WATER TREATMENT Membrane Technology

Operator Training Manual

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CHAPTER 1 1

Introduction to Membrane Technology 1

CHAPTER 2 4

Four Types of Membrane Filters 4

CHAPTER 3 7

Membrane Materials, Modules and Systems 7

CHAPTER 4 12

Ultrafiltration 12

CHAPTER 5 18

Reverse Osmosis 18

CHAPTER 6 20

Factors Influencing Reverse Osmosis Performance 20

CHAPTER 7 23

Membrane Types and configuration 23

CHAPTER 8 33

Components of Reverse osmosis unit 33

CHAPTER 9 39

RO Plant Operation 39

CHAPTER 10 41

Chemical Cleaning 41

CHAPTER 11 50

Troubleshooting 50

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Chapter 1

Introduction to Membrane Technology

Introduction Membrane separation processes have been used for years, but they have not come

to play an important role in producing potable water supplies until the past 10

years. RO (reverse osmosis) and ED (electrodialysis) are replacing phase change

desalting technologies for supplying water to coastal and island communities all

over the world. Nanofiltration is becoming an economical alternative to the

traditional water softening processes. Tremendous improvements have been made

in recent years and the utilization of membrane technology has dramatically

increased in water treatment. Today membrane technology is used in wastewater

treatment for water recycling.

Table 1 .l lists several undesirable water contaminants, the conventional solutions

for them, and corresponding membrane processes that can do the job. There are

many variations on these conventional processes that could be included, but the

ones listed are sufficient to illustrate that there are membrane process alternatives

available to address most water problems.

What is a Membrane?A membrane is a film. A semi-permeable membrane is a very thin film that

allows some types of matter to pass through while leaving others behind. Some

membranes are porous and separate materials based on size compared to the size

of the pores. Others are dense films with no apparent pores that separate matter

based on differences in diffusion rates through the membrane.

Membranes are divided into indistinct classes based on the size of the materials

they retain. MF membrane is very porous; it allows water, dissolved salts,

colloidal materials, and particles that are smaller than the pores to pass through.

On the other end of the spectrum, RO membrane is a dense film with no pores,

only spaces in its polymeric structure that are large enough to allow water and

other small, uncharged molecules to pass through.

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Constituent of

concern

Conventional process Membrane process

1. Turbidity

2. Suspended Solids

3. Biological

contamination

Coagulation /flocculation/settling

Media Filtration

Disinfection

Microfiltration

1. Colour

2. Odour

3. Volatile organics

Activated carbon

Chlorine +Media filtration

Aeration

Ultrafiltration

1. Hardness

2. Sulphates

3. Manganese

4. Iron

5. Heavy Metals

Lime softening or Ion Exchange

Ion Exchange

Oxidation and filtration

Same as above and ion exchange

Coagulation/flocculation

Nanofiltration

1. Total Dissolved

solids (TDS)

2. Nitrate

Distillation

Ion Exchange

Reverse Osmosis

(Report no 29 from US Bureau of Reclamation technical service center)

There are three primary mechanisms for the separation and transport of water and

solutes across a membrane: sieving, convection, and solution diffusion.

Dead-End, Cross flow, and Transverse Flow OperationDead-end, cross flow, and transverse flow operation refer to the direction of flow

across the membrane. The diagrams illustrate the differences between them. In

dead-end filtration, there is no liquid waste stream; all feed water passes through

the membrane. Over time, particles build up in the membrane structure or on the

surface, limiting water passage. As the filter cake accumulates, the AP builds until

the maximum is reached, and then the membrane must be replaced or cleaned.

In cross flow operation, the feed stream flows parallel to the membrane surface,

limiting filter cake thickness and density. A part of the stream permeates through

the membrane, leaving a zone of high particle concentration at the surface.

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Particles and/or solutes are drawn back into the bulk feed stream by the flow of

lower concentration water past the surface.

Transverse flow operation is used with tubular membranes configured such that

the feed stream flows past them at right angles. The product stream permeates to

the interior of the tubes. The higher turbulence across the membrane in this

configuration enhances filter cake disruption and thereby maintains a higher

productivity rate than cross flow or dead-end filtration operated under equivalent

conditions.

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Chapter 2

Four Types of Membrane Filters

IntroductionMembrane processes used today for water and wastewater include both pressure-

driven processes and electrically driven processes. Pressure driven processes

includes microfiltration, ultrafiltration, nanofiltration and reverse osmosis.

Electrically driven processes include electrodylasis reversal and

electrodeionization. Each of these technologies utilizes a membrane barrier that

allows the passage of water but removes contaminants. These membrane systems

are available in different physical configuration that include spiral wound

(consisting of flat sheet membrane material wrapped around a central collection

tube), hollow fiber (consisting of hollow fiber material), Tubular and ceramic.

Apart from the configuration, there are other factors, which may vary, in different

classes of filtration.

They are

1. Membrane material

2. Hydraulic mode of operation

3. Operational driving forces (i.e. pressure or vacuum).

Microfiltration Microfiltration is basically filtration through a coherent medium with a nominal

pore size range from slightly below 0.1 m to slightly above 1m size. The size

mentioned above is for the pore size of the membrane media and not of any cake

that gets accumulated on the medium. Thus, in terms of pore size, microfiltration

fills in the gap between ultrafiltration and granular media filtration. Membranes

for microfiltration, as defined above, have been available and in use for decades.

The 0.45-micron membranes used for silt density index measurements are

microfiltration membranes.

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Microfiltration is used to remove particles, bacteria, and colloids from feed

streams in water treatment systems. Ion exchange resins, ultrafilters, and RO

membranes are all susceptible to fouling by microorganisms and colloids in the

feed. Even after ion exchange, feed water to RO membranes must be filtered with

a microfilter to remove fine resin particles. Micro filtration removes particles in

the range of approximately 0.1-Micron to 1 micron. In Microfiltration suspended

particles and large colloidal particles are rejected while macromolecules and

dissolved solids pass through the MF membranes.

UltrafiltrationUltra filtration is midway between MF and NF. The method of separation is same

as in MF that is by sieving. UF retains much smaller particles than MF like

macromolecules like colloidal silica protein etc but allows dissolved solids to pass

through. For UF, pore sizes generally range from 0.01 – 0.05 m (nominally

0.01m) or less, decreasing to an extent at which the concept of a discernable

“pore” becomes inappropriate, a point at which some discrete macromolecules

can be retained by the membrane material. In terms of a pore size, the lower

cutoff for a UF membrane is approximately 0.005 m.

UF membranes are more commonly classified based on molecular cut off

(MWCO) because of their ability to retain large organic macromolecule.

