. Material of construction to be compatible with seawater environment. The equipment op-

erates at low pressure, therefore, plastic materials of construction are preferred.

2. Low energy requirement for operation (low pressure loses)

3. Low pressure requirement for backwash (otherwise dedicated pumping equipment for

backwash may be required)

4. Low equipment cost.

Example of strainer equipment being used in seawater application is shown on Figure 3.6.

http://hyorim.trustpass.alibaba.com/productshowimg/108906656-101544063/Intake_Facility_Traveling_Band_Screen_Center_Flow_.html#inSearch

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Figure 4.1.2. Automatic disc strainers. (Arkal Filtration Systems)

7.2. Dissolved air flotation (DAF)

DAF is considered as very effective process for removal of light particles from water. It could be

applied as initial treatment step to remove algae from seawater feed, if present in large concentra-

tions.

DAF process involves number of process steps:

1. Destabilization of colloidal particles by coagulation and flocculation.

2. Saturation under pressure of fraction of raw water with air.

3. Injection of air saturated water into the stream of treated raw water and release of air micro

bubbles.

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4. Attachment of destabilized colloidal particles to the air micro bubbles and their rise to the

surface.

5. Hydraulic or mechanic collection of floating solids and discharge from the system.

6. Collection of subnatent clear water and its direction to the storage or filtration unit.

Schematic diagram of DAF system, integrated with media filtration unit, is shown on Figure 4.2.1.

Raw water entering the system is acidified followed by addition of coagulant. After a rapid mixing

step, water flows into two stages flocculation unit. From flocculation unit water enters the DAF sec-

tion. At the entrance to the DAF (contact zone), the influent is combined with the pressurized recir-

culation stream, saturated with air. The air saturated recirculating stream flow rate is about 10% of

the total water flow in the DAF unit.

AIR

ACID COAG POLY

RAW WATER

INLET RAPID

MIX

STAGE 1

FLOCCULATOR

STAGE 2

FLOCCULATOR

DAFF TANK

TO SEA

TO TREATED

WATER STORAGE

AIR

SATURATOR

FLOAT

RECYCLE

PUMPS

Typical DAFF Plant Schematic

UnitedKG (AU)

Figure 4.2.1. Schematic diagram of DAF system integrated with multi media filtration unit (courte-

sy UnitedKG)

The sudden pressure drop at the mixing point of two streams results in release of air, which forms

large quantity of micro bubbles: 10 – 100 micron size. The bubbles with attached flock particles rise

to the surface, forming dense layer of captured particles (float). The float layer flows to the collec-

tion and discharge channel and leaves the DAF unit.

The clarified water body passes through the multimedia filtration layer and flows to the storage tank.

In the DAF configuration without media filtration unit, the clarified water is collected at the bottom

of the filtration unit and overflows to the clear well.

The flock particles, lifted to the surface by attached air bubbles, form a dense layer of floating sol-

ids, known as float. The float is removed through a mechanical skimming unit (mechanical removal)

or by solids overflow to the collection through (hydraulic removal). Mechanical removal results in

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waste stream with solids concentration of 2 – 3%. The hydraulic removal produce wastewater with

lower solids concentration in the rate of 0.5 – 1%.

A 10% fraction of the clarified water effluent flow is pumped to the air saturators.

The air saturators are configured as pressure tanks filled with plastic spheres filling, as shown on

Figure 4.2.2.

The saturator vessel is configured based on the water flow rate of 60 – 80 m3/m2/hr and air pressure

of 4.5 – 5.5 bar. The objective of operation of saturator is to achieve dissolved air concentration lev-

el of about 100 ppm.

The representative design parameters of DAF system are listed in Table 4.1.

The solubility of air in water is governed by Henry’s law. The solubility is function of temperature

as shown on Figure 4.2.3.

The target of 100 ppm of air concentration in the recirculation stream is selected to create a dis-

solved air excess concentration of about 10 ppm in DAF influent, after mixing both streams together.

It has been shown that turbidity of DAF effluent reaches plateau at air concentration of about 10

ppm, as shown on Figure 4.2.4. Therefore, concentrations higher than 10 ppm would only increase

energy consumption without additional reduction of concentration of colloidal particles.

The relation between mass concentration of air released at the entry of the DAF cell (CDAF), concen-

tration of air in the recirculating stream (CR) and concentration of dissolved air in the raw water in-

fluent (CIN) is given by equation 4.1.

CDAF = (CR – CIN)*r/(1+ r) (4.1)

Where: r – is recirculation fraction of DAF influent flow

Accordingly, required concentration of air in the recirculation flow is given by equation 4.2.

CR = CIN + CDAF*(1+r)/r (4.2)

CIN of the surface water is usually close to 24 g/m3. Pressure required in the air saturation units is

determined based on target CDAF, recirculation ratio air solubility vs. pressure shown on Figure

4.2.3.

Picture of DAF system operating at Tuas, Singapore, SWRO desalination plant is shown on Figure

4.2.5. The picture shows DAF-multimedia filters basins and air saturator tanks.

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DAF air saturator

Saturated water outlet

Pressurized water

inlet, 60-80 m/h

(24.5 gpm/ft2)

Air transfer packing

Pressurized air

inlet, 4.5 – 5.5 bar

(65 – 80 psi)

1- 1.5 m

(3 – 5 ft)

Figure 4.2.2. Configuration of DAF air saturator

0

40

80

120

160

200

0.0 1.0 2.0 3.0 4.0 5.0 6.0

Air pressure, bar

Dis

so

lced

air

co

ncen

trati

on

, p

pm

P = KC10 C

30 C

Figure 4.2. 3. Relation between air pressure and concentration of dissolved air in water.

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0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4 5 6 7 8 9 10 11 12

Air concentration, ppm

DA

F e

fflu

en

t tu

rbid

ity,

NT

U

Figure 4.2.4. Relation between turbidity of DAF effluent and concentration of dissolved air in water.

DAF system

UnitedKG (AU)

Figure 4.2.5. Picture of DAF installation at the Tuas, Singapore, SWRO desalination plant.

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Although DAF is well known water treatment technology, number of SWRO desalination plants

that utilize DAF technology is limited. DAF by itself can not produce effluent with quality suffi-

cient for RO applications. It has to be followed by additional filtration equipment: multimedia filtra-

tion or membrane filtration. Therefore, utilization of DAF is only considered when raw water

source has high frequency of presence of algae or similar nature particles.

Operation of DAF is associated with additional energy usage, in the range of 1 – 3 KWh/1000 m3.

Table 4.2.1. Representative design parameters of DAF system

Design parameter Value or range

Hydraulic loading rate, m3/m2/hr 10 – 20

DAF tank length, m < 11

DAF tank with to length ration < 1

DAF tank surface area, m2 90 – 110

Maximum DAF tank capacity, m3/hr 1,000 – 2,000

Basin depth, m 1.5 – 3.0

Contac zone detention time, sec 60 – 240

Recycle ratio, % 6 – 10

Recycle system pressure, bar 4 – 6

Saturator hydraulic loading rate, m3/m2/hr 60 – 80

Saturator packing depth, m 1 – 1.5

Air bubble size, micron 10 – 100

Float (sludge) concentration, % 1 - 3

7.2.1. Example of calculation of operating cost of the DAF unit.

Operating expenses of the DAF unit includes:

1. Usage of acid for acidification of seawater

2. Usage of ferric coagulant

3. Energy of pumping water to the saturator

4. Energy for pumping air to the saturator

5. Maintenance parts

Dosage of acid will depend on pH required for effective flocculation. Dosing rate will depend on

required pH, water alkalinity and temperature. For the propose of preliminary cost estimation a dos-

ing rate of 20 ppm of sulfuric acid can be used.

Dosing rate of ferric coagulant will depend on results of “Jar Test” or pilot unit operation. For pre-

liminary cost estimation a dosing rate of 10 ppm of ferric sulfate can be used.

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Energy (kwhr) required for pumping is calculated based on flow rate of water pumped to the satura-

tor and pressure:

E = 0.0274* Qs*Ps/(M*P) (4.3)

Where: Qs is water flow to the saturator (m3/hr), Ps is the saturator pressure (bar), M is efficiency

of the motor, P is efficiency of the pump

Energy required by air blower to saturate the reciculation stream with air to the designed air concen-

tration level is given by equation 4.4.

E = 0.133* QS*AS/(1000*A)*(((Ps +1)0.283) – 1 )/(M*B*VFD) (4.4)

Where: QS is the water flow through the saturator, AS is air saturation concentration in water at the

outlet from the saturator (g/m3), usually the same as CR – equation 4.1, A is air density (1.2041

kg/m3 @ 20 C), Ps is the saturator pressure (bar), M is efficiency of the motor, B is efficiency of

the blower and VFD is efficiency of the VFD.

Table 4.2.2. Example of energy usage of air saturation unit for a DAF system for the raw water

flow of 1000 m3/hr.

Process Parameter Value

Water flow to Saturator, m3/hr 100.0

Water pressure, bar 5.0

Air saturation concentration, ppm 100.0

Air density, kg/m3 1.2041

Pump efficiency 0.75

Motor efficiency 0.90

Blower efficiency 0.55

VFD efficiency 0.98

Energy for water pumping, kwhr 20.30

Energy for air pumping, kwhr 1.50

Total energy, kwhr 21.80

Table 4.2.3. Design parameters of a DAF unit, 100,000 m3/day effluent capacity

Process Parameter Units Value

Effluent capacity m3/day 100,000

Suspended solids concentration ppm 10

Water loses with float % 2

Designed hydraulic loading m3/m2/hr 20

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Cell area m2 60

Cell length m 8

Cell with m 7.5

System area m2 212.5

Calculated number of cells 3.5

Actual number of cells 4

Actual hydraulic loading m3/m2/hr 17.7

Recycle ratio % 10.0

Recycle flow m3/hr 425.0

Saturator loading rate m3/m2/hr 70.0

Total saturator cross section m2 6.1

Number of saturators 4.0

Saturator tank diameter m 1.4

Air conc. in raw water g/m3 24.0

Designed air concentration in the DAF unit g/m3 10.0

Air concentration in recirculation Stream g/m3 134.0

Required pressure in the saturator bar 6.0

Energy of pumping water to saturator kWhr 99.4

Energy of pumping air to saturator kWhr 9.5

Energy usage KkWhr/m3 0.03

Sulfuric acid dosing rate ppm 21.00

Sulfuric usage t/day 2.23

Ferric dosing rate ppm 10.0

Ferric usage t/day 1.02

Solids to disposal @20% concentration t/day 10.20

7.3. Coagulation and flocculation

Coagulation and flocculation is a combined process of destabilization and conglomeration of colloi-

dal particles to facilitate more effective removal in media filtration process. Colloidal particles in

the water stream are negatively charged and electrostatic repulsion helps to maintain them in sus-

pension. Coagulants are positively charged hydrolyzed metal salts that neutralize negative charges

of suspended colloids and help to aggregate them into larger, heavier, more filterable solids. For co-

agulation mainly ferric or alum salts are used. During hydrolysis of these salts a complex polynu-

clear, positively charged species are formed in a matter of seconds. The solubility of these species is

low and they form dense, suspended flock. The action of coagulants is threefold: they adsorb col-

loidal particles on the flock surface, neutralize negative charges that surround colloidal particles and

also enmesh suspended particles in the body of the flock formed. The effective quantity of coagu-

lant required is specific to water composition, type of colloidal particles, water pH and temperature.

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An excessive quantity of coagulant could have the undesirable effect of increasing the stability of

colloidal particles. Excessively high concentration of coagulant may increase dispersion of colloids,

due to reversal of surface charges: formation of high density, positive charges on the colloids sur-

face and mutual electrostatic repulsion. The transition of Zeta Potential and Turbidity with increas-

ing dose of coagulant is illustrated on Figure 4.5. As shown on Figure 4.3.1, the preferred range of

zeta potential to achieve low turbidity of the effluent is between 0 to - 20 mV. Zeta Potential values

outside this range will result in increased stability of colloidal matter and higher turbidity.

The initial estimation of the required dosing rate of coagulant and optimum pH range for the pro-

cess is determined by conducting a “jar test” (described in chapter 4.3.3.1). The dosing rate of inor-

ganic coagulant is usually in the range of 1 – 30 ppm and pH in the range of 6 - 8. Following the

results of the jar test the adjustment of coagulation process parameters is conducted during the ini-

tial stages of commercial system operation.

For RO applications ferric salts are preferred over aluminum due to the lower tendency of forming

deposits in membrane elements. The solubility of hydrolyzed species of aluminum and iron depends

Picture 4.3.1.. Transition of Turbidity and Zeta Potential with increasing dose of coagulant (courtesy

Peter Hillis).

on pH. The solubility is at a minimum at about pH 6 for aluminum hydroxide and at about pH 8 for

ferric hydroxide. The solubility of hydrolyzed ferric compounds is much lower than of correspond-

0

4

8

12

16

20

-30 -20 -10 0 10 20 30 40

zeta potential (mV)

turb

idity (

NT

U)

.

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ing aluminum species. The minimum solubility for Fe is about 10-9 mol/l compared to 10-6 mol/l for

Al. Therefore, if metal salt coagulation is applied, the pretreatment system effluent, and subsequent-

ly RO feed water will have lower concentration of ferric ions then the potential concentration of

aluminum ions at the corresponding conditions. Accordingly, in case of ferric coagulant there will

be lower potential for precipitation as a result of feed water pH changes and/or due to the increase

in concentration of dissolved species that occurs in the RO process.

As mentioned already coagulation is a very rapid process requiring just a few second to complete.

However, effective coagulation required intensive mixing to bring the coagulant in contact with a

large number of colloidal particles. In majority of applications this is usually achieved by incorpo-

rating coagulation tanks with mechanical mixers, either vertical rotating blades or horizontal paddle

mixers. In RO systems in line coagulation, using static mixers positioned downstream of the coagu-

lant injection point, is also being used.

The configuration of the coagulation – flocculation system will depend on configuration of filtration

unit downstream. Systems that utilize pressure filters usually relay on in line coagulation - floccula-

tion, using static mixers. Use of static mixers and in line flocculation, avoids reduction of pressure

of the pumped stream to the atmospheric pressure, which would require repumping.

Static mixers are compact devices that can be incorporated into feed line piping. The mixing effi-

ciency of static mixers is depended on the flow rate through the mixer. Therefore, the dispersion of

coagulant could be lower at partial flow. Use of static mixer introduces head loss of 0.5 – 1m. For

efficient flocculation downstream of static mixer, a straight segment of pipe is required, about 20

pipe diameters long.

The gravity filtration systems are configured to provide gravity flow through the whole system, af-

ter initial pumping boost, through a cascade of overflowing steps. In gravity filtration systems, the

initial coagulation step is accomplished either in static mixers or in tanks equipped with rapid mix-

ers. The flocculation is usually conducted in tanks open to atmosphere with slow mixing.

Mechanical mixing consists of coagulation tank with a mixer that can create velocity gradient (G)

of about 300 sec-1. The tank size is designed for a retention time of 0.5 – 3 min. The power required

for mechanical mixer is in the range of 2 – 2.5 HP/1000 m3/day.

The velocity gradient G, is related to coagulation or flocculation tank volume V, power rating of the

mixer motor P and water viscosity , according to equation 4.5

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G = [P/( * V)]0.5 (4.5)

For the coagulation process the G value is selected in the range of 300 – 800 s-1. For the flucculation

process the G values are lower, usually below 150 s-1.

During coagulation, metal salts hydrolyze and dissociate. The conversion of metal coagulants to the

hydrolyzed form consumes alkalinity in the water. Therefore, the raw water pH is reduced (0.1 – 0.3

pH units), in proportion to the coagulant dosing and alkalinity present, according to the following

equations:

FeCl3 + 3HCO3- = Fe(OH)3 + 3Cl- +3CO2 (4.6)

Fe2(SO4)3 + 6HCO3- = 2Fe(OH)3 + 3SO4

= +6CO2 (4.7)

AlCl3 + 3HCO3- = Al(OH)3 + 3Cl- + 3CO2 (4.8)

Coagulation can be also conducted using long chain synthetic organic polymers, which could be of

nonionic, anionic or cationic types. The nonionic and anionic polymers destabilize colloids by

bridging particles together. The cationic type polymers have a dual action of bridging and neutrali-

zation the negative surface charges of the colloids. Cationic organic polymers can be used as prima-

ry coagulants. However, in RO pretreatment systems polymers are usually used as additives to en-

hance the effectiveness of metal based coagulants by binding flock particles together. In most cases,

polymers are applied at low dosing rage, below 1 ppm, directly injected to the feed water down-

stream of the dosing point of the metal coagulant, at the location where hydrolyzed metal flock has

been already formed. If polymers are used at a high dosing rate, and a carryover from the sand

filters occurs, cationic polymers may react with anionic scale inhibitors and form a fouling layer on

the membrane surface.

Flocculation, which follows coagulation, is a process of flock formation during gentle mixing. Floc-

culation is a slower process than coagulation and takes number of minutes to complete. During

flocculation, colloidal particles and some fraction of dissolved organics are being attached to the

flock body, and are eventually retained on the filtration layer in the granular media filters.

Flocculation in conducted in tanks equipped with mixers that are able to create velocity gradient of

30 – 120 sec-1. The rotating velocity of the mixers is slow, in the range of 20 – 60 rpm. The depth of

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flocculation tanks is 3.5 – 4.5 m and the working volume such that the retention time during floccu-

lation will be 10 – 30 min.

The schematic configuration of coagulation flocculation unit is shown on Figure 4.3.2.

Flocculators

Ferric

dosing

Acid

dosing

Media filters

Static

mixer

Figure 4.3.2. Schematic diagram of configuration of coagulation – flocculation unit

According to diagram shown in Figure 4.3.2, there is in line coagulation, utilizing static mixer, fol-

lowed by flocculation in two flocculation chambers equipped with blade mixers. Preliminary speci-

fication of coagulation – flocculation systems are provided in Table 4.3.1. The unit flow capacity is

designated for a filtration system of nominal effluent capacity of 100,000 m3/day. Such a system

requires about 104,000 m3/day effluent to produce sufficient excess of filtrate for filters backwash.

Table 4.3.1. preliminary specifications of coagulation – flocculation unit. Nominal flow capacity

100,000 m3/day.

Net effluent system capacity m3/day 100,000

Influent flow rate m3/hr 4,335

Common manifold diameter mm 864

Flow velocity m/sec 2.05

Static mixer diameter mm 864

Number of flocculation lines 4

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Number of flocculators in series in each line 2

Floccculator cell length m2 6.0

Flocculator cell with m 4.3

Flocculator cell depth m 6.00

Operational volume of flocculator cell m3 129.0

Lotal volume of flocculators m3 1109.4

Flocculation time min 15.4

Coagulant dosing rate ppm as Fe2(SO4)3 10

Coagualnt usage t/day (100%) 1.04

Accid dosing rate ppm as H2SO4 20.0

Acid usage t/day (96%) 2.17

7.4. Granular media filtration

In the granular media filtration process, suspended solids are removed through attachment to the

filtration media particles and through blockage/capture by the filtration cake. The preferred process

of filtration is capture of suspended solids with significant bed penetration as opposed to surface

filtration, since the latter results in faster increase of pressure loss and therefore shorter filter runs.

In a single medium filtration bed, after number of backwash runs, fine size filtration media particles

are aggregated at the top of the bed. This reduces penetration of suspended solids and therefore,

mainly results in surface bed filtration. A graduation of the filtration bed from coarse to fine parti-

cles of the filtration bed can be achieved in dual media configuration by placing fine, high specific

gravity, filtration media as the lower filtration layer and coarse, low specific gravity, filtration media

as a top layer. Filtration media selection that provides coarse to fine filtration bed configuration, in-

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cludes anthracite (specific density 1.5 – 1.75 t/m3, effective size around 1.5 mm. as a top layer and

silica sand (specific density: 2.55 – 2.65 t/m3, effective size around 0.6 mm, as a bottom filtration

layer.

The value of effective size (ES) means that size of 90% of filter media particles in the given lot is

larger than the value indicated.

Another important parameter of filtration media is uniformity coefficient (UC). The uniformity co-

efficient is expressed as a ratio of ES value corresponding to 60% passage over ES value corre-

sponding to 10% passage:

UC = ES60/ES10 (4.9)

The lower the values of UC for a given lot, the closer together are sizes of particles forming the lot.

The practical values of UC specifications for filtration media are in the range of 1.3 – 1.5.

Example of filtration media (anthracite and sand) specifications is provided in Table 4.4.1.

Table 4.4.1. Range of specifications parameters of filtration media.

Parameter Designation Anthracite Filtration sand

Effective size ES = D10 1.5 – 1.8 0.5 – 0.7

Uniformity coefficient UC = d60/d10 1.3 – 1.5 1.3 – 1.5

Specific gravity SG 1.6 – 1.8 2.4 – 2.6

Hardness Moh scale 3 – 4 6 – 7

The operation of sequence of operation of media filters is based on capture of suspended solids in

media layer and formation of filtration cake on the surface. Increasing load of solid particles in me-

dia results in increase of pressure drop across the media. At the end of filtration cycle, the flow of

water through the filter is reversed. Usually using filtrate, the flow through the filter is from the out-

let port to the inlet port, expanding the filtration layer and flushing out the solids particles captured.

The duration of filtration cycle is usually determined based on the operating time, the standard

length being 24 hr. The backwash process usually includes sequence of steps, which are finalized

rinsing filter in forward direction to create initial coating on filtration particles and improve solids

capture efficiency of the filter. Once the quality of filter effluent reaches the required level, the filter

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is connected back to the common effluent piping manifold.

Effective backwash requires that the filter layer will be fluidized, which corresponds to the filtration

bed expansion of about 30%. The bed expansion occurs as the drag forces of backwash flow in-

creases of gravity forces. The backwash flow rate required for bed expansion will depend on water

viscosity. The required flow rate will be higher at increased water temperature (reduced water vis-

cosity and lower drag forces). At the temperature range of 20 C, the backwash flow rate for bed ex-

pansion will be about 40 m/hr for silica sand and about 50 m/hr for anthracite.

The backwash flow rate (VB) required for a given rate of bed expansion can be calculated

according to the equations below:

VB = *Re/(w*d) (4.10)

Where: is dynamic viscosity of water, Re is Reynolds number, w is density of water an

d is representative media particle size.

Re = – V (1 – )/2l + 1/(2l) [2V(1 – )2 + 4l

1/2 (4.11)

Where: V is head loss coefficient due to viscous forces, dimensionless, l is head loss coe

fficient due to inertial forces, dimensionless, is porosity, dimensionless and is backwash

calculation factor, dimensionless.

The recommended range of the above parameters is provided in Table 4.4.2.

Table 4.4.2. Recommended values of filtration media parameters

Filtration medium V l

Sand 110 – 115 2.0 – 2.5 0.40 – 0.43

Anthracite 210 – 245 3.5 – 5.3 0.47 – 0.52

= gw(p – w)d33/2 (4.12)

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Where: g is gravity acceleration (9.81m/sec) and p – particle density

Example of calculation of required backwash flow rate (VB)

Filter media anthracite

Bed depth 1.0 m

Required expansion 30%

Media particle size 1.5 mm

Media porosity 0.50

Media density 1700 kg/m3

Water temperature 15 C

V 230

l 4.4

expanded bed

(9.81*1000)*(1700 – 1000)*(0.0015)3 *(0.615)3]/(0.001139)2 = 3405

Re = -230(1 – 0.615)/(2*4.4) + 1/(2*4.4)*[(230)2(1 – 0.615)2 + 4*(4.4*3405)]0.5 = 19.3

VB = 0.001139*19.3*3600/(1000*0.0015) = 52 m/hr

7.4.1. Pressure filter

There is a variety of media filtration equipment configurations used in potable and waste water fil-

tration. In RO applications the frequently used filter types are pressure or gravity down-flow fil-

ters in single or two stage configuration.

The pressure filters are cylindrical pressure vessels filled with a layer(s) of filtration media. The fil-

ters could be configured for horizontal or vertical operation (Figure 4.4.1 and 4.4.2). Maximum di-

ameter of filter is limited to about 3 – 4 m, due to logistic of transportation of large pressure vessels

from the manufacturer to the project site. The shell length of horizontal pressure filters usually does

not exceed 12 m.

The important features of the pressure filters configurations are the top distributors for the uniform

entry of influent to the filter and bottom collectors of the filtrate. The influent distributors are usual-

ly configures as a grid of perforated laterals. For the filtrate collection laterals could be used or false

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bottom with nozzles could be utilized. Examples of filter nozzles are shown on Figure 4.4.3.

Pressure filters are equipped with air relief valves, installed at the highest point of the filter shell. In

horizontal filters, which are divided into chambers, each chamber should be equipped with a sepa-

rate air relief valve.

The filters that utilize air scouring to enhance media backwash, could have separate port and inter-

nal manifold for air distribution for this purpose. In systems utilizing false bottom and nozzles, the

air is injected into the filtration layer through the filtrate collection nozzles.

The filter shell should have man port for loading the media and filter maintenance and a smaller

viewing port to enable observation of the condition of the surface of the media during normal opera-

tion and during the backwash.

Filters that utilize bottom laterals for filtrate collection should have the volume below laterals filled

with concrete. This is to eliminate presence of areas of stagnant water, below the bottom collectors.

In the filtration step, raw water enters the filter shell through the inlet port and, flows through dis-

tributors over the surface of filtration layer. Water infiltrates through the media and is collected as a

filtrate by the bottom laterals or nozzles (in the “false bottom” configuration).

In the backwash mode the flow directions are being reversed. The filtrate collector serves as the en-

try port for the filtrate used as backwash water. The backwash water eventually exits the filter

through the top, influent port, and is directed to the backwash water storage tank or to the outfall.

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Figure 4.4.1. Drawing of a vertical pressure filter (courtesy of Tonka Company)

Additional step that follows the backwash and produces water stream for disposal is rinsing of the

media. The rinse flow is applied in the same direction as the flow direction during regular filtration

step, with the exception that filtrate produced flows to waste and not to the filtrate storage tank.

Accordingly the valves arrangement at the influent port should allow entry of raw water during the

filtration step and discharge of backwash water to waste during filter backwash.

The valves arrangement at the bottom effluent port should allow exit of filtrate during filtration step,

entry of filtrate during backwash step and discharge off speck filtrate to waste during filter media

rinse. Media rinse that follows the backwash step, is conducting at a normal filtration rate till the

turbidity of filtrate declines to the required level.

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Figure 4.4.2. Drawing of horizontal filter (courtesy of Tonka Company)

Figure 4.4.3. Examples of filter nozzles (web page of FTR, Istanbul)

In RO application the filtration system operates at constant output. The control of the filtration rate

is accomplished by adjusting the throttling of the valve located on the effluent line. As shown on

figure 4.4.4, the differential head (DH) is a sum of head losses (HL) in the filtration bed and flow

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resistance of the outlet valve.