Nanofiltration Nanofiltration refers to specialty membrane process which rejects particles in the

approximate size range of 1 nanometer (10 Angstrom) and hence the term

Nanofiltration. NF operates in the realm between UF and reverse Osmosis.

Organic molecules with molecular weights greater than 200-to 400 are rejected

also dissolved solids are rejected in the range of 20 to 98 %. Salts which have

monovalent anions have rejection of 20 to 80 % (for example Calcium Chloride

and Sodium Chloride) whereas salts which have divalent anion have rejection

rate of almost 98 %(example Magnesium sulphate).

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Reverse Osmosis

Reverse Osmosis is the finest level of filtration available. The RO membrane acts

as barrier to all dissolved salts and inorganic molecule as well as well organic

molecules with molecular weight greater than 100.Water molecules pass freely

through the membrane. Rejection of Dissolved salts is typically 95 to 99 %.

Comparing NF and RO membranesMost RO and NF membranes available today are very similar. The

characterization is done basically on the basis of rejection characteristics.

Generally and in this discussion TDS removal is required the membranes are

considered to be RO membrane and when softening is required the membranes

are considered to be NF membrane. RO membranes are utilized if the TDS is

above 1500 and if the percentage of TDS is made of monovalent ions then NF is a

better solution.

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Chapter 3

Membrane Materials, Modules and Systems

Introduction

There are variety of membrane materials and modules and associated system that

are utilized by various classes of membrane filtration. All membrane filtration

technologies utilize a membrane barrier that allows passage of water but removes

contaminants. These membranes may is either made of some organic material or

may be some material like zircon or ceramic. The membrane media is generally

manufactured as flat sheets or as hollow fibers and then configured into

membrane modules, Though not a rule but it is common to see that most MF/UF

employ hollow fiber module and NF/RO use spiral wound membrane.

Membrane materials & Properties

Membrane materials for filtering aqueous solutions are either made of organic

polymer or may be of some inorganic substance like zircon/ceramic. Normally the

membrane material is made from synthetic polymer. Most common materials are

cellulose acetate, polyamide and poly sulphonate (TFC). There can be significant

impact on design and operation of filtration system because of membrane

material. For example any strong oxidant like chlorine can destroy the membrane

and hence on such waters proper pretreatment should be provided. Polyamide and

TFC membranes are more sensitive to chlorine than CA Membrane. CA

membranes are more pH sensitive and work in a very narrow pH range. PA and

TFC membranes work on a broader pH range. Mechanical strength is also

important because it allows a higher TMP (Trans membranes Pressure) which

means more operational flexibility. Cross filtration also allows cleaning operation

to be done from bot feed and filtrate side of the membrane.

It is more common to find wider type of materials being used for manufacturing

MF/UF membranes then NF/RO membranes. MF/UF membranes are

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manufactured from Cellulose acetate (CA), polyvinyl fluoride (PVDF),

Polyacrylonitrile (PAN), polypropylene (PP), polysulfone (PS) and other

polymers like polyethersulfone (PES). The choice of these materials will depend

upon the intended use because each of them has different properties. NF /RO are

generally manufactured from cellulose acetate or polyamide membrane.

Membrane ModulesMembranes are generally manufactured as flat sheet or as hollow fiber and then

configured into one of several different types of membrane modules. Membranes

can be made from many different materials and can be formed into a wide variety

of configurations. The most common are depth filters, plate and frame, spiral

wound, pleated, tubular, and hollow fiber. No configuration is better under all

circumstances; there are advantages and disadvantages inherent in each of them.

One must select the best for a particular situation. Depth filters are used in MF to

trap coarser particles. They are dense, thick walled cylinders made from spun

polymer strands. They are formed such that the outside has a more open structure

than the inside so that as water passes from the outside in, larger particles are

trapped first.

1. Depth filters are dense, thick walled cylinders made from spun polymer

strands and so formed that the outside has more open structure for trapping the

coarser particle from water as it passes from outside in. They are basically

used in microfiltration for trapping coarser particles.

2. The plate and frame configuration is less widely used in water treatment. It

finds wider application in food industries. It is also used in electrodialysis (an

electrical driven ion exchange membrane process). It is also used in high

solids content micro- and ultra-filtration because the units can be taken apart

and cleaned very thoroughly.

3. The spiral wound configuration, which is used in the whole range of filtration

processes, provides a greater surface area per module than pleated or tubular

configurations in the same processes, but is much more difficult to clean.

4. Tubular and hollow fiber membranes are made from a variety of materials as

mentioned above They are designed for cross-flow filtration. The pressure

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vessels are similar for tubular and hollow fiber membranes as those used for

cross flow pleated cartridges but they operate in reverse direction With tubular

and hollow fiber membranes, feed water flows from inside the membrane tube

or fiber and is filtered to the outside. The port on the side of the module is the

permeate port and the reject comes out the end. Tubular configurations can be

used for MF, UF, and sometimes NF membranes. They are easy to clean but

have a low surface area to volume ratio. Tubular configurations are used for

high flow /high solids content feed streams.

General Concept and few Definition

Certain concepts and definition are common for all membrane filtration system

and understanding these common definitions will prove helpful in system

designing and operation and maintenance of the system

Cross flow: - Flow of solution parallel or tangential to membrane surface. It helps

in counteracting concentration polarization.

Concentrate – the continuous waste stream (typically consisting

of concentrated dissolved solids) from a membrane process,

usually in association with nanofiltration (NF) and reverse

osmosis (RO) processes. In some cases also used to describe a

continuous bleed stream of concentrated suspended solids

wasted from microfiltration (MF) and ultrafiltration (UF) systems

operated in a crossflow (or feed-and-bleed) hydraulic

configuration

Dead End Filtration – term commonly used to describe the deposition mode

hydraulic configuration of membrane filtration systems; also synonymous with

“direct filtration”

Deposition Mode – a hydraulic configuration of membrane filtration systems in

which contaminants removed from the feed water accumulate at the membrane

surface (and in microfiltration (MF)/ultrafiltration (UF) systems are subsequently

removed via backwashing)

Feed: - Starting solution to processed (Treated)

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Flux: -Membrane filtration system throughput is typically characterized by the

system flux, which is defined as the filtrate flow per unit of membrane filtration

area, as shown in drawing.

Feed-and-Bleed Mode – a term used to describe a variation of the suspension

mode hydraulic configuration of membrane filtration systems in which a portion

of the crossflow stream is wasted (i.e., bled) rather than recirculated

Hydrophilic – the water attracting property of membrane material

Hydrophobic – the water repelling property of membrane material

Recovery: - The recovery of membrane unit is defined as the amount of feed flow

that is converted to filtrate flow, generally expressed in percent The recovery as

defined in most cases does not account for the use of filtrate for routine

maintenance purpose such as chemical cleaning and backwashing.

General flow balance for all Membrane filtration: - A general flow balance

can be written which is applicable to all membrane filtration.

Qf = Qc+Qp

Transmembrane Process: - Membrane filtration like MF, UF and MCF require a

driving forces to transport water molecules across the membrane barrier. This is

the pressure gradient across the membrane, or the TMP. TMP is defined by the

pressure on the feed side of the membrane minus the filtrate pressure, commonly

called the backpressure.

TMP = Pf -Pp

Membrane units can be 1 ft by 1 ft or 1 meter by 1 meter and the flow can be in gallon per day or M3/day

Unit membrane area

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Where Pf = Feed Pressure and Pp = Filtrate pressure.

Fouling - Interaction between substances in the feed and the membrane that

reduces flux.

Permeate- Solution that permeates (passes through the membrane. Also called

filtrate or product water.

Plugging - It means accumulation of particulates in the membrane passages that

restrict flow.

Retenate/concentrate: - Residual solution containing the concentrated

contaminants

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Chapter 4

Ultrafiltration

Introduction

Ultrafiltration is a pressure-driven process, which uses semi-permeable synthetic

membranes to separate certain chemicals and materials from water. Various

membrane configurations are available. Some are fibers or tubes with the active

membrane being found on the inside, while others are flat sheets, which can be

stacked in frames or rolled into a spiral configuration. They all function by a

similar mechanism.

How it works

Membrane separation technology is based upon molecular size. The semi

permeable membrane in an ultrafiltration system has pore sizes in the range of

0.0025 to 0.01 microns. The pores at the membrane surface are so small that they

will allow only water and small dissolved chemical to pass through, while

stopping bigger molecules and particles. Due to very small pore size, the water

will flow through the membrane only b applying pressure. Generally in most

cases a minimum of approximately one kg /cm2 (15 psi) at least is required.

Pressure is applied to one side of the membrane so that water and low molecular

weight compounds in the waste stream flow through the pores as permeate, while

the larger molecules and suspended solids flow across the membrane and become

part of the concentrate.

In an ultrafiltration system, water flows parallel to the membrane surface, as

compared to the perpendicular flow of ordinary filtration. The cross flow motion

of the water within an ultrafiltration system allows high filtration rates to be

maintained continuously, whereas the constant build up of solids along the filter

surface can cause blockage in an ordinary perpendicular filtration system.

A phenomenon of ultrafiltration is known as concentration polarization. This is a

buildup of chemical contaminants at the membrane surface as the water filters

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through to the other side. The condition is not permanent and can be removed by

flushing the membrane with either more dilute feed or with clean water, or

permeate

Configuration of Ultrafiltration MembraneThere are three primary configurations of ultrafiltration membranes: tubular,

hollow fiber and spiral wound. The tubular membrane is generally used in small

flow, high solids loading applications. The construction of this membrane allows

easy cleaning; therefore it is the membrane of choice when severe fouling is

expected. The hollow fiber design consists of a membrane wound into a hollow

cylinder. Cylinder diameters vary; the expected solids loading govern the size of

cylinder necessary for a specific application. Hollow fiber MF/UF is the most

common configurations used today due to its ability to handle tough waters at

reasonable cost. Hollow fiber UF membrane consists of several thousand hollow

fibers, typically 0.5 –1.0m in diameter, bundled into a membrane element. These

systems can be either “outside in” or “inside –out”. In the outside in system the

feedwater enters from the outside of the hollow fibers and is pushed through the

membranes. The treated water is collected from the inside of the membrane

leaving the impurities behind. In inside out the water enters the bore of the fiber

and is filtered radially through the fiber wall. The filtrate is then collected from

outside the fiber.

The third configuration of ultrafiltration membranes is the spiral-wound, and it is

usually used for high volume applications. The spiral membrane is constructed by

rolling a flat membrane that is netted together with specially designed spacer

material. This type of membrane cannot be mechanically cleaned, and is usually

reserved for applications where TSS loading is low or has been reduced by pre-

filtration.

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Cross-Flow UFDuring the filtration cycle, exactly the same quantity of raw water is fed through a

preliminary filter into the closed loop, as of permeate and bleed leaving the plant.

With the aid of the cross-flow pump and reject pressure valve, the membrane

cross-flow and feed pressure gradient for the UF module is adjusted. To prevent

the concentration of retained water constituents becoming too high during the

filtration process, part of the retained substance flow is bled off from the loop.

The permeate is either passed into the permeate tank or leaves the system as

product. At the end of the filtration cycle, backwashing takes place. This can be

carried out either as conventional backwashing (permeate only) or chemical

backwashing (permeate and backwashing chemical).

With the backwash pump, permeate is forced through the UF module in the

reverse filtration direction at twice to three times the transmembrane pressure

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(TMP) and so removes the water constituents retained by the membrane. The

concentrate thus produced is discharged as wastewater from the filtration unit.

Operation of Ultrafilters Membrane Unit

Ultrafiltration differs from conventional filtration plants in that filtered product is

continuously removed from the membrane surface. Fouling material is removed

from the surface by two different processes

Flushing This is usually an automatic process by which certain amount of flux recovery is

achieved either by backflushing or forward flushing

Chemical Cleaning While automatic backflushing is effective in maintaining the plant capacity in

short term, a periodic chemical cleaning is usually required to prevent gradual

degradation in membrane flux Chemical cleaning in Place (CIP) sequence take

some 2 to 4 hours when the plant can not be used

Four operating modes are possible 1. Service mode -100 % recovery Flux recovery – Backflush

2. Service mode - Feed and bleed Flux recovery - Backflush

3. Service mode - Feed and bleed Flux recovery - Fast forward flush

4. Service mode - Recirculation Flux recovery – Backflush

In single pass system during service run the recovery is hundred percent with all

the feed passing through the membrane as permeate. A small permeate bleed fills

the backflush/CIP tank ready for backflush sequence. During the backflush

sequence, permeate is pumped in reverse direction through the membrane to the

drain. Backflush frequency depends on water quality

Feed & Bleed OperationIn feed and bleed type some amount of concentrate is continually passed to drain.

Flushing can be achieved either by backflushing or by forward flushing.

RecirculationIn this case the concentrate is returned to feed water pump inlet or to the feed

tank. This is only done during chemical cleaning.

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Maintaining CapacityRegular flushing and chemical cleaning are essential for maintenance of capacity.

New membrane may have very high flux initially but with colloidal feed water

fluxes reduces rapidly. Automatic flush arrest the decline in membrane

performance experienced on colloidal feed waters. Eventually the membrane

performance levels of and the plant operate in equilibrium. During further

operation there will overall decline in performance and this will indicate that

chemical cleaning is required. Chemical Cleaning frequency can be once a week

to once per three months depending on water quality.