Figure 4.4.4. Configuration of effluent flow control in a pressure filter

Initially, when the filtration bed resistance is low, the valve is only partially open. As the head loses

increases during the filtration run, the valve is gradually open to maintain constant output. When the

fitter reaches the point that valve position is close to being fully open, filter enters the backwash

step to remove collected matter from the media and reduce flow resistance to the original value.

Otherwise, filter output will start to decline.

The filtrate for the backwash can be provided from a storage tank (clear well) or can be generated

internally from filters that are in filtration mode. Such configuration for backwash with internal fil-

trate supply is shown in Figure 4.4.5.

During the filtration step the effluent valve is open and the valves to the waste are closed. To start

the backwash, the effluent valve of the filtration unit composed of filters 1 – 4 will close. Then for

the filter entering the backwash, the inlet valve will close and valve to waste will open. In configu-

ration shown on Figure 4.4.5, if filter # 1 enters to backwash, filters 2 – 4 will produce filtrate that

will flow through the filtrate collecting manifold back to filter 1 in a reverse direction. When the

backwash is completed, the valve to waste will close, effluent valve of the filtration unit will open.

The inlet valve of filter # 1 will open also, returning filter to normal flow direction. Most likely the

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initial filtrate will be send to waste till turbidity of the effluent will reach the level compatible with

requirements of the RO feed.

Figure 4.4.5. Configuration of vertical pressure filters with valves required for utilization of internal

source backwash water.

Figure 4.4.6. Configuration of horizontal four chambers pressure filter with valves required for uti-

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lization of internal source backwash water.

The backwash described above can only be conducted if the backwash flow requirement of the filter

being backwashed does not exceed the combined filtrate production rate of the filters remaining in

operation.

Another alternative configuration for backwash of pressure filters, utilizing water supply from high

service line is shown on Figure 4.4.7. In this configuration the backwash starts with closing Influent

valve and opening the valve directing flow to waste for a set time. The backwash water from the

distribution line will enter the filter in reverse flow, backwashing the filter bed.

Waste

Effluent to

distributiion

Influent

Figure 4.4.7. Alternative configuration for backwash of pressure filters from the high service line.

The operating inlet pressure of the pressure filter is equal to driving head plus the pressure required

by the equipment operating downstream of filter outlet. The design inlet pressure is usually 20 –

30% higher.

For example a pressure filter having driving head of 5 m (0.5 bar) and required discharge

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pressure of 20 m (2 bar), will have designed inlet pressure of 3 bar (2.5 * 1.2).

The design of length of filter shell for vertical filters and shell diameter of horizontal filters is based

on the height of filtration layers in the filter. For vertical filter, starting from the bottom, gravel me-

dia will be loaded to cover the filtrate collection laterals up to the level of about 20 cm above the

laterals. The filtration layer will be about 80 cm to 1 m deep. Above it, there should be allowance

for filtration bed expansion during backwash of about 30% (25 cm), additional 30 cm freeboard. To

this combined length of 160 cm (20 + 80 + 25 + 25) one should add the shell length required for

installation of inlet water distributor, additional 25 cm. The same length of 25 cm will be added for

the bottom collector. This result in a total length of the strait filter shell of 200 cm. Additional

heights will be added to include bottom and top elliptical heads, 40 cm each, making total height of

filter of 280 cm. The actual filter structure will be higher than this due to filter foundations and filter

supporting frame.

The horizontal media filters are sized in a similar way. The sizing of the filtration system is based

on the nominal filtration rate and the backwash rate. The horizontal filters are usually divided into

four compartments, each being backwashed separately. The system is divided into number of filters

that would provide sufficient operational flexibility during filters backwash and filters maintenance.

Example of results of sizing of filtration system utilizing horizontal media filters is included in Ta-

ble 4.4.3.

Table 4.4.3. Design parameters of media filtration system utilizing horizontal filters. System efflu-

ent capacity 100,000 m3/day.




Nominal filtration rate m/hr 12.0

Actual filtration rate m/hr 12.4

Required filtration area m2 347

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Number of filters in operation 10

Number of filters installed 11

Nominal filtrate flow per filter m3/hr 417

Actual filtrate flow per filter m3/hr 441

Filter diameter m 3

Filter shell length m 12

Inlet/outlet pipe diameter m 0.25

Gravel layer depth m 0.3

Sand layer depth m 0.3

Anthracite layer depth m 0.9

Media expansion allowance m 0.4

Free board m 0.4

Width of media surface m 3.0

Filtration area per filter m2 35.7

Filtration compartments per filter 4.0

Filtration area per chamber m2 8.9

Backwash rate m/hr 45.0

Backwash flow per chamber m3/hr 401.4

Backwash duration min 20.00

Backwash volume m3/backwash 133.8

Daily backwash volume m3/day 5351.8

Filtration interval hr 23.5

Filtrate volume per filtration cycle m3 9791.7

Nominal recovery rate % 94.5

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Gravel volume m3/filter 7.2

Sand volume m3/filter 9.0

Anthracite volume m3/filter 31.7

Gravel, total volume m3 83.3

Sand, total volume m3 104.5

Anthracite, total volume m3 366.2

In RO applications pressure filters are applied in small in medium size systems, seldom exceeding

capacity of few thousands m3/day. However, in Spain it is common to use pressure filters also in

large seawater RO desalination systems. Example of one such installation is the SWRO desalination

plants at Carboneras, Spain. The plant has permeate water capacity of 120,000 m3/day. Configura-

tion of this plant, including location of horizontal pressure filters is shown on Figure 4.4.8. The fil-

ters are located on both sides of main RO building, feeding the symmetrically divided two lines of

RO trains.

Carboneras, Spain 120,000 m3/day

Figure 4.4.8. Configuration of SWRO desalination plant at Carboneras, Spain

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7.4.2. Gravity filters

The gravity filters have the configuration of rectangular tanks, usually made of concrete. The tank

has connections for entry of influent and exit of filtrated effluent. In addition there is a connection

for air supply used air scouring of filtration media during backwash step. In the same manner as

with pressure filters, during the backwash step the flow direction is reversed, supplying the back-

wash flow through the effluent exit port.

An example of a gravity filter configuration is shown on Figure 4.4.9. In this diagram the influent

water to the filter is supply through a side channel. Filtrate leaves the filter through the effluent out-

let located at the bottom of the filter. The filter bottom is covered with blocks with slits that provide

support to the filtration media and also collects filtrate. The exit port and the filtrate collecting

blocks serve as an entrance of the backwash water. In some filter configurations, the blocks are re-

placed with a raised concrete floor with evenly spaced filter nozzles (shown on figure 4.4.9)

Backwash operation is sometimes augmented by air. The air port is a separate port for injecting the

compressed air to the underdrain. The backwash water is collected by the wash water troughs, lo-

cated above the level that the media expands during the backwash. Some gravity filters utilize

washing of media surface using water jets.

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Wash water

troughs

Influent &

backwash water

channel

Underdrain blocks & media

retaining plate

Backwash supply

& filter effluent

Dual filter media

Air

backwash

Header

Surface wash, not

shown

Figure 4.4.9. Schematic configuration of a gravity media filter.

The dimensions of gravity filters cells are determined by the surface area which is in the range of

25m – 100 m2 per filter cell. The number of filtration cells for a given system is determined by the

logistic of operation, the filtration and backwash steps. The preffered configuration is not increase

filtration rate of filters in operation by more that 10 – 15%, while one filter is off line due to back-

wash. Another consideration is logistic operation of the RO membrane trains. In very large systems

it is sometimes convenient to have media filter cells divided into two groups, so the desalination

system can have flexibility to operate effectively at 50% of production capacity.

The usual range of length to with ratio of filter cell dimensions is in the range of 2 – 4. The depth of

the filter cell should be such to accommodate design depth of filtration layer, provide required driv-

ing head and safety free board. In dual media gravity filter the combined depth of filtration layers

usually does not exceed 2 m. The depth of sand layer is in the range of 0.5 – 1.0 m and the depth of

anthracite layer in the range of 0.5 – 1.0 m. Usually, the depth of top anthracite layer is larger than

the depths of underlying sand filtration layer. Depending on type and configuration of filtrate col-

lectors, the media layer could be supported by layer of gravel, 0.25 – 0.4 m thick.

The dimension range of filtration layers in gravity dual media filter is shown on Figure 4.4.10.

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Figure 4.4.10. Configuration of filtration layers in a gravity filter

Figure 4.4.10 shows schematically the inlet – outlet connections of the gravity filter. Raw water en-

ters the filter through inlet port of channel above the filtration media. The filtrate exits the filter at

the bottom effluent port. Through this port the filtrate can be directed to the storage clear well or

send back to the head of the system in case that quality of filtrate is not within the limits. This port

also serves as an inlet port for the backwash water. The backwash water is usually collected through

backwash water troughs and sent through the dedicated outlet port to the backwash water storage

tank.

In gravity filters the filtration process is driven by hydrostatic level difference between water level

in the filter and water level in the clear well. These conditions are illustrated schematically in Figure

4.4.11.

If the level in the clear well is the same or higher than the level of the top surface of filter media

than the filtration driving head is regarded as positive driving head. If the level in the clear well is

below the level of the surface of top filtration layer, this difference of levels is designated as nega-

tive driving head. It is advisable that the positive driving head be as large as possible, as the pres-

ence of negative driving head could result if formation of vacuum and release of air in the filtration

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bed. Some filtration systems are configured to relay only on a positive driving head for filter opera-

tion.

Driving

head

Filtrate Clearwell

Feed

Figure 4.4.11. Schematics of filtration driving head in the gravity filter.

The driving head determines the duration of the filtration run. It should be sufficiently high to com-

pensate for the flow resistance of clean filtration bed and additional friction loses due to solids ac-

cumulation.

In desalination applications, the gravity filters operate at constant flow rate during the filtration cy-

cle (the same way as pressure filters). The filtration rate is controlled by opening of the valve in-

stalled on the effluent line. The valve is initially partially closed and its opening is increased with

operating time. At the end of filtration cycle, determined by the filtration time, the filter is taken out

of operation, into a backwash step. In gravity filters the backwash step is usually longer than in

pressure filters. It is composed of number of steps and lasts 40 – 60 min.

The backwash steps include, partial draining of the filter, repeated segments of reverse filtrate flow,

air scouring, removal of backwash water and rinsing of the filter media. After quality of filtrate re-

turns to the specified values, the filter cell is returned to the regular operation cycle.

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Example of design parameters of a gravity filter of nominal filtrate capacity of 100,000 m3/day is

provided in table 4.4.4.

Table 4.4.4. Design parameters of media filtration system utilizing gravity filters. System effluent

capacity 100,000 m3/day.


Number of filtration cells 10



Coagulant dosing rate ppm as Fe2(SO4)3 10

Nominal filtration rate m/hr 12.0

Total filtration area m2 356.9

Actual filtration rate m/hr 12.1

Filtration rate during backwash step m/hr 13.5

Filter cell with m 4.3

Filter cell length m 8.3

Filtration area per filter cell m2 35.7

Filter cell height m 6.0

Inlet/outlet pipe diameter m 0.23

Underdrain height m 0.40

Gravel layer depth m 0.25

Sand layer depth m 0.80

Anthracite layer depth m 1.20

Media expansion allowance m 0.68

Free board m 0.40

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Media level relative to filter floor m 2.65

Filtration driving head m 3.35

Filter cell draining time min 10.00

Backwash # 1,duration min 6.0

Backwash # 1,flux m/hr 50.0





Air scouring, duration min 2.0

Air flow rate m/hr 50.0

Air volume per backwash m3 59.5

Media settling time min 10.0

Forward rinse min 15.0

Rinse water volume m3/backwash 107.1

Rinse volume per filtration cycle m3 1070.7

Total off line time min 60.0

Backwash water volume m3/backwash 297.4

Daily backwash volume m3/day 2974.2

Filtration interval hr 23.0

Filtrate volume per filtration cycle m3 104040.0

Nominal recovery rate % 96.1

Total gravel volume m3 89.2

Total Sand volume m3 285.5

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Anthracite volume m3 358.1

Coagulant usage t/day (100%) 1.04

The gravity filters usually require extensive site preparation, sometimes including soil stabilization

and excavation to built concrete filter cells. An aerial picture of layout of gravity filters in sea-

water desalination plant is shown on Figure 4.4.12.

The relative level of filters and the clear well is very important as the clear well is the threshold

point of hydraulic profile of the pretreatment system. The relative level of clear well will affect the

energy required for transfer of raw water from intake to the media filters and energy of pumping of

filtrate to the suction of high pressure pumps. Access to the clear well will determine configuration

of transfer pumps that could be used: vertical or horizontal. The horizontal pumps are usually less

expensive but require side wall access to the clear well.

Palmahim 90,000 m3/day RO seawater plant

Figure 4.4.11. Aerial picture of sweater RO desalination plant showing layout of gravity filters

(courtesy GES Engineering).

7.4.3. Solids management system

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Almost all media filtration systems utilize coagulation process to improve capture efficiency of sus-

pended solids.

The solids together with coagulant are flushed from the filters during the backwash step. Due to

presence of metal coagulant (usually ferric hydroxide), the backwash stream can not be disposed

directly to the ocean. The backwash stream represents 3 – 10% of the volume of the influent to the

desalination system. The backwash stream contains about 0.01 – 0.05% concentration of solids.

Prior to disposal, dispersed solids have to be concentrated into sludge with solids concentration

above 20%. The concentration process includes solids thickening in a clarifier and dewatering in a

filter press. Figure 4.4.13 shows schematically the concentration and dewatering process of the

backwash stream.