Monitoring Parameters in UF There are three purposes for instrumentation in UF treatment processes. These are

to monitor:

1. Hydraulic performance,

2. Retention performance

3. General water quality.

Hydraulic monitoring requires measurement of the feed and permeate pressure,

feed, permeate, recycle and backwash flows, and temperature of the feed stream.

Retention monitoring requirements depend on the source water and treatment

objectives. Operators should be able to tell, with a glance at the monitoring

instrumentation, whether the product water is within specifications or not. Surface

water treated for drinking purposes should be monitored for turbidity and particle

count of the feed and permeate.

General water quality monitoring for UF includes conductivity, pH, and chlorine

residual.

Equipment Maintenance Ultrafiltration plants have number of components that should be routinely

checked for correct operation and serviced as necessary.

These include

1. Pressure regulators

2. Control Valves

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3. Pumps

4. Pressure Switches

5. Membranes.

6. General Maintenance like leaks, motor fuses blown out etc.

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Chapter 5

Reverse Osmosis

Osmosis Osmosis is a process, which can be defined as a passage of liquid from weak

solution to a more concentrated solution across a semipermeable membrane.

The semipermeable membrane allows the passage of liquid (solvent) but does not

allow solids (solutes) to pass through

Principle of OperationIn natural osmosis water in a dilute solution passes through a semipermeable

membrane and into the more concentrated solution in an attempt to equalize the

salt concentrations on both sides of the membrane. This process continues until an

equilibrium is reached, (depicted in Figure 1) in which the difference in fluid head

between the concentrated and the dilute solutions is equal to the osmotic pressure

difference of the two solutions. Its magnitude is proportional to the amount of

dissolved salts in the solutions and to the temperature of the solutions.

Reverse OsmosisIf the osmotic pressure is overcome by applying an external force to the

concentrated solution (depicted in Figure 2), the natural tendency of the water to

flow from the dilute solution to the concentrated solution is overcome. Thus, pure

water as well as any dissolved gases it may contain are forced out of the

concentrated solution through the semipermeable membrane, while the salts or

dissolved solids are held back. This is called Reverse osmosis. (RO)

Mineral RejectionThe purpose of demineralization is to separate minerals from water. The ability of

membrane to reject mineral is called mineral rejection.

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RecoveryRecovery is defined as the percentage of feed flow that is recovered as product

water.

Reverse Osmosis Process

The simplified reverse osmosis process is shown in Figure. With a high-pressure

pump, pressurized saline feed water is continuously pumped to the module

system. Within the module, consisting of a pressure vessel (housing) and a

membrane element, the feed water will be split into a low saline product, called

permeate and a high saline brine, called concentrate or reject. A flow regulating

valve, called concentrate valve, controls the percentage of feedwater that is going

to the concentrate stream and the permeate which will be obtained from the feed.

In the case of a spiral wound module consisting of a pressure vessel and several

spiral wound elements, pressurized water flows into the vessel and through the

channels between the spiral windings of the element. Up to seven elements are

connected together within a pressure vessel. The feedwater becomes more and

more concentrated and will enter the next element, and at last exits from the last

element to the concentrate valve where the applied pressure will be released.

The permeate of each element will be collected in the common permeate tube

installed in the center of each spiral wound element and flows to a permeate

collecting pipe outside of the pressure vessel.

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Chapter 6

Factors Influencing Reverse Osmosis Performance

IntroductionWe all know that the performance of any membrane depends on the composition

of feed water but given a standard feed water, performance varies with pressure,

water temperature, level of water recovery, and the oxidation potential of the feed

water. Pressure, temperature and recovery are parameters that are factors of feed

water composition. Oxidation potential off feed water can effect the material of

the membrane and hence the kind of pretreatment required will also depend upon

the oxidant potential of feed water. Assuming that chlorine is used an oxidant,

dechlorination either by chemical or by granular activated carbon filter is a must.

The above factors greatly influence the permeate flux and salt rejection which are

the key performance parameters of a reverse osmosis process. They are mainly

influenced by variable parameters, which are as follows:

1. Pressure

2. Temperature

3. Recovery

4. Feed water salt concentration

Pressure

Refer to the equation for water flux Fw = A (p-)

Where Fw = water flux in gram / sq. cm /sec

A= water permeability constant in gm/sq. cm –sec atm5

p = pressure differential applied across the membrane atm

= Osmotic differential applied across the membrane atm

The equation above shows that the water flux is directly proportional to the

applied pressure. Increasing the feed pressure reduces the permeate TDS and

increases the permeate flux. As feed water is forced at greater velocity at higher

pressure more foulants in the feed stream interact at the membrane surface.

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Increasing the pressure could also lead to scaling because the salts, which remain

on the membrane surface, increase the local osmotic pressure. Ions diffuse away

when the surface pressure exceeds the main stream osmotic pressure. Salts have

to become more concentrated for diffusion to take place at higher osmotic

pressure. This can lead to precipitation of slightly soluble salts.

Temperature

Water passes through the membrane with lower applied pressure at higher

temperature than is required at lower temperatures. This is because the viscosity

of water increases with decreasing temperature, larger TMPs (by application of

increased pressure or vacuum) are required to maintain constant flux.

If the temperature increases and all other parameters are kept constant, the

permeate flux and the salt passage will increase. Temperature also impacts energy

consumption in RO feed pumps, since temperature influences flux and flux as an

impact on NDP

Recovery

The recovery is defined as the percentage of feed flow that is recovered as the

product water. Mathematically

Recovery =(Product flow /Feed Flow) *100

Two parameters, which determine Recovery is product water (Permeate) quality

and the solubility limits of minerals in the brine. The first can happen because of

exceeding product quality criteria with excessive recovery. The second is due to

concentration polarization. Concentration polarization means concentration of

brine to a degree where minerals get precipitated on membrane surface. Due to

concentration polarization there is a tendency for sparingly soluble salts to

precipitate and also for deposition of particulate matter.

Effect of pH

The pH of the feed water can affect the membrane structure and the scale

formation potential of the concentrate stream. CA membranes are pH sensitive

and operate under a very narrow pH range of 4-6. Infact there are some CA

membranes, which operate at a pH range of 5.5 to 6. If exposed to a pH outside

this range, hydrolysis occurs. Hydrolysis results in a lessening of mineral

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rejection capability. Thin film composite membranes generally have a much

broader operational pH range, some as large as 2 to 11. The pH of the feed water

may need adjustment to control scaling of the concentrate conveyance system. For

example, silica solubility increases dramatically above pH 7.7, and at higher

temperatures. A silica-scaling problem could be controlled by either raising the

pH or the temperature of the feed water. Calcium carbonate, on the other hand, is

more soluble at low temperatures and at a pH less than 8.0. Lowering the pH,

temperature or adding anti-scalants can relieve a carbonate-scaling problem.