Pumps

to RO

Backwash

pumps

Filtrate

Clear well

Filters backwash

water storage

Lamella

clarifiersBelt press

Polymer

dosing

Polymer

dosing

Figure 4.4.13. Schematic diagram of filtration system including solids management unit.

The backwash water is initially stored in the backwash water storage tank. From the storage tank it

is transfer to a clarifier. If necessary a polymer is added to this stream to improve settling of the sol-

ids. The upper clarified, fraction (overflow stream) from the clarifier is transfer back to the head of

the pretreatment system. The lower fraction, which is sludge of solids concentration of 0.5% – 1.5%

is transfer to the filter press. The press reduces water content in the sludge to less than 80%. The 20%

solids sludge can be disposed to the landfill. The effluent produced during press dewatering is re-

tuned to the backwash water storage tank.

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The solids management system is essential part of modern pretreatment system based on media fil-

tration. It has to be design and sized to enable uninterrupted operation of the pretreatment system.

The sizing of the equipment also includes sufficient capacity of storage and transportation to the

landfill to account for periods when landfill is not operational (weekends and holidays).

An example of design parameters of solids management unit is provided in Table 4.4.5.

Table 4.4.5. Design parameters of solids management unit for a filtration system. Filtration system

capacity 100,000 m3/day

Net effluent filtration system capacity m3/day 100,000

Filtration system influent flow rate m3/hr 4,335

Suspended solids concentration in raw water ppm 5

Filter effluent coagulant dosing rate ppm as Fe2(SO4)3 10.0

Daily filter solids loading kg/day 1,076.8

Total filtration area m2 357.0

Backwash flux m/hr 50

Total duration of backwash flow min 10

Daily filter backwash volume m3 2,975

Solids concentration in the backwash flow % 0.036%

Operational volume of the filter backwash water equalization tank

m3 446

Operational capacity of the clarifier m3/hr 248

Clarifier sludge flow (@1% solids) m3/hr 4.5

Clarified effluent flow m3/hr 243.5

Daily sludge production (@20% solids) t/day 5.4

Filter press discharge liquid flow m3/hr 4.3

Filter press polymer dosing rate ppm 20

Filter press polymer usage kg/d (100%) 2.2

7.4.4. Pretreatment system design method

The pretreatment system design methods starts with evaluation of quality of raw water, follows by

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the evaluation of effective pretreatment alternatives, bench scale testing and field operation of a pi-

lot unit.

The evaluation of raw water quality should address seasonal fluctuation of salinity, water tempera-

ture and the physical water quality parameters, such as suspended solids and turbidity. Important

quality indicators include indicators of biological activity, such as TOC, COD, bacterial count and

presence of algae. The relevant information includes complete as possible chemical composition of

raw water. More detailed discussion on quality of raw water for RO application is included in chap-

ters 1.2 – 1.7.

The selection of pretreatment technology is based on raw water source and quality, along the guide-

lines listed in Table 3.2.

Bench testing and pilot operation is recommended only in selective application cases. These include

very variable and unpredicted raw water quality or very stringent requirements on quality of product

water.

One of the more common bench scale testing is the “Jar Test” a procedure enables preliminary op-

timization of the coagulation and flocculation process. Jar test equipment is shown on Figure 4.4.14.

It includes number of beakers equipped with stirrers. The optimization of coagulant dosing rate is

conducted by adding sequentially increasing quantity of coagulant to each beaker containing raw

water sample. Following addition of coagulant the stirrers start to operate for 05 – 1.0 min at high

speed, to simulate coagulation. After this initial period the stirring velocity is decreased to simulate

gentle mixing during flocculation.

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Figure 4.4.14. Jar test equipment utilized in estimation of the required dosing rate of alum based

coagulant. (Courtesy Peter Hillis).

After a defined period of 5 – 20 min, the stirrers are stopped, allowing flock to settle. Appearance of

flock and clarity of water in the beakers are indicators of effective flocculation and adequate dosing

rates. In the example shown on Figure 4.4.14 the effective dosing rates are in the range of 2 – 5

mg/l. The dosing rate of 1 mg/l is not sufficient to develop noticeable flock particles. At dosing rates

higher than 5 mg/l, the flock is small and most likely the excess of metal ions in solution increases

stability of colloidal particles (effect of charge reversing on colloidal particle surfaces).

The Jar Test apparatus is useful tool to evaluate condition of coagulation – flocculation, both during

the process of the pretreatment system design and also later on during the system operation, to op-

timize performance of the commercial unit.

Some types of Jar Test equipment are configured to test also effect of diffused air flotation (DAF) in

addition to testing of the coagulation process.

Operation of pilot unit provides more complete representation of treatment process as the operation

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is conducted under real site conditions. However, field test pilot program is significantly more ex-

pensive than the bench testing. In addition to the cost of equipment there is cost of site preparation,

providing water supply, utilities, permitting and manpower for operation and equipment mainte-

nance. The pilot unit to test the pretreatment process could be configured as to mimic configura-

tion of commercial unit and test operation of the complete desalination system. Such pilot unit

would include pretreatment and RO unit. Sometimes, sufficient results could be obtained just by

operation of the pretreatment equipment only.

Schematic configuration of pilot unit for evaluation pretreatment process based on gravity media

filtration is shown on Figure 4.4.15.

Flocculator

Ferric

dosing Acid

dosing

Media filter Static

mixer

M

Vacuum

pump

Filtrate

storage

Spent

backwash

tank

To RO unit

Media

filter

FlocculatorFiltrated

water

Spent

backwash

Figure 4.4.15. Schematic diagram of a pilot unit for testing of a gravity filtration process.

The pilot unit configuration shown on the above figure contains all components of the commercial

pretreatment system with the exception of the solids management equipment.

The pilot equipment could be of any size as long as the size of equipment used will not affect effi-

ciency of treatment process or results. In the pilot unit that includes an RO unit, the size of pre-

treatment unit will be dictated by the water demand of the RO unit downstream. Otherwise, pre-

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treatment pilot unit could be quite small. Small size pilot unit have advantage of being not only less

expensive to purchase but also more convenient to operate. It is advisable to operate pilot unit

though a complete cycle of raw water quality fluctuation to test the selected pretreatment process at

most adverse conditions at the future site of the commercial system.

The results of bench testing, and possibly pilot testing, are good source of information for pretreat-

ment system design. In case that commercial desalination system operated in vicinity of the future

system site, their configuration and operating parameters are good reference for the design process.

Also vendors of pretreatment equipment are good source reference information on configuration

and process parameters.

The number of possible pretreatment process alternatives has to be reduced to 2 – 3 options, which

should be evaluated in respect of required results of effluent quality, references, design difficulties

and capital and operating cost.

The evaluation process includes development of preliminary configuration, process and flow dia-

gram including process parameters (mass and flow balance) and general specification of major

equipment.

Once the pretreatment process has been selected, it is important to have alternative selection of ma-

jor equipment in order to arrive to a competitive prices of final system.

7.5. Cartridge filtration

The role of cartridge filters in RO system is mainly to protect equipment located downstream

(pumps and membrane elements) from sudden appearance of particulate matter in feed water. Such

conditions could be sometime experienced due to sudden sand or silt release from wells or from

sand filters (in system utilizing sand filters). Use of cartridge filters as a feed water filtration step for

colloidal matter removal is usually prohibitively expensive in respect of the cartridge replacement

cost.

Extensive field experience shows that RO systems treating well water, with cartridge filtration as

the only filtration step, operated successfully over the years. In some isolated cases that the plants of

this configuration experienced release of silt and/or sand from wells, and cartridge filters do not op-

erated properly, the particulate matter eventually ended up reaching membrane elements. In almost

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all reported cases of such events, the solids intrusion and accumulation was limited to lead elements

only. This condition was eventually rectified by flushing of lead elements (in reverse flow direction,

after turning them around and moving to the end of the system) and replacing some of them.

Cartridge filters for RO applications have nominal porosity in the range of 5 – 15 micron. The pre-

ferred porosity rating of filtration cartridges is 5 micron. Feed water flow through cartridge filters

should not exceed 1 m3/hr per 25 cm of cartridge length. The schematic configuration of cartridge

filter housing is shown in Figure 4.4.16.

Max flow,m3/hr

Number of25 cm

cartridges

Diameter,cm

Height,cm

In-outdiameter,

cm

Weight,kg

3.5 3 33 70 5.0 57

7.0 6 38 70 5.0 82

27.0 24 38 130 7.5 100

86.0 76 60 150 10.0 200

CARTRIDGE FILTER CONFIGURATION

Figure 4.5.1. Schematic configuration of cartridge filtration unit.

In the above configuration the filtration cartridges are mounted in vertical position. In large capacity

plants horizontal cartridge filters (Figure 4.4.17) are sometimes being used.

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Figure 4.5.2. Picture of horizontal housing of cartridge filter.

As shown in the above picture, the entry of the feed water is at the housing vessel wall and the exit

of filtrated water at the end of the vessel. The manual valves on the exit and entry are always in

completely open position, except for the events of cartridge replacement. The lid of the cartridge

vessel is mounted on hinge. This arrangement enables system operator to open the lid without need

of a lifting device, as it is necessary with vertical cartridge housing.

Horizontal cartridge housing in a process of cartridge replacement is shown on Figure 4.4.18.

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Figure 4.5.3. Horizontal cartridge filter housing in open position.

The important part of cartridge filter housing is the baffle, which protects cartridges from direct im-

pingement of suspended particles. Filtration cartridges are usually made of soft polymeric materials.

Exposure to direct impingement of hard particles could result in abrasion of cartridges and eventual-

ly loss of integrity.

One big advantage of pretreatment filtration configuration, limited to cartridge filters only, is reduc-

tion of exposure of feed water to outside environment, which is very convenient in treatment of an-

aerobic water sources. Anaerobic water sources (for example water from deep Floridian aquifer)

may contain variable quantity of hydrogen sulfide and usually sulfate reducing bacteria are present

as well. Had this water been exposed to air there would be high probability of hydrogen sulfide

being partially oxidized to elemental sulfur according to the following reaction:

2H2S + O2 = 2H2O + 2S (4.13)

Elemental sulfur has very limited solubility in water or water based solutions and once deposited in

feed channel of RO elements can not be removed. At the early stages of RO technology develop-

ment attempts were made to oxidize hydrogen sulfide presented in feed water with strong oxidants,

prior to RO. This process configuration almost always ended up in either fouling of membrane ele-

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ments with elemental sulfur or in oxidative damage of membrane barrier. The design approach that

provides stable system performance is to maintain anaerobic conditions of the water through the RO

system. After the RO unit, hydrogen sulfide is removed from permeate (and sometimes from con-

centrate as well) either by aeration or oxidation. If location of the RO system is close to urban cen-

ters the degasifiers can not vent off gasses to the air. The common solution in case of hydrogen sul-

fide aeration is to follow degasifiers with an absorption system. In such a system hydrogen sulfide

is absorbed on an iron based catalyst and eventually disposed as a solid waste.

Recently some system designer introducing alternative equipment in place of cartridge filtration as

a safety filters. The new safety filters are 20 micron strainers of the same configuration as are being

used in protecting membrane filtration systems (Figure 4.1.2, Chapter 4.1). This equipment is

somewhat more expensive than the conventional cartridge filters, however, no cartridge replace-

ment is necessary. Therefore in systems that experience high rate of cartridge replacement, use of

this type of equipment could be cost effective.


Utilization of membrane pretreatment for RO applications is increasingly growing. In the RO sys-

tems, designed for salinity reduction of secondary treated municipal wastewater effluent, membrane

filtration is the most commonly used pretreatment technology. Also in systems treating waters for

industrial applications, either treatment of wastewater or production of process makeup water,

membrane filtration is quite frequently being applied. In RO systems treating surface water for po-

table applications, either brackish or seawater feed, membrane filtration is considered as an emerg-

ing technology, with great potential but still being usually more expensive than the conventional

pretreatment. The obvious benefit of membrane filtration is the existence of membrane barrier that

is preventing suspended particulate to pass through, regardless of the quality of the raw water.

However, membrane pretreatment systems are more complex than the “conventional” pretreatment

equipment and require more energy to operate.

In RO system treating secondary effluent, past application of conventional pretreatment resulted in

unacceptably high fouling rates of RO membranes. Application of membrane pretreatment, that

practically removed all colloidal matter from the feed water, improved dramatically stability of op-

eration of RO membranes in wastewater reclamation systems.

In RO systems treating surface water, well design conventional pretreatment is usually capable of

producing RO feed water of acceptable quality. However, in cases when quality of raw water is poor

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with high degree of seasonal fluctuations, use of membrane pretreatment may improve stability of

performance of RO membranes and also be cost effective.

The membrane filtration technology that is used as pretreatment process in RO applications has the

following attributes:

1. The filtration process is conducted at low pressure, usually not exceeding 1 bar. Energy re-

quirement of this process is in the range of 0.05 – 0.15 kWhr/m3.

2. The filtration process is conducted through the membrane barrier that rejects all suspended

particles in the feed water.

3. Membranes are exclusively in a hollow fiber configuration.

4. The operation consists of sequence of steps. Filtration step, conducted at 100% recovery rate

(direct filtration), lasted 20 – 60 min. During the direct filtration step foulant layer is built on

the membrane surfaces, resulting in permeability decline.

5. The foulant deposits are removed and membrane permeability restored by reversing water

flow, i.e. pushing filtrate through the membrane in reverse direction for a short duration of

0.5 – 1 min.