However, if the concentrate is saturated in both silica and carbonate, changes in

temperature or pH can cause one or the other to precipitate. Care must be taken to

find the best condition to prevent scaling.

Oxidants

Oxidants like chlorine and ozone are added to water to control microbiological

growth and to improve taste and odor. These oxidants can affect membrane.

Chlorine is the most widely used oxidant but ozone and UV are also being used.

Most membranes used in RO application are sensitive to oxidant. CA membranes

are more tolerant to chlorine compared to other membranes. Irrespective of the

type of membrane used some method of biological treatment is still needed.

Chlorine is added to water supplies to control biological growth, to improve taste

and odor, to remove iron and manganese, and to speed the decomposition of

vegetable and animal matter. CA membranes are also biodegradable and it is

advisable to have feed water chlorine held between 0.2 ppm to 0.5 ppm (Reverse

Osmosis by Zahid Amjad). CA membranes are also temperature sensitive and

hence at higher temperature chlorine and bacteria becomes more aggressive. This

limits the use of CA membrane. Non-cellulosic thin film composite membranes

are not tolerant to oxidation; yet some method of biological treatment is still

needed. Systems that use chlorine with thin film composite membranes require

dechlorination just ahead of the RO unit.

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Chapter 7

Membrane Types and configuration

Types of Membrane Increased use of Reverse Osmosis process and other membrane process has led to

development of variety of membranes. Membranes most widely used in water

treatment are:

1. Cellulose Acetate or CA membrane

2. Polyamide membrane or PA membrane

3. Thin film composite membrane or TFC membrane

Cellulose Acetate Membrane (CA)CA membranes are constructed of cellulose acetate or cellulose triacetate or

blend. One problem this material has is slow chemical decomposition through a

process called hydrolysis. The use of acids to prevent scaling increases the rate of this

form of membrane decay. As mentioned earlier CA membrane are also biodegradable

and hence bacterial protection must be provided. Advantage of CA membrane is that

they are chlorine tolerant and can be used with feed water having residual chlorine of

less than 0.5 mg/liter. Though these membranes achieved acceptable results with

brackish water but not with seawater due to compressibility of membrane at high

pressure.

Its use is also limited for the following reason: -

1. Works under a narrow pH range.

2. Temperature sensitive

3. Performance limitation

Polyamide Membrane (PA)Polyamide membranes are made of aromatic polyamides, PA membranes works

in a broader pH range and hence it is more resistant to hydrolysis and has better

salt rejection and organic rejection and they are non biodegradable. Despite this

chemical stability, these membranes cannot tolerate any residual oxidant. The

feed water to PA membrane should be free from chlorine as they are subject to

attack by Chlorine. If chlorination is required to reduce the amount of biological

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suspended solids, then dechlorination must be complete if polyaromatic amide

membranes are used.

Thin film Composite MembraneThe above two membranes are of asymmetric structure are dense, thin layer (salt

rejecting layer) supported by a thick, porous layer. They are both composed of the

same polymer.

TFC membranes consist of three layers and are based on polyamide membrane

and consist of poly sulfone membrane as a support for very thin polyamide layer.

Advantages of TFC membrane are: -

1. High flux rate

2. Wider pH range

3. More resistant to chlorine than PA

4. High silica rejection

The disadvantage they are very intolerant towards chlorine.

Configuration of membraneMajor configurations of membranes are: -

1. Spiral wound

2. Hollow fiber

3. Tubular frame

4. Plate & Frame

Spiral Wound In spiral wound configuration are assembled from flat sheet polymer Membrane

and spacers are wound around the permeate collection tube to produce flow

channels for permeate and feed water. These are the most commonly used in

Industrial application. The advantages of this configuration are: -

1. Simpler plumbing system

2. Easier maintenance

3. Greater design freedom

4. Less prone to fouling

5. Can withstand higher level of pre-treatment upsets.

24

Spiral wound elements are constructed from flat sheet membranes. The "spiral-

wound" element is wound around a central permeate collection tube and separated

by thin spacer materials. The membrane material may be either made of cellulose

acetate (CA) or of composite membrane (TFC). In CA membrane the two

different layers are of the same polymer and is referred to as asymmetric. In case

of composite membrane the two layers are completely different polymers, with

the porous substrate often being polysulphone. One of the advantages of spiral

wound membrane is that the design yields a high membrane packing density.

Specific packing density depends on the size of spacer material used. Spacer

thickness can be adjusted depending on the application.

In the spiral wound design a flat membrane envelope is formed closed on the

three sides with a supporting grid inside. This is the central permeate tube. The

open side is sealed to the tube Each leave consists of two membrane sheets placed

back to back and seperated by a fabric spacer called permeate carrier. A single

spiral-wound module 8 inches in diameter may contain up to approximately 20

leaves, each separated by a layer of plastic mesh called a spacer that serves as the

feed water channel. The leaves are rolled spirally around the product tube. Each

end of the unit is finished with a plastic molding called an “Anti-telescopic

Device”

Feedwater flows axially through the spiral over the membrane surface, which is

roughly a path parallel to the central tube. A portion of feed water permeates

through the membrane envelope in a spiral path and enters the central tube via the

perforation leaving behind any dissolved and particulate contaminants that are

rejected by the semi-permeable membrane. There are two flows one which is the

filtered water which permeates and collected from the permeate port. This is

known as the permeate or product water. The other, which does not permeate

through the membrane layer, continues to flow across the membrane surface,

becoming increasingly concentrated in rejected contaminants and is called the

reject and is removed from the reject port.

Recovery is a function of the feed-brine path length. In order to operate at

acceptable recoveries, spiral systems are usually staged with three to seven)

25

membrane elements connected in series in a pressure tube (or housing). The brine

stream from the first element becomes the feed to the following element, and so

on for each element within the pressure tube. The brine stream from the last

element exits the pressure tube to waste or to feed another tube. Permeate from

each element enters the permeate collector tube and exits the tube as a common

permeate stream.

Internal construction of a spiral-wound membrane

26

Hollow Fine Fiber Hollow fiber contains a bundle of extremely small diameter membrane tubes,

which creates a tremendous membrane area in a small volume. Feed water is fed

to the center of vessel through a porous pipe. The ions get rejected through these

hollow fibers when water permeates the wall and is discharged as product water.

Concentrated reject is collected at the opposite end of the vessel.

Tubular MembraneTubular membranes are inserted into or coated onto the inside surface of a porous

tube and are designed to withstand the operating pressure. Diameter of tube may

vary depending on the application. At times it may also differ depending on the

manufacture. One of the biggest advantages of this is that the device can be used

with many different membrane types and provides opportunity for repairing the

membrane, which becomes an advantage in many applications.

Feed water enters the end of the tube permeates through the membrane and is

collected and discharged through a concentrator concentrated reject water leaves

through the end of porous tube.

Plate & Frame type

The fourth and the least used configuration in water treatment are the plate and

frame type. A series of flat sheet membranes seperated by alternating filtrate

spacers and feed /concentrate spacers. This configuration provides very low

27

surface area to volume ratio and hence considered in sufficient and is therefore

seldom used in water treatment

RO Plant Layout Most RO systems use Spiral wound membrane. The incoming water after proper

pretreatment is pumped into the membrane system. The spiral wound membranes

are loaded into pressure vessel, which is made of stainless steel, or fiber

reinforced plastic. A standard pressure vessel can hold six to seven elements but

to accommodate other number of elements they are custom manufactured.

The elements are arranged in series in a pressure vessel such that the concentrate

from each preceding element represents the feed water for next.

A brine seal around the outside of the feed end of each element separates the

feedwater from the concentrate and prevents feed water from bypassing the

membrane element. For a single RO/NF element the recovery is generally

considered to be less than 15 % and recovery for six elements is generally taken

to be 15 %.

In one of the manual called “Desalting process” by bureau of reclamation typical

recoveries given are

No of elements per vessel

Maximum recovery per stage (for standard pressure)

Maximum recovery per stage (for low) pressure

4 40 355 50 456 55 507 60 608 75 N/A

A diagram of typical of typical pressure vessel containing spiral wound modules

is shown.

28

Membrane Element ConstructionThe membrane element is constructed of parts mentioned below

The Center Tube (CT) The center tube is also called permeate collection tube. The membrane “leaves’,

permeate carrier and spacer material is wound around this. The CT provides

structural strength to the element.

Anti telescopic device (ATD)Anti telescopic device are attached to the end of element. This can be either

bonded or loosely attached top the CT to form an integral part of interconnector.

ATDs fills space between elements in a pressure vessel. It also facilitates in flow

of feedwater from one element to next, which helps in preventing pressure build

up.

The Interconnector (IC)The IC connects the center tubes of neighboring elements and directs the flow to

one or both ends of pressure vessel. The IC is connected to CT by O rings.

Product end adapter (EA)The product end adapter is (EA) is used at both ends if the permeate volume is

high. Otherwise it is provided at only one end. It is more common to find

membrane element with one end plug. The other is sealed with dead end plug

(DEP).

Permeate carrier This is a sheet of material inserted between the backsides of the membranes,

forming a membrane envelope to promote the flow of permeate toward the center

tube for discharge at the ends of the vessel.

29

Feed spacer

This is the material inserted between neighboring membrane surfaces to create the

best possible flow conditions over the membrane.

Reverse osmosis membrane staging configurations.A group of pressure vessels operating in parallel collectively represents a single

stage of treatment in a NF/RO system. The system recovery is based on the

number of stages. We know that the system recovery is generally based on feed

water quality but a rough estimation gives for a single stage –55 % recovery, for

two stage –75 % and for three stage – maximum of 90 %. There are two types of

staging generally employed in Reverse osmosis system-Reject or concentrate

staging and Product staging. Reject staging is used more in drinking water

treatment and in low salinity water. This is done so that most of the raw feed

water will eventually be recovered as product water. Product staging also called

permeate staging is used to treat highly saline waters. It is used more commonly

in industries where ultrapure water is required.

One should not confuse Array with stages. Array is a combination of two or more

stages in series and generally identified as a ratio of pressure vessels in sequential

stages. The ratio of number of vessels per stage defines the array. Array can also

be defined in relative term.

For example let us consider a two-stage system containing 24 vessels and 144

elements in first stage and 12 vessels and elements in the second stage. We can

call it as 24: 12 array or 2:1 array in relative term.

Product StagingProduct staging is true series operation of two or more reverse osmosis membrane

systems. In permeate staging configuration the permeate from a stage becomes the feed

water for the subsequent stage. In most cases the second stage always requires its own

pressurizing pump, taking suction from storage tank of the first stage reverse osmosis

system. Though this configuration is mostly used in Industries it is and can also be used

in drinking water when the salinity is high.

30

Reject staging. Reject staging is used when the low salinity of the raw water permits a high

water-recovery ratio. Most membrane module manufacturers have a minimum

allowable brine reject flow for any given membrane of their manufacture. The

manufacturer's recommended maximum feed water flow rate and minimum

recommended brine reject flow can be used to calculate a maximum

recommended single stage recovery fraction by use of the following equation:

F – B/ F =R

Where:

F = Maximum recommended feed flow per module

B = Minimum recommended brine reject flow per module

R = Maximum recommended recovery rate

This maximum single stage water recovery is one means of evaluating a

membrane module being considered for Ro desalination of low salinity. When the

reject stream is still diluted enough for further concentration after the maximum

recommended recovery ratio is reached, the brine can be piped directly into

another membrane module for further water recovery. This is accomplished by

combining the brine flow from a number of first stage modules onto a fewer

number of secondary membrane modules. It is occasionally possible to further

concentrate the brine on a third reject stage

31

Reject staging

Product staging

32

Chapter 8

Components of Reverse osmosis unitReverse osmosis unit comprises of following components. The schematic is

shown below. The unit includes the following components.

1. Pressurization pump

2. Piping

3. Concentrate control valve

4. Sample valve

5. Flush connection

6. Cleaning connection

7. Permeate rinse valve

8. Permeate draw back tank

9. Energy recovery device

10. Membrane

33

Pressurization pumpThe pressure required for RO unit can range from 100 psi to about 1200 psi The

feed pump used for pressurizing feed water is called the pressurizing pump and is

either a centrifugal pump or a positive displacement pump. Centrifugal pumps

are used for lower pressure. Positive displacement pumps are preferred for higher

pressure. For seawater desalination it is the preferred pump.

The output of centrifugal pump may be throttled by use of throttling valve. This is

often done for new system or after membrane cleaning. The output of positive

displacement pump is not throttled. The pump discharge line should contain a

pressure relief mechanism.

Piping Choice of piping material is generally based on the salinity of water. For seawater

desalination high-grade stainless steel is used for high-pressure lines.

Brackish water use 304 or 316 SS and low pressure plant (House old RO) use PVC piping.

Pressure vessel housingSpiral wound membranes are housed in pressure tube generally called as pressure

vessel housing. The elements are connected in series and held in Pressure vessel.