6. This backwash step periodically includes addition of cleaning chemicals, mainly hypo-

chlorite, in a process called chemical enhanced backwash (CEB).

7. At variable frequency of once every few weeks or months, permeability restoration is con-

ducted by applying cleaning in place (CIP)

7.6.1. Configurations and components of membrane pretreatment system

The membrane filtration system could be either pressure driven or operating under vacuum. The

membrane unit in pressure driven system consists of encapsulated membrane modules and water is

driven through the membrane by feed pressure, developed by a feed pump or hydraulic head.

In vacuum driven membrane filtration system, membrane bundles, that form membrane modules,

are immersed in tanks. Water is driven through the membranes by negative pressure created by suc-

tion of filtrate pumps.

Block diagram of submersible membrane filtration system is shown in Figure 4.6.1. The difference

of configuration of pressure driven membrane filtration system is replacement of membrane tanks

with membrane racks and filtrate pumps with feed pumps as indicated in Figure 4.6.2..

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Feed

pumpsStrainer

Static

mixerMembrane tanks

Filtrate

pumps

Filtrate

clearwell

Dosing

systems 1

Heat

exchanger

Backpulse

tank

Backpulse

pumps

Dosing

systems 3

Blowers

Compress

air

Cleaning and

neutralization

unit

Dosing

systems 2

Solids

handling

Vacuum

pumps

Electric

system

Control

system

Figure 4.6.1. Block diagram of submersible membrane filtration system

Feed

pumpsStrainer

Static

mixerMembrane racks

Filtrate

clearwell

Dosing

systems 1

Heat

exchanger

Backwash

tank

Backwash

pumps

Dosing

systems 3

Blowers

Compress

air

Cleaning and

neutralization

unit

Dosing

systems 2

Solids

handling

Electric

system

Control

system

Figure 4.6.2. Block diagram of pressure driven membrane filtration system

System components that are common to both membrane configurations are:

- Membrane units (racks or submersible trains)

- Micro strainers

- Cleaning chemicals storage and dosing units

- Feed pumps or filtrate pumps

- Blowers and/or compressors

- Membrane backwash unit

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- Chemical cleaning and neutralization unit

- Filtrate storage and distribution unit.

- Solids management unit (if coagulant is being used)

- Electric power supply unit

- Process control unit

7.6.2. Settling and screening

Membrane filtration systems are very robust in respect of solids concentration in the feed water. The

quality of filtrate is very little affected by the variation of quality of the feed water. However, with

increased concentration of solids in the feed water the membrane system has to operate at lower fil-

trate flux rate and the filtration time between backwash steps is being reduced. These conditions

result in reduced filtrate flow rate and reduced recovery rate.

Therefore, if raw water turbity exceeds 20 – 30 NTU for significant period of time, it is more effec-

tive to include settling (clarification) unit ahead of membrane filtration unit. As indicated previously,

if suspended solids are of biological nature (algae) light particles, then use of DAF would be a more

effective solution than clarification through settling.

All filtration systems treating surface water include screens as safety measure to prevent large size

particles to reach membrane elements. For applications involving pressure drive membrane filtra-

tion technology, the screening devices are rated at 80 – 100mm.

The immersed, vacuum driven, technology can tolerate larger particle sizes than the pressure driven

systems. The opening size of screens used in submersible system could be as large as 0.5 – 2 mm,

depending on configuration of membrane module.

The sequence of operation of membrane filtration systems is fully automatic, controlled by PLC.

Therefore, the screening units used in membrane filtration systems are self cleaning, automatically

operated.

An example of screening device, used frequently in membrane filtration system is automatic filter

manufactured by Arcal, shown on Figure 4.6.3.

The filtration device consists of set of grooved discs held together by a clamp. Water is passing

through grooves between the discs. For membrane filtration applications the grooves opening size is

80 or 100 m. During filtration the trapped solids are accumulating in grooves between discs, in-

creasing flow resistance and the differential pressure. Once the differential pressure reaches the pre-

set limit, usually in the range of 0.3 bar, the filtration unit is disconnected from the effluent mani-

fold, clamping device holding the discs opens and the accumulated solid matter is flushed from the

filtration unit.

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Figure 4.6.3. Microstrainer configuration offered by Arkal.

Configuration of a microstariner unit, based on Arkal equipment, that corresponds to system capaci-

ty of 200,000 m3/day is shown on Figures 4.6.4 and 4.65. Unit specifications are listed in Table

4.6.1.

Table 4.6.1. Specifications of microstariner unit. Output capacity 200,000 m3/day

Number of modules 4

Number of filters in module 12

Unit footprint 13.9 m X 10.1 m = 140.4 m2

Pressure loss during operation 0.015 – 0.30 bar

Backwash flow rate 960 m3/hr

Backwash duration 20 second

Backwash volume 43 m3/filtration cuycle

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Number of filtration cycles per day 10 – 12

System recovery rate 99.7% - 99.8%

Backwash pressure required 3.5 bar

Figure 4.6.4. Side view of microstrainer assembly. System width 13.9 m

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Figure 4.6.5. Top view of microstrainer assembly. System length 10.1 m

In addition to strainer configuration described above there are commercial strainers that utilize

screens in form of baskets or cylindrical cartridges.

One example of such a filter is strainer offered by Boll Filter Corporation. Schematics of system

with boll strainer is shown on Figure 4.6.6. The filter contains tubular filtration elements made from

wedged metal wires. Backwash is conducted by rotating arm that moves sequentially between filtra-

tion elements, disconnecting them from the effluent manifold and connecting to backwash discharge

line.

Majority of components of disc filter is made of polypropylene. The material of construction of

screen filters is mainly metal, which could be a reason of concern in operation in highly corrosive

environment.

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Figure 4.6.6. Wedged screen strainer configuration

In membrane filtration system, microstrainers operate as one unit producing screened water to the

membrane filtration system.

7.7. Filtration membranes and membrane unit configuration

7.7.1. Fundamentals of the membrane filtration water transport process

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The water is driven through the membrane by difference of pressure between feed and permeate

sides of the filtration membrane.

The driving force is defined as trans membrane pressure (TMP) and it is calculated through the fol-

lowing equation:

TMP = (Pf + Pc)/2 – Pp (4.7.1)

Where Pf is feed pressure

Pc is concentrate pressure

Pp is permeate pressure

Accordingly, specific permeability, SP is calculated according to the following equation:

SP = Q/(Am * TMP) (4.7.2)

Where Q is instantaneous filtrate flow

Am is total membrane area in the system

Example of calculation of TMP and permeability for filtration membrane module is included in Ta-

ble 4.7.1.

As can be seen from the results displayed there is significant difference of TMP and SP values for

new membrane (ex-factory results) and for the membrane in operation at field conditions. At field

conditions the TMP is higher and water permeability significantly lower. The membrane filtration

devices operate at conditions of 100% recover rate (dead end operation). Therefore, during opera-

tion at field conditions the membrane surface is always fouled by deposits, which reduces the water

permeability rate.

The water permeability rate is affected by temperature. The effect of temperature on water permea-

bility is result of changes of water viscosity and changes of pore size of filtration membrane.

The effect of temperature on water permeability is expressed as temperature correction factor (TCF).

It is calculated by the following equation:

TCF = exp(-0.031*(T – 20))

Where T is water temperature expressed in C.

The reference temperature for calculation of TCF is 20 C.

Table 4.7.1. Example of the permeability results at ex-factory test and during field operation

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Test parameter New membrane Field conditions

Pp, bar (psi) 0.25 (3.6) 0.70 (10.1)

Pc, bar (psi) 0.15 (2.2) 0.60 (8.5)

Pp, bar (psi) 0.10 (1.5) 0.15 (2.2)

TMP, bar (psi) 0.10 (1.5) 0.50 (7.2)

Q, l/hr (gpd) 3,500 (22,000) 5,100 (32,300)

Am, m2 (ft2) 46.5 (500) 46.5 (500)

SP, l/m2-hr (gfd/psi) 750 (29) 219 (8.9)

In graphic form changes of TCF are provided in Figure 4.7.1.

Figure 4.7.1. Temperature correction factor vs. water temperature

Temperature correction factor

0.0

0.5

1.0

1.5

2.0

0 5 10 15 20 25 30 35 40

Feed water temperature

TM

P C

orr

ecti

on

Facto

r

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The membrane filtration represents almost absolute barrier to suspended particles. The filtration

spectrum, shown on Figure 4.7.2. indicates particles separation capability of different filtration

technologies. The figure shows distinct difference of projected particle separation sizes by utrafiltra-

tion (UF) and membrane filtration (MF). This is due to difference of pore sizes range of UF and MF

membranes. However, after short exposure to feed water a dynamic membrane is formed on the

membrane surfaces, making separation properties of UF and MF membranes quite similar.

um

A

RELATIVE

SIZE OF

COMMON

MATERIALS

MOLECULARWEIGHT

0.001

10

0.01

100

0.1

1000

1.0

10 4

10

10 5

100 1000

10 6 10 7

100 200 5,000 20,000 100,000 500,000

Aqueous salts

Metal ions

Sugars

FILTRATION

TECHNO-

LOGY

Pyrogens

Virus

Colloidal silica

Albumin protein

Bacteria

Carbon black Paint pigment

Yeast cells

Milled flour

Beach sand

Pollens

Reverse Osmosis and NF

Ultrafiltration

Microfiltration

Particle filtration

THE FILTRATION SPECTRUM

Figure 4.7.2. Separation size range of filtration technologies.

7.7.2. Membrane material and membrane configurations

The sequence of operation of UF and MF membranes require high durability of membranes and

membrane modules.

The important properties of membrane material are:

- Narrow pore distribution or sharp molecular weight cut off (MWCO)

- High polymer strength: elongation, high burst and collapse pressure

- Good polymer flexibility

- Permanent hydrophilic character

- Wide range of pH stability

- Good tolerance to strong oxidants

- Acceptable cost

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There is no single membrane material that has all of the above properties. However, number of

membrane materials provide most of the listed properties and are preferred in commercial applica-

tions.

These are:

- PVDF (polyvinylidiene fluoride)

- PS (polysulfone)

- PES (polyether sulfone)

- PAN (polyacrilonitrile)

Majority of commercial membranes for filtration applications are made from the above listed poly-

mers.

Majority of membranes are configured as capillary devices. Very limited number of filtration mem-

branes is configured as flat sheet, plate and frame, or spiral modules.

The capillary membranes can operate as pressure driven or submersible, vacuum driven units.

In the pressure driven category there are two possible flow directions:

1. Feed water can be pumped through the lumen and filtrate collected outside the capillary: in-

side out operation (Figure 4.7.3 a), also called pressure driven inside (PDI).

2. Feed water can be applied under pressure outside the capillary and filtrate collected through

the capillary lumen: outside in operation (Figure 4.7.3 b) also called pressure driven outside

(PDO).

Figure 4.7.3. Filtrate flow direction in pressure drive capillary membranes: a – PDI, b - PDO.

According to flow directions through the membrane, the membrane elements are configured differ-

ently and operating parameters are different. The main difference are related to different size of ac-

tive membrane area per fiber length, difference of backwash flux rate and utilization of air scouring

for foulants removal.

Both PDI and PDO configurations are commercially viable technologies that provide comparable

a b

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performance at field conditions at similar process economics.

The selection of a given filtration technology configuration for specific application is usually based

on the designer preferences.

The differences between membrane modules in these two flow configurations are listed in Table 4.7.

2.

Table 4.7.2. Attributes of PDI and PDO membranes configuration

Filtration direction Inside out Outside in

Designation PDI PDO

Membrane barrier On the lumen surface Outside fiber surface

Backwash flux rate 3 – 4 times forward flux 1.5 – 2 times forward flux

Air scouring No Yes

Advantages Barrier protected inside fiber High membrane area per mod-

ule

Advantages Sufficient backwash with fil-

trate only Reduced backwash volume re-

quired

Advantages Small volume of cleaning solu-

tion Small volume of cleaning solu-

tion

The other significant version of membrane filtration technology is the submersible, vacuum driven

process. In the submersible process the only practical water flow direction alternative is outside in

configuration.

In the past, the submersible technology has been used preferentially for high turbidity waters and

for large capacity systems. Today both the pressure driven and submersible, membrane filtration

technologies have converged in respect of performances and economics and they are competing for

the same markets and applications.

Summary of process attributes of pressure driven and submersible technologies are provided in Ta-

ble 4.7.3.

Table 4.7.3. Summary of process parameters of pressure driven and submersible technologies.

Pressurized Immersed

Treats low to medium feed turbidity Treats medium to high feed turbidity

Ability to compensate feed temperature fluctua-

tion in wide range Ability to compensate feed temperature fluctu-

ation in narrow range

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Can operate at high flux rates Flux rate limited by available TMP

Limited ability to treat high turbidity water Limited ability to treat high turbidity water

Small footprint Larger footprint

Cost effective in small to medium capacity sys-

tems Cost effective in medium to large capacity sys-

tems

Small volume of cleaning solution Larger volume of cleaning solution

Suitable for prefabricated systems Can retrofit gravity filters

7.7.3. Membrane filtration process

The membrane filtration process operates in a stepwise manner. The sequence of operation depends

somewhat on type of technology: pressure driven or immersed.

7.7.3.1. Pressure driven technology.

The sequence of operation of pressure driven technology includes:

- Filtration

- Forward flush

- Backwash

- Chemical enhanced backwash (CEB)

- Air scouring (for PDO only)

- Cleaning in place (CIP)

The operation sequence starts with a filtration step, usually conducted in a direct flow mode (also

called dead end filtration), i.e. operation at 100% recovery. This step is shown schematically on

Figure 4.7.4.

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Figure 4.7.4. Filtration step – direct flow mode of operation

The filtration step lasts between 30 – 60 min. During the direct flow step, membrane surface is be-

ing covered with particles. The water permeability declines, requiring increase of the feed pressure

in order to maintain the design filtrate flow rate.

The next step that follows is restoration of permeability of the membrane. This is accomplished

through removal of foulants that accumulated on the membrane surfaces by applying backwash of

the membrane module. In the backwash step, filtrate is pushed in reverse direction, through the

membrane to dislodge foulant form membrane surfaces and remove them from the membrane filtra-

tion modules. The backwash step may include application of air scouring or addition of chemicals

to increase efficiency of foulants removal. The chemical most frequently used during chemical en-

hanced backwash (CEB) step is hypochlorite added together filtrate backwash flow. Depending on

quality of feed water, usually the CEB is conducted once to few times per day.

Schematics of backwash step, including listing of process parameters is shown on Figure 4.7.5.

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Figure 4.7.5. Backwash step.

The forward flush, applied in addition to backwash for permeability restoration, involves rapid

opening of the concentrate valve for number of seconds to flush solids that has accumulated in the

membrane module. The forward flush is usually applied during the periods of very high turbidity of

the feed water.

CIP also utilized for recovery of water permeability is applied at much lower frequency than the

backwash step, usually once every few months. In differentiation to CEB, which is applied on the

filtrate side of the membrane, the CIP is applied from the feed side. The cleaning solutions used

during the CIP step include solutions of citric acid, mineral acids, NaOH and hypochlorite.

Membrane filtration units that operate for potable water production have to verify integrity of mem-

brane barrier.

The verification of membrane barrier integrity is described in ASTM procedure (ASTM D6908-03)

The frequency of application of integrity test is specified in the operational permit of the membrane

filtration system. The unusual frequency of integrity testing is once per day. The integrity test pro-

cedure in commercial systems is based on measuring decay profile of air pressure applied on the

feed side of the membrane filtration system.

The sequence of integrity test procedure is shown schematically in Figure 4.7.6.

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Figure 4.7.6. Integrity test sequence.

The integrity is calculated through determination of pressure decay rate (PDR) according to equa-

tion below

PDR = (Pi-Pf)/t

Where Pi – initial pressure

Pf – final pressure

t – time interval

PDR = PDR (measured) – rate of diffusion

Vacuum decay rate (VDR)

VDR = VDR (measured) – rate of diffusion

During determination of PDR, the rate of pressure decline due to diffusion of pressurized air

through water filled pores should be subtracted from the total measured PDR. The pressure decline

due to air diffusion is function of membrane surface. Therefore, the number of membrane modules

that could be tested for integrity as a one unit is limited by this phenomenon.

In majority of commercial systems the integrity test is conducted automatically, controlled by PLC,

which also automatically evaluates the results and displays warnings if the test results are outside

the range of sufficient membrane integrity.

7.7.3.1.1. Configuration of pressure driven membrane filtration unit

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The schematics configuration of pressure driven membrane filtration unit is shown in Figure 4.7.7

and system layout shown in Figure 4.7.8.

The basic components include:

- Raw water supply source

- Feed water pump

- Optional dosing systems of oxidant (sodium hypochlorite) and coagulant

- Micro strainer

- Membrane unit(s)

- Filtrate storage tank

- Backwash system

- Storage and dosing systems for cleaning chemicals

- Filtrate pumping system

Figure 4.7.7. Configuration of pressure driven membrane filtration unit

In addition, the filtration system will include cleaning system, control system and power supply unit.

Pressure driven system with modules operating in outside – in configuration (PDO), will also in-

clude air compressor for conduction membrane air scouring step.

During the filtration step, feed pump pumps raw water through the micro strainer to the membrane

filtration units. The membrane unit will operate with concentrate valve closed (100% conversion).

The filtrate is directed to the filtrate clear well. Filtrate pumping system transfers filtrate to distribu-

tion or to the next processing step. As the membrane surfaces being fouled and water permeability

decreases, the feed pump develops higher feed pressure to maintain the design filtrate flow. At pre-

sented time interval, after 20 – 60 min of operation in filtration step, membrane units enter sequen-

tially into backwash step. In the backwash step, set of valves isolates the membrane unit from the

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pressurized feed line, connect filtrate line to the backwash system and connects feed or concentrate

line to discharge. The backwash is initiated by starting the backwash pump and pumping filtrate in

reverse direction for a period of 30 – 60 seconds. The filtrate dislodges foulants form the membrane

Figure 4.7.8. Pressure driven membrane filtration system (courtesy Simens Water Technologies)

surfaces and they are discharged from the membrane unit through feed or concentrate connections.

The configuration and status of valves during filtration and backwash steps is shown on Figures

4.7.9. a – f.

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Figure 4.7.9. a. Valves position during filtration step

Valves During Backwash - Top

Figure 4.7.9. b. Valves position during backwash step

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Valves During Backwash – Bottom

Figure 4.7.9. c. Valves position during backwash step

Valves During CEB

Figure 4.7.9. d. Valves position during chemical enhanced backwash step

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Valves During Water Draining

Figure 4.7.9. e. Valves position during system draining step

Valves During Air pressurizing

Figure 4.7.9. f. Valves position during air pressurizing step

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The air pressurizing step is utilized by one of the manufacturer of pressure driven membrane mod-

ules (Hydranautics – PDI configuration) to increase effectiveness of filtrate backwash step. It is

called air enhanced backwash (AEB). During the AEB air is introduced on the feed side of the

membrane to drive off feed water from the module. At this conditions, the filtrate backwash that

follows, flows against empty feed volume, therefore at higher flow velocity resulting in better re-

moval of deposits from membrane surface.

Another manufacturer of pressure driven membrane modules (Siemens – PDO configuration) ap-

plies both air scouring and compressed air to drive filtrate back through the membranes.

The process steps during the backwash follows the schedule listed in Table 4.7.4.

Table 4.7.4. Air assisted backwash of the pressurized (PDI) membrane filtration system

Step 1 Normal filtration The unit is in filtration, the feed control valve is open.

Step 2 Drain to back-

wash level

Filtration stops. The unit drain valve is opened and water is

drained from the unit until the backwash level is reached.

Step 3 Aeration and liq-

uid backwash

Low pressure air enters the shell side to provide air scouring

to remove solids from the membrane fibers. The flow meter

on the discharge of the blower is used to monitor the back-

wash air scour flow. Compressed air pushes the filtrate in the

lumen side to the shell side of the fibers, thus achieving liquid

backwash.

Step 4 Post aeration Air scouring continues after the liquid backwash ends. The

lumen side remains pressurized with the compressed air.

Step 5 Shell draining The top shell side of the fibers is vented, while the lower

backwash discharge valve is opened. The backwash waste

drains under the influence of gravity out of the array. Site hy-

draulics should be such that shell draindown can be achieved

in 45 seconds.

Step 6 Shell fill - flush Feed enters the shell side via the lower feed manifold and ex-

its the unit via top backwash discharge valves.

Step 7 Resume to filtra-

tion ready status

Feed continues to enter the shell side via the lower feed mani-

fold, however the top backwash discharge valve is closed and

the filtrate exhaust valve is opened. This forces water into

the fibers. Once the unit is refilled with feed water, the unit

returns to filtration (or enters STANDBY).

7.7.3.1.2. Sizing of pressure driven membrane filtration unit

The sizing of membrane filtration unit consist of:

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- Determination of process parameters (filtration flux, backwash flux)

- Defining duration of operating steps.

- Calculation of time allocation for each process step

- Calculation of gross filtrate capacity and membrane area required

- Defining membrane train size and number of membrane trains

An example of operational sequence in pressure driven membrane unit is provided in Table 4.7.5.

The values of duration and frequency of operational steps are provided as a range. The actual value

selected for design will depend on quality of raw water or previous experience of similar operation.

Table 4.7.5. Sequence of operation of pressure driven membrane filtration unit.

Process step Objective Duration Frequency

Forward flow Permeate production 15 – 60 min Continuous

Backwash Foulants removals 30 – 60 sec Every 15 – 60 min

Chemicals enhanced

backwash (CEB)

Foulanlts removal 1 – 15 min Once – twice a day

Cleaning in place Foulants removal 2 – 4 hr Every 1 – 6 months

Integrity test Verification of mem-

brane integrity

20 min Every 1 – 7 days

Example of water quality parameters of surface water source is provided in Table 4.7.6.

Table 4.7.6. Water quality parameters of surface water source

Parameter Value

Turbidity, NTU 0.5 – 5

Total suspended solids, ppm 2 – 5

Water temperature, C 2 - 25

An example of filtrate flux and schedule of operating sequence for membrane filtration system

treating surface water of quality parameters listed in Table 4.7.6 is provided in Table 4.7.7.

Defining of operational schedule allows calculation what fraction of the operating time the mem-

brane system is on line, producing filtrate. It is evident that the filtrate capacity of the unit should be

planned to provide design output and to produce sufficient filtrate for backwash and cleaning.

Table 4.7.7. Example of operational parameters and schedule of pressurized membrane filtration

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system

Operating step Parameter value

Design filtrate flux, l/m2/hr 85

Design backwash flux, l/m2/hr 300

Backwash frequency, min 30

Backwash duration, min 1.0

Off line time due to backwash, min 1.5

Integrity test frequency per day 1

Integrity test duration, min 20

CEB frequency per day 1

CEB duration, min 10

Total off line time per day, min 97 (integrity + CEB + backwash)

Backwash time per day, min (1440 – 20 – 10)/(30 + 1.5) = 45

Backwash off line time, min 1.5*45 = 67

Off line time due to monthly cleaning,% 1

Membrane trains on line time fraction (1440 – (20+10+67+0.01*1440))/1440

= 0.92

According to the values listed in Table 4.7.7, the scheduled off line time represents close to 8% of

the available time. This off line time requires compensation of filtrate production capacity in sizing

of the membrane filtration unit.

The sizing of the membrane filtration unit is based on schedule of the process steps, configuration

of membrane elements selected and the designed filtrate flux rate.

Example of sizing of membrane filtration system of filtrate capacity of 200,000 m3/day according

to parameters listed in Table 4.7.7 is provided in Table 4.7.8.

Table 4.7.8. Example of sizing calculations of 200,000 m3/day pressurized membrane filtration sys-

tem

Design process parameter Value

Filtrate capacity required for backwash, % 45*300/[85*(1440-97)] = 11.8

Capacity to compensate off line time, % 97/(1440 – 97) = 7.2

Time for membrane cleanings, % 1

Contingency capacity, % 5

Total additional capacity, % 25 (11.8 + 7.2 + 1 + 5)

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Installed filtration capacity, m3/d 200,000* 1.25 = 250,000

Membrane area required, m2 250,000,000/(24*85) = 122,550

Number of membrane modules (46m2/module) 122,550/46 = 2,664

Rack size, membrane modules per rack 192

Number of racks required 14

Number of racks installed 14 + 2

Instantaneous feed flow per rack, m3/day 192*46*85*24/1000 = 18,020

Backwash volume, m3/day/rack 192*46*300*45/(60*1000) = 1,990

CIP interval, days 20

CIP frequency per day 14/20 = 0.7

CIP volume per day, m3/day (CIP tank = 20 m3) 0.7*20 = 14

Maintenance wash (MW) interval, days 5

MW frequency per day 14/5 = 2.8

MW volume per day, m3 (CIP = 20 m3) 2.8*20 = 56

Total waste volume, m3/day 14*(1990 + 14 + 56) = 28,840

System recovery rate, % 100* (1 –

28,840/(14*192*46*85*24*0.92/1000) = 88

As expected the recovery rate is function of filtration intervals (time between backwash events),

filtrate flux and backwash flux. Increase of filtration intervals and filtrate flux and reduction of

backwash flux will increase recovery rate.

The rate of usage of chemicals determines the size of storage tanks and capacity of dosing equip-

ment. Example of sizing of chemical dosing equipment is listed in Table 4.7.9.

Table 4.7.9. Example of sizing of chemicals dosing equipment in pressurized membrane filtration

system

Chemical reagent Quantity

Sodium hypochlorite for CIP

Sodium hypochlorite concentration, % 12.5

Sodium hypochlorite specific gravity 1.2

Dosing concentration, ppm 1,000

Daily volume of CIP solution, m3 14

Daily volume of hypochlorite for CIP, l 14*1000/(1000*1.2*0.125) = 93.3

Sodium hypochlorite for MW

Dosing concentration, ppm 200

Daily volume of MW solution, m3 56

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Daily volume of hypochlorite for MW, l 56*200/(1000*1.2*0.125) = 75.7

Total volume of hypochlorite, l/day 169

Storage volume of hypochlorite for 30 days, m3 ~ 5.0

Citric acid for CIP

Citric acid concentration, % 50

Citric acid specific gravity 1.25


Daily volume of citric acid, l 14*20000/(1000*1.25*0.5) = 448

Storage volume of citric acid for 30 days, m3 ~ 14.0

Sodium hydroxide for CIP

Sodium hydroxide concentration, % 40

Sodium hydroxide specific gravity 1.2


Daily volume of sodium hydroxide, l 14*1000/(1000*1.2*0.4) = 29

Storage volume of citric acid for 30 days, m3 ~ 1.0

The system will be composed of subunits providing functional operational capability according to

listing in Table 4.7.10.