Generally six elements are housed in one vessel. With later design of spiral

wound membrane seven 40-inch long elements have been placed in single unit. A

seventh element housing is useful if it is necessary to increase the system

recovery marginally.

RO ArrayThe array structure is determined during the design process by the hydraulics of

the system. The most common arrangement has six membranes in each pressure

vessel. This arrangement recovers as permeate 50% of the water fed to it. For a

34

75% recovery, two stages are required. When using two stages, 50% of the feed is

recovered in the first stage. The remaining 25% is recovered by the second stage,

as it yields a 50% recovery on the concentrate from the first stage. The largest

number of stages normally used is a three-stage unit. If pressure vessels with

fewer than six elements are used the recovery per stage is decreased

Valves Various valves are used in RO. The major valves are

1. Pump inlet

2. 1st stage inlet

3. Product outlet

4. Reject outlet

5. System flush inlet & outlet

6. Stage isolators

7. Cleaning connection

8. Permeate rinse

9. Sample valves

Concentrate control valve A regulating valve located in the concentrate line (reject) provides a means of

applying backpressure to membranes. Positioning this valve in conjunction with

pump discharge valve will set the permeate and concentrate flow rate.

Sample Valve Sample valve is located on feed, permeate and concentrate line. It is so located

that sampling is possible during all mode of operation like servicing, flushing or

cleaning.

Cleaning connectionAll units should have cleaning connections for each bank of permeators or

pressure vessels connected in parallel, isolation valves for each bank would allow

for one bank to soak while the upstream or downstream bank is cleaned.

35

Permeate RinseProvision for sending the permeate from one bank or unit proves useful when troubleshooting. Some process require that permeate achieve quality by rinsing after shutdown periodIrrespective of the type of membrane employed, the basic building block from which an RO Plant is constructed consists of a high-pressure pump and a, membrane module or permeator and the other components mentioned above.However, in order to obtain a reliable flow and quality of product, other components are required. These include the following items, some of which are optional depending on the feedwater quality.

Feedwater Iron ControlIron, if present in its oxidized trivalent state, can build up on the RO membrane, thus reducing productivity. Typical approaches for its removal include such options as a manganese greensand iron filter, a softener, and sodium hexameta phosphate injection.

Feedwater Scaling Tendency ControlScale in the form of calcium carbonate or calcium sulfate, even if their concentrations are low enough to be soluble in the feedwater, may be precipitated on the RO membrane due to the high concentration of solids on the brine side of the membrane. Typical options for their control include softening, acid injection, and sodium hexameta phosphate injection. It should be pointed out here that if the acid injection option is chosen, the product water will have a low pH due to the inherent production of carbon dioxide and 1ts complete passage through the RO membrane. The concentration of solids on the brine side of the membrane and thus the scaling tendency may be controlled to some extent by the control of the relative brine concentrate and product flow rates. These rates are typically adjusted such that a recovery ratio (i.e., ratio of product flow rate to feed flow rate) of 50-75% is maintained.

Feedwater Chlorine ControlThe presence of free chlorine or its absence in the feedwater is somewhat of a "Catch..22" situation. If it is not present, bacteria will most likely be present and the concentrated nutrients on the brine side of the RO membrane will ensure their continued growth and subsequent fouling of the membrane. However, if free chlorine is present in concentrations greater than 1 ppm, the membrane will deteriorate. Thus, carbon filtration or sodilJ1l sulfite injection may be used to remove chlorine, whereas sodium hypochlorite may be injected to add chlorine.

Feed water suspended solid control Adequate prefiltration must be provided to ensure continuous high -productivity. Generally, prefiltration to 5 microns is adequate. However, in cases where there are significant quantities of colloidal solids or solids less than S microns in

36

diameter (i.e., SxlO-4 cm), additional means of suspended solids removal, such as polyelectrolytes or filter aids, must be used.

Feedwater Temperature ControlFeedwater temperature should not exceed 30 C or be lower than 0 C in order to preserve the RO membrane. However, feedwater temperatures higher or lower than 25 C will give, respectively, greater or less product. For example, there is a 15% increase in product flow at 30 C and a 15% decrease in product flow at 20 C, both compared to operation with feedwater temperature Of 25 C

High Pressure Pump ProtectionDue to the high discharge pressure, the amount of possible pretreatment equipment upstream of the high-pressure pump and the fact that it is usually a multi-stage pump, it must be protected against loss of suction. Therefore, a pressure switch at its suction and a pump trip mechanism should be provided.

RO membrane Cleaning SystemInspite of the precautions taken to prevent fouling of the RO membrane, periodic cleaning is required. Therefore, a chemical injection system or cleaning connections should be provided for cleaning of the brine side of the membrane.

Pressure ControlIn order to maintain the required pressure to make the RO unit work (usually in the order of 400 psi), a pressure control valve is required on the brine concentrate line. And in order to prevent an unexpected increase in product pressure if the membrane should rupture, a relief valve is required on the product line.Additional InstrumentationIn order to properly monitor and control the routine operation of an RO Plant, the following additional instrumentation is preferred:1. Pressure indicators for the inlet and outlet of the prefilter, the discharge of the

high pressure pump, the brine concentrate and the product lines;2. Temperature indicator for the high pressure pump inlet;3. Flow meters with adjusting valves in the brine and product lines to control

their relative flow rates, thus the design recovery ratio;4. Conductivity probe pH probe, monitors, and recorder for the product line;5. Grab sample points throughout the pretreatment system, in the brine

concentrate line, and in the product line6. Level control in the product storage tank to trip the high-pressure pump on

high level.

37

This complete RO Plant is schematically represented in Figure

38

Chapter 9

RO Plant Operation

IntroductionAfter the water has been properly pretreated and desired quality of water is

obtained it is fed to the membrane through high-pressure pump

Feedwater to the RO system is first pumped through a fine filter. This is a

replaceable cartridge element filter nominally rated at 5-10 microns. The purpose

of this filter is to remove any turbidity and particulate matter from feed water

before it enters the RO system and is not meant for routine filtration. The filtered

raw water than flows to high pressure pump, which feeds the water at a pressure

of 400 psi through the RO membrane unit. Valves and pressure gauges between

the cartridge filter, the high-pressure pump and membrane modules control the

flow.

The RO system consists of stages. The raw water is pumped through the first

stage, which contains twice the number of membrane modules as the second

stage. The first stage purifies 50% of purified water fed to the system and rejects

the remaining 50% that contain all the impurities. The reject from the first stage is

then passed through the second stage, which again purifies 50 % and rejects 50 %.

The reject of the second stage now contains all the impurities removed by both

stages. Thus the total flow through the system is 75% purified water and 25%

reject water.