Table 4.7.10. Listing of subunits and major equipment in pressurized membrane filtration system

Equipment Quantity Sizes/comments

Feed water system

Centrifugal, end suction pumps 16 800 m3/hr, 3 bar, VFD con-

trolled motor

Automatic backwashing strainer 16 800 m3/hr

Instrumentation Lot Turbiditymeter, pressure indi-

cators and transmitters, temper-

ature transmitter

Valves Lot Manual, butterfly

Membrane unit

Trains frame and piping 16 Corrosion protection coating

Membrane modules 3,072 46 m2/element

Set of automatic valves 16 lots Inlet, outlet, waste discharge,

air release

Instrumentation 16 lots Magnetic flow meters, turbidi-

tymeter, pressure indicators and

transmitters

Chemical cleaning system

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CIP/MW tank 2 20 m3

Immersion heated 2 120 kw

Centrifugal, end suction pump 2 800 m3/hr, 3 bar, stainless steel

wetted parts

Instruments 2 lot Magnetic flow meter, ORP sen-

sor/transmitter, CIP tank level

transmitter, CIP temperature

transmitter, pressure gauges

Valves 2 lot Valves: isolation , flow control,

air release, pressure release,

chemical injection isolating

valve

Chemical dosing systems

Sodium hypochlorite

Transfer pumps 2+2 750l/hr

Storage tank 1 5 m3

Calibration column 2

Valves and piping 1 lot

Citric acid

Transfer pumps 1 + 1 1500l/hr

Storage tank 1 14 m3



Sodium hydroxide


Storage tank 1 1 m3



7.7.3.1.3. Operating cost of pressurized membrane filtration unit

The operating cost includes the following cost items:

- Power

- Chemicals

- Labor

- Maintenance

- Membrane replacement

The combined power usage of the filtration system is based on power usage of individual equip-

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ment according to the scheduled operation time.

Example of energy usage in membrane filtration system of filtrate flow capacity of 200,000 m3/day

is provided in Table 4.7.11. As expected the major energy user is feed pump that delivers the feed

water and provides sufficient pressure to drove water through the membranes.

Table 4.7.11. Breakdown of energy usage in pressurized membrane filtration system. Filtrate ca-

pacity 200,000 m3/day

Power compo-nent

Efficiency M (P)

Operating pressure, bar

Flow, m3/hr Active, % Power, kW

Feed pump 0.95 (0.80) 2.0 14*750 =

10,500

92.0 706.9

BW pump 0.95 (0.80) 2.4 2,650 23.0 53.5

Cleaning pumps

0.95 (0.80) 1.7 750 8.3 3.9

Cleaning tank heater

8.3 10.0

Compressor 12.0 1.0

Valve actuators 3.3 1.0

Instrumentation &PLC

100.0 20.0

Total 796.3

Annual usage 6,626,808 kWhr/y

Specific energy usage

0.091 kWhr/m3

Another significant contributor to the water operating cost is cost of chemicals used in the process

for recovery of membrane permeability. These are chemicals used in the course of CEB and CIP.

The calculation of quantity of chemicals used is based on volume CEB and CIP and concentration

of chemicals used. An example of calculation of volumes required is provided in Table 4.7.12.

Summary of chemicals usage in a 200,000 m3/day membrane filtration system is provided in Table

4.7.13. The generic chemicals used in permeability recovery procedure include sodium hypochlorite,

sodium hydroxide, sulfuric acid and citric acid.

If coagulation – flocculation process is utilized in the filtration process, then quantity and cost of

coagulant (usually ferric salts) should be included in calculation of water cost.

Table 4.7.12. Summary of volumes required for CEB and CIP in pressurized membrane filtration

system

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Procedure Volume used,

m3/day/train

Number of treatments

in system per year

Total volume used,

m3/year

CEB1(1/4 backwash) 1990/4 = 500 14*365*0.95 = 4,950 2,475,500

CEB2 (1/40 backwash) 1990/50 = 50 4,950 247,550

CEB3 (1/40 backwash) 1990/50 = 50 4,950 247,550

CIP1 20 04*14*(365/20) = 102 2,040

CIP2 20 04*14*(365/20) = 102 2,040

CIP3 20 02*14*(365/20) = 51 1,020

Table 4.7.13. Summary of chemicals usage in pressurized membrane filtration system. Filtrate ca-

pacity 200,000 m3/day.

Process step Chemical Dosage, ppm t/year (as 100%) $/year

CEB1 NaOCl 50 123.8 123,800

CEB2 H2SO4 1,000 247.6 37,140

CEB3 NaOH 1,000 247.6 247,600

CIP1 Citric acid 20,000 40.8 81,600

CIP2 NaOH 3,500 7.1 7,100

CIP3 NaOCl 1,000 1.0 1,000

Total 497,240

Chemicals cost $0.007/m3

Summary of total operating cost is provided in Table 4.7.14.

Table 4.7.14. Summary of operating cost of pressurized membrane filtration system

Cost component $/year $/m3

Power@C10kWhr 662,681 0.0091

Chemicals 497,240 0.007

Operation and maintenance 480,000 (8 opera-

tors@$60,000/y)

0.006

Membrane replacement 658,285 (16 trains,7 years

membrane life)

0.009

Total 2,298,206 0.031

7.7.3.2. Configuration of immersed, vacuum driven, membrane filtration system

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The schematic configuration of immersed, vacuum driven, membrane filtration unit is shown in

Figure 4.7.10 and a layout of immersed unit is shown on Figure 4.7.11.

The basic components include:

- Raw water supply source

- Optional dosing systems of oxidant (sodium hypochlorite) and coagulant

- Micro strainer

- Membrane tanks

- Membrane unit(s)

- Filtrate pump

- Backwash unit

- Air scouring unit

- Storage and dosing systems for cleaning chemicals

- Filtrate storage tank

- Filtrate pumping system

In addition, the filtration system will include cleaning system, control system and power supply unit.

During the filtration step, feed pump pumps raw water through the micro strainer to the membrane

filtration units. The membrane unit will operate with concentrate valve closed (100% conversion).

CEB1 CEB2 CEB3

Air blower

Vacuum pump

Backwas

h pump

Filtrate

pump

CIP1 CIP2

Strainer 100 m

Filtrate

storage &

backwash

tank

Cleaning chemicals Backwash chemicals

Figure 4.7.10. Configuration of immersed, vacuum driven, membrane filtration unit

The filtrate is directed to the filtrate clear well. Filtrate pumping system transfers filtrate to distribu-

tion or to the next processing step. As the membrane surfaces being fouled and water permeability

decreases, the filtrate pump develops higher suction pressure to maintain the design filtrate flow. At

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presented time interval, after 20 – 60 min of operation in filtration step, membrane units enter se-

quentially into backwash step. In the backwash step, set of valves isolates the membrane unit from

the feed line, connecting filtrate line to the backwash system. The backwash is initiated by reducing

water level in the membrane tank and then starting the backwash pump and pumping filtrate in re-

verse direction for a period of 30 – 60 seconds. The reverse filtrate flow dislodges foulants form the

membrane surfaces to the tank volume. At the end of backwash step, water in the membrane tank is

drained (deconcentration step) and then the tank is refilled again with the feed water. `

The layout drawing on Figure 4.7.11. is of submersible system with 10 tanks, each containing racks

with connected membrane elements in configuration of clovers of four elements connected in paral-

lel. The equipment shown on the layout includes chemical storage tanks in containments, located at

the lower right corner. In clockwise direction there are two rows of five membrane tanks with the

manifolds and filtrate pumps located between the tanks. Each row of tanks is equipped with one

membrane lifting device. The compressed air system is located at the upper right corner. Low pres-

sure blowers are located to the right of the air compressors. Cleaning tanks and CIP recirculation

pumps are located between blowers and chemicals storage area.

Figure 4.7.11. Layout of immersed membrane filtration system (courtesy Siemens Water Technolo-

gies)

In system configuration shown in Figure 4.7.11 all the equipment including membrane storage tank

is located above the ground, mounted on a concrete slab. In alternate configuration the membrane

tanks and filtrate pumps could be located below the ground surface level. The total system area is

about 1200 m2.

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7.7.3.2.1. Sizing of immersed, vacuum driven, membrane filtration unit.

The sizing of immersed membrane filtration unit is conducted in similar way as described in Chap-

ter 4.7.5 for the vase of pressurized membrane filtration unit.

The process operational steps of the immersed unit are listed in Table 4.7.15. The only significant

difference of operational steps is the draining of tanks (deconcentration step) conducted at the end

of the backwash sequence.

Table 4.7.15. Sequence of operation of immerse, vacuum driven, membrane filtration unit.

Process step Objective Duration Frequency

Permeation Permeate production 15 – 60 min Continuous

Backwash & tank de-

concentration

Foulants removals 15 – 60 sec Every 15 – 60 min

Chemicals enhanced

backwash (CEB)

Foulanlts removal 1 – 15 min Twice a day – once per

week

Cleaning in place Foulants removal 2 – 5 hr Every 1 – 6 months

Integrity test Verification of mem-

brane integrity

20 min Every 1 – 7 days

An example of filtrate flux and schedule of operating sequence for membrane filtration system

treating surface water of quality parameters listed in Table 4.7.6 is provided in Table 4.7.16.

Defining of operational schedule allows calculation what fraction of the operating time the mem-

brane system is on line, producing filtrate. It is evident that the filtrate capacity of the unit should be

planned to provide design output and to produce sufficient filtrate for backwash and cleaning.

Table 4.7.16. Example of operational parameters and schedule of immersed membrane filtration

system

Operating step Parameter value

Design filtrate flux, l/m2/hr 40

Design backwash flux, l/m2/hr 160

Backwash frequency, min 30

Backwash duration, min 1.0

Off line time due to backwash, min 3.5

Integrity test frequency per day 1

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Integrity test duration, min 20

CEB frequency per day 1

CEB duration, min 10

Total off line time per day, min 177 (integrity + CEB + backwash)

Backwash time per day, min (1440 – 20 – 10)/(30 + 3.5) = 42

Backwash off line time, min 3.5*42 = 147

Off line time due to monthly cleaning,% 1

Membrane trains on line time fraction (1440 –

(20+10+147+0.01*1440))/1440 = 0.87

According to the values listed in Table 4.7.15, the scheduled off line time represents close to 13% of

the available time. This off line time requires compensation of filtrate production capacity in sizing

of the membrane filtration unit.

The sizing of the membrane filtration unit is based on schedule of the process steps, configuration

of membrane elements selected and the designed filtrate flux rate.

Example of sizing of membrane filtration system of filtrate capacity of 200,000 m3/day according

to parameters listed in Table 4.7.16 is provided in Table 4.7.17.

Table 4.7.17. Example of sizing calculations of 200,000 m3/day immersed membrane filtration sys-

tem

Design process parameter Value

Filtrate capacity required for backwash, % 45*160/[40*(1440-177)] = 14.2

Capacity to compensate off line time, % 177/(1440 – 177) = 14.0

Time for membrane cleanings, % 1

Contingency capacity, % 3

Total additional capacity, % 31 (14.2 + 14 + 1 + 3)

Installed filtration capacity, m3/d 200,000* 1.31 = 262,000

Membrane area required, m2 262,000,000/(24*40) = 272,916

Number of membrane modules (28m2/module) 272,916/28 = 9,747

train size, membrane modules per tank (1 train per

tank)

720

Number of tanks required 14

Number of tanks installed 14 + 2

Instantaneous feed flow per tank, m3/day 720*28*40*24/1000 = 19,350

Backwash volume, m3/day/rack 720*28*160*42/(60*1000) = 2,260

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CIP interval, days 20

CIP frequency per day 14/20 = 0.7

CIP volume per day, m3/day (CIP tank = 20 m3) 0.7*20 = 14

Maintenance wash (MW) interval, days 5

MW frequency per day 14/5 = 2.8

MW volume per day, m3 (CIP = 20 m3) 2.8*20 = 56

Total waste volume, m3/day 14*(2260 + 14 + 56) = 32,620

System recovery rate, % 100* (1 –

32,620/(14*720*28*40*24*0.87/1000) = 86

As in the previous example the recovery rate is function of filtration intervals (time between back-

wash events), filtrate flux and backwash flux. Increase of filtration intervals and filtrate flux and

reduction of backwash flux will increase recovery rate.

The rate of usage of chemicals determines the size of storage tanks and capacity of dosing equip-

ment. Example of sizing of chemical units was provided previously and is included in Table 4.7.9.

The system will be composed of subunits providing functional operational capability according to

listing provide in Table 4.7.18.

Table 4.7.18. Listing of subunits and major equipment, immersed membrane filtration system

Equipment Quantity Sizes/comments

Feed water system

Automatic backwashing strainer 2 + 1 6,800 m3/hr

Instrumentation Lot Turbiditymeter, pressure indi-

cators and transmitters, temper-

ature transmitter

Valves Lot Manual, butterfly

Membrane unit

Membrane tanks and piping 16 Made of FRP or concrete with

ckorrosion protection coating

Membrane modules 11,520 46 m2/element

Centrifugal, end suction filtrate pumps 16 850 m3/hr, negative suction

head 1 bar, discharge head 2

bar, VFD controlled motor

Set of automatic valves 16 lots Inlet, outlet, waste discharge,

air release

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Instrumentation 16 lots Magnetic flow meters, turbidi-

tymeter, pressure indicators and

transmitters

Chemical cleaning system

CIP/MW tank 2 20 m3

Immersion heated 2 120 kw

Centrifugal, end suction pump 2 800 m3/hr, 3 bar, stainless steel

wetted parts

Instruments 2 lot Magnetic flow meter, ORP sen-

sor/transmitter, CIP tank level

transmitter, CIP temperature

transmitter, pressure gauges

Valves 2 lot Valves: isolation , flow control,

air release, pressure release,

chemical injection isolating

valve

Chemical dosing systems

Sodium hypochlorite

Transfer pumps 2+2 750l/hr

Storage tank 1 4 m3



Citric acid


Storage tank 1 4 m3



Sodium hydroxide


Storage tank 1 1 m3



7.7.3.2.2. Operating cost of immersed membrane filtration system

The operating cost includes the following cost items:

- Power

- Chemicals

- Labor

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- Maintenance

- Membrane replacement

The combined power usage of the filtration system is based on power usage of individual equip-

ment according to the scheduled operation time.

Example of energy usage in membrane filtration system of filtrate flow capacity of 200,000 m3/day

is provided in Table 4.7.11. As expected the major energy user is feed pump that delivers the feed

water and provides sufficient pressure to drove water through the membranes.

Table 4.7.19. Breakdown of energy usage in immersed membrane filtration system. Filtrate capacity

200,000 m3/day

Power compo-nent

Efficiency M (P)

Operating pressure, bar

Flow, m3/hr Active, % Power, kW

Feed pump 0.95 (0.80) 2.0 14*810 = 11,340

87.0 721.0

BW pump 0.95 (0.80) 2.4 3,225 23.0 65.0

Cleaning pumps

0.95 (0.80) 1.7 750 8.3 3.9

Cleaning tank heater

8.3 10.0

Compressor 12.0 1.0

Air blower 50.0 30.0

Valve actuators 3.3 1.0

Instrumentation &PLC

100.0 20.0

Total 848.0

Annual usage 705,056

kWhr/y

Specific energy usage

0.0097 kWhr/m3

Another significant contributor to the water operating cost is cost of chemicals used in the process

for recovery of membrane permeability. These are chemicals used in the course of CEB and CIP.

The calculation of quantity of chemicals used is based on volume CEB and CIP and concentration

of chemicals used. An example of calculation of volumes required is provided in Table 4.7.12.

Summary of chemicals usage in a 200,000 m3/day membrane filtration system is provided in Table

4.7.13. The generic chemicals used in permeability recovery procedure include sodium hypochlorite,

sodium hydroxide, sulfuric acid and citric acid.

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If coagulation – flocculation process is utilized in the filtration process, then quantity and cost of

coagulant (usually ferric salts) should be included in calculation of water cost.

Table 4.7.20. Summary of volumes required for CEB and CIP in immersed membrane filtration sys-

tem

Procedure Volume used,

m3/day/train

Number of treatments

in system per year

Total volume used,

m3/year

CEB1(1/4 backwash) 2260/4 = 600 14*365*0.95 = 4,950 2,970,500

CEB2 (1/40 backwash) 2260/50 = 50 4,950 247,550

CEB3 (1/40 backwash) 2260/50 = 50 4,950 247,550

CIP1 20 04*14*(365/20) = 102 2,040

CIP2 20 04*14*(365/20) = 102 2,040

CIP3 20 02*14*(365/20) = 51 1,020

Table 4.7.21. Summary of chemicals usage in immersed membrane filtration system. Filtrate ca-

pacity 200,000 m3/day.

Process step Chemical Dosage, ppm t/year (as 100%) $/year

CEB1 NaOCl 50 148.5 148,500

CEB2 H2SO4 1,000 247.6 37,140

CEB3 NaOH 1,000 247.6 247,600

CIP1 Citric acid 20,000 40.8 81,600

CIP2 NaOH 3,500 7.1 7,100

CIP3 NaOCl 1,000 1.0 1,000

Total 522,940

Chemicals cost $0.0075/m3

Summary of total operating cost is provided in Table 4.7.22.

Table 4.7.22. Summary of operating cost in immersed membrane filtration system

Cost component $/year $/m3

Power@C10kWhr 705,056 0.0097

Chemicals 522,940 0.0075

Operation and maintenance 480,000 (8 opera-

tors@$60,000/y)

0.006

Membrane replacement 822,857 (16 trains,7 years

membrane life)

0.012

Total 2,530,853 0.035

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The above examples of calculations of power consumption, chemicals consumption and operating

cost are not specific for any commercial membrane filtration technology. The present calculations

are based on generic process parameters and just provided here to demonstrate the approach to this

type of calculations.

In calculations conducted for a commercial project the specific membrane module configuration,

filtration process sequence and operating parameters specified by equipment provider will have to

be applied. The results will have to be confirmed with provider of membrane filtration equipment to

receive their approval and eventually warranty for system performance and operating cost, once the

project is secured.

7.7.4. Comparison of conventional and membrane filtration technologies as pretreatment

for seawater RO desalination systems

The conventional pretreatment technology is based on suspended solids removal through coagula-

tion, flocculation and media filtration. The process is well established and in majority of cases is

capable to produce RO feed water of the required quality in respect of SDI and turbidity. The con-

ventional process is very effective as pretreatment. However the quality of the effluent could be

significantly affected by the quality of the raw water.

The membrane filtration process is more equipment intensive than the conventional pretreatment

process. Therefore maintenance requirements are much higher than of the conventional system. As

in the case of conventional system, deterioration of raw water quality will affect operation of the

filtration system. However, due to presence of the membrane barrier, filtrate quality will be much

less affected by the quality of raw water, as compared to operation of the conventional system at the

same conditions.

Schematic configuration of a conventional pretreatment system is shown on Figure 4.7.12. The pre-

treatment system components include coarse strainer, storage and dosing systems for chemicals

(hypochlorite, acid, coagulant and polymer), static mixer, flocculation system followed by gravity

filters. The filtration effluent is collected in the clear well. The filter backwash unit consists of

backwash pumps and air blower (for air scouring). The backwash stream is treated in solids man-

agement unit. The auxiliary units include electric supply unit and process control system.

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Intake

pumpsStrainer

Static

mixerMedia filters

Filtrate

clearwell

Dosing

systems

2

Backwash

pumps

Blowers

Solids

handling

Electric

system

Control

system

Seawater

source

Dosing

systems

3

Dosing

systems

1

Dosing

systems

4

Flocculation

Figure 4.7.12. Configuration of conventional pretreatment system for SWRO

Feed

pumpsStrainer

Static

mixerMembrane tanks

Filtrate

pumps

Filtrate

clearwell

Dosing

systems 1

Heat

exchanger

Backpulse

tank

Backpulse

pumps

Dosing

systems 3

Blowers

Compress

air

Cleaning and

neutralization

unit

Dosing

systems 2

Solids

handling

Vacuum

pumps

Electric

system

Control

system

Figure 4.7.13. Configuration of immersed membrane pretreatment system for SWRO

The configuration of membrane filtration pretreatment system is shown on Figure 4.7.13. The pre-

treatment system components include fine strainer, storage and dosing systems for hypochlorite,

static mixer, flocculation system followed by membrane tanks with membrane modules. The filtra-

tion effluent pumped through the membrane by filtrate pumps is collected in the clear well. The fil-

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trate line is equipped with air receivers and vacuum pumps. The membrane backwash unit consists

of filtrate backwash storage tank, backwash pumps and air blower (for membranes air scouring) and

storage and chemicals dosing system(s) for CEB’s. The membrane filtration system also includes

cleaning (CIP) unit with circulation pump, heat exchanger and cleaning solution neutralization sys-

tem. The auxiliary units include electric supply unit and process control system.

Addition of coagulant to feed water improves stability of permeability in membrane filtration sys-

tem. However, there is tendency not to use coagulation in RO seawater applications to avoid neces-

sity of inclusion of solids management system, which is necessary in the conventional pretreatment

systems.

The relative advantages of conventional filtration and membrane filtration systems are summarized

in Table 4.7.23.

Table 4.7.23. Summary of comparison of relative advantages of multimedia and membrane filtra-

tion systems for SWRO applications.

Selected equipment and cost

component Conventional pretreatment Membrane filtration pretreat-

ment

Microstrainers No Yes

Flocculators Yes Yes/No (1)

Filtration media Anthracite + sand Membranes

Filtration basins 100% 50 – 60%

Solids handling Yes Yes/No (1, 2)

Cartridge filters Yes Yes/No (3)

Footprint 100% 50 – 60%

Additional energy use + 0.05 – 0.08 kwhr/m3

Chemicals usage + 50 – 100%

Membrane replacement + 1 – 2 c/m3

Equipment maintenance cost + 100%

(1) Frequently membrane filtration pretreatment systems operate without addition of coagulant.

(2) Solids handling unit only required if coagulant is being used

(3) In RO system utilizing membrane filtration pretreatment, cartridge filtration equipment is

frequently omitted from the treatment process.

Comparison of estimated capital cost of two pretreatment systems, one including multimedia filtra-

tion and the second including membrane filtration, based on filtrate capacity of 200,000 m3/day is

provided in Table 4.7.24.

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Table 4.7.24. Estimated capital cost of multimedia filtration and membrane filtration units. Filtrate

capacity 200,000 m3/day.

System cost com-ponent

Conventional pretreatment technology Membrane filtration pretreatment technology

Cost component, $M

Cost compo-nent, %

Cost component, $M

Cost compo-nent, %

Microstrainers 1.2 2.33

Filter media 1.70 4.09 5.00 9.69

Static mixer 0.15 0.36 0.15 0.29

Flocculators 0.30 0.72

Filter system 0.70 1.68

Filter piping 6.00 14.44 4.00 7.75

Membrane equip-ment 15.00 29.08

All equipment in-stallation 6.00 14.44 6.00 11.63

Concrete tanks 8.60 20.70 6.20 12.02

Civil works 2.00 4.81 1.50 2.91

Chemical systems 0.50 1.20 0.40 0.78

Electric and instru-mentation 6.00 14.44 7.00 13.57

Solids handling 4.50 10.83

Subtotal equip-ment 36.45 87.72 45.25 87.72

Engineering (5%) 1.82 4.39 2.26 4.39

Construction man-agement (8) 2.92 7.02 3.62 7.02

Commissioning (1%) 0.36 0.88 0.45 0.88

Subtotal indirect 5.10 12.28 6.34 12.28

Total 41.55 51.59

$/m3/day 208 258 +24

The results of estimation included in Table 4.7.22 indicate capital cost of membrane filtration pre-

treatment to be about 20 – 25% higher than the conventional pretreatment. Higher capital cost of

membrane pretreatment system is expected as it includes more expensive equipment components

compared to single stage multimedia filtration. However, the above evaluation did not accounted for

much larger footprint of multimedia filters and therefore an additional land cost, which could be

significant at some sites.

7.7.5. Offering of commercial membrane filtration technology

Offering of commercial membrane filtration products includes both membrane elements and com-

plete systems. Some suppliers offer just membrane elements or systems, some offer both. The fol-

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lowing list of suppliers, included in Table 4.7.25, illustrates variability of approach to market place

by different suppliers

Table 4.7.25. Listing of established suppliers of commercial membrane filtration products.

Provider Market ap-proach

Membrane type

Membrane ma-terial

Module con-figuration

Well estab-

lished

Aquasource Systems UF CA PDI +

Dow/Omexell Elements & sys-

tems MF PVDF PDO

Hydranautics Elements UF PES PDI +

Inge Elements UF PES PDI +

Koch Systems UF PS PDI +

Norit Elements & sys-

tems UF PES PDI +

Pall/Asahi Systems MF PVDF PDO +

Polymem Systems UF PS PDO

Siemens (Memcor) Systems MF PVDF SUB

+

Siemens (Memcor) Systems MF PVDF PDO

+

Toray Elements MF PVDF PDO

Zenon Systems MF PVDF SUB +

The important information included in Table 4.7.23 is related to membrane material. Majority of

manufacturers provide membrane made of PVDF, followed by PES and PS. There is large variety of

membrane modules configurations. Larger number of manufacturers offers pressurized products,

while immersed products are only offered by two. One of the major providers, Siemens, offers both,

the pressurized and the immersed products.

Unlikely the offering of membrane elements for RO applications the offering of membrane filtration

products is not standardized in dimensions and configurations. The variability of membrane filtra-

tion products is illustrated in the following pictures of products offers by various providers.

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Figure 4.7.14. Membrane filtration train with Hydracap modules – Hydranautics

Figure 4.7.14. Membrane filtration train with Xiga modules – Norit

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Figure 4.7.15. Membrane products offered by Inge

Figure 4.7.16. Membrane products offered by Pall – Asahi.

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Figure 4.7.17. Pressurized membrane filtration products (CP) offered by Siemens.

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Figure 4.7.18. Immersed membrane filtration products (CS) offered by Siemens.

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Membrane filtration membranes made of ceramic materials has been manly used for specialty ap-

plication that could justify high cost membranes and membrane modules.

In the last few years the cost of ceramic membranes has declined and in parallel membrane ele-

ments of increased membrane area per elements has been offered.

The ceramic membranes has unique properties that make it very attractive for filtration applications.

• The membrane is permanently hydrophilic

• It has practical ability to operate at high permeate flux (~200 lmh) at field conditions

• The membrane can be preserved in dry condition

• The membrane can be clean and sanitized with hot water, high concentration oxidant solu-

tion, extreme range of pH

• It has long useful membrane life, significantly longer than polymeric membranes

The disadvantages are

• Lack of standardization of membrane configurations

• The membrane is mechanically strong but brittle

• High cost of membrane

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Figure 4.7.21. Representative summary of configuration of ceramic membrane modules

desalination process engineering part i · desalination process engineering part i mark wilf ph. d....

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