The RO system removes 90 to 95% of dissolved solids in the raw water. The

exact percent of product purity, product recovery and reject water depends on the

amount of TDS in feedwater, temperature and on maintenance.

Checklist for RO Operation

1. Before starting the RO pump check the cartridge filter. If not properly

installed in can damage the High-pressure pump or foul the membrane

39

element. It should be replaced whenever the headloss exceeds the

recommended specification or if the effluent turbidity exceeds 1NTU.

2. Start and check the scale inhibitor feeder equipment and adjust feed rate to

desired dose (2 to 5mg/L)

3. Remember that if scale inhibitor is not added membrane can get scaled

because of calcium salts and other inorganic. Inhibitor commonly employed is

Sodium hexameta phosphate (SHMP).

4. Add chlorine. The dose rate is so adjusted that the chlorine residual is between

1 to 2. For small plants chlorine is dosed through metering pump but in large

plant gas chlorinators are used.

5. Depending on the type of membrane and the recommendation adjust the pH to

desired level and bypass the feedwater till pH is adjusted. The acid dosing

pump should stop when the high-pressure pump stops. In most RO plants acid

dosing start and stop is directly linked to High-pressure pump.

6. Check the suction pressure. The high-pressure pump should not start till the

desired pressure is reached. Low pressure tripping is generally included in all

RO plants.

7. RO pump should trip if the discharge pressure is higher than recommended.

High-pressure switch is provided for tripping for protecting the membranes.

8. Adjust permeate and concentrate flow to establish the desired recovery rate.

9. Check the following after the desired flow rates have been achieved.

Check the differential pressure (p) = Feed pressure – concentrate pressure.

This should be noted. Bigger plants have recorder, which records this

continuously. Increase in p indicates that the system requires cleaning. p

should not increase more than 414 kpa or 4.1 kg / Sq cm

10. With the system online monitor all flows, pressure, level and the quality of

water being produced.

11. It is very necessary and also proves very useful if pretreatment is monitored

and records maintained. More problems in RO membranes are caused by

faulty pretreatment.

40

Chapter 10

Chemical Cleaning

IntroductionReverse osmosis (RO) is now an well-accepted unit operation in water treatment. One of the major drawbacks of RO system is that the membrane can foul with the contaminants being removed from feed water. Membrane fouling can result from the formation of a fouling layer on the membrane surface, or from internal changes of the membrane material. Scaling is a form of fouling that occurs when dissolved species are concentrated in excess of their solubility limitThe fouled membrane can cause reduction in flux rate and operating efficiency, leading to unscheduled shutdown, lost in production time, replacement of membrane and resulting in downtime and additional expense. The cleaning of fouled membrane should be as soon as possible or there is all possibility of membrane getting damaged beyond repairA number of pre-treatment methods are employed to reduce the fouling potential of a membrane treatment feedwater. These methods include various types of conventional filtration, disinfection and chemical treatment. Chemical agents can be added to slow the formation of precipitates. Acidification is used to prevent the formation of carbonates of low solubility, such as magnesium carbonate. An ion exchanger is sometimes used to trade cations of low solubility salts for cations that are more soluble, for example, sodium sulfate may be traded for calcium sulfate. No matter which method is used, most RO treatment systems must be cleaned regularly.

Types of FoulantA membrane treatment system can be fouled by virtually anything present in the water being fed to the unit. However, for common treatment systems, such as reverse osmosis (RO), the foulants may be generally categorized as 1. Inorganic fouling2. Organic Fouling3. Particulate Deposition or colloidal fouling4. Biofouling

Inorganic Deposit or foulingThis generally happens due to inorganic salts of low solubility. These may enter the treatment system in particle form, or they may precipitate inside the system as a result of concentration changes occurring in the feed water as permeate is recovered through the membrane. Metal hydroxides are another example of an inorganic foulant. The common culprits are iron hydroxide Fe (OH) 3 and aluminum hydroxide Al (OH) 3.

41

Organic Compounds. Organic compounds make up the next general category of foulants. Surface water sources, such as rivers and lakes, may contain naturally occurring organics, such as humic acids. Clarified water may contain leftover polymers, and wastewater may contain any number of organic compounds. The mechanism behind organic fouling depends upon the size and chemical nature of the specific foulant. Large molecular weight compounds may act more as particles and may plug the feed spacer in the membrane element. This plugging may be worsened if inorganic particles, such as clays and metal hydroxides, are also present.Low molecular weight organics may foul the surface of the membrane through chemical interaction. As an example, chlorinated phenols will adhere to the surface of an RO membrane by means of hydrogen bonding. In this situation, a small concentration of the chlorinated phenol in the feed water can cause a large loss of flux in the treatment system.

Colloidal foulingColloidal fouling of reverse osmosis elements can seriously impair performance by lowering productivity and sometimes-salt rejection. An early sign of colloidal fouling is often an increased pressure differential across the system. The source of silt or colloids in reverse osmosis feed waters is varied and often includes bacteria, clay, colloidal silica, and iron corrosion products. Pretreatment chemicals used in a clarifier such as alum, ferric chloride, or cationic polyelectrolytes can also cause colloidal fouling if not removed in the clarifier or through proper media filtration

Biological Organisms or foulantsAll raw waters contain microorganisms: bacteria, algae, fungi, viruses and higher organisms. In most RO system bacteria causes the majority of problems. The reasons are that bacteria can easily adapt to the environment inside the membrane treatment system. Biological fouling of the membranes may seriously affect the performance of the RO system. The symptoms are an increase of the differential pressure from feed to concentrate, finally leading to telescoping and mechanical damage of the membrane elements

Types of Membrane Cleaning SolutionsDeposits from membrane surface is removed either by mechanical or chemical means. An example of mechanical means is by flushing with high velocity water.It is more common to use chemical cleaning method. There are large numbers of chemical agents or formulation for removing scale and other deposits Broadly they can be classified asAcidAlkalizesChelantsFormulated Products

42

Cleaner Scale/Metal Oxides

Colloidal/Particulate Biological Organic

Hydrochloric acid 0.5 % (WT)

X

Citric Acid 2 % (Wt) and ammonium Hydroxide(pH 4.0)

X

Phosphoric Acid 0.5 % (wt)

X

Sodium Hydroxide pH(11-11.9)

X X

Trisodium sulphate or sodium tripoly Phosphate 1% (wt), Sodium salt of EDTA 1% and sodium Hydroxide –pH(11.0-11.9)

X X

Sodium hydrosulphite 1%(wt) & Detergent

X

Citric Acid 2.5%(wt) & ammonium bi fluoride 2.5%(wt)

X X

Membrane Cleaning Clean membranes are critical for maintaining the efficient operation of RO System Chemical cleaning of membrane treatment systems is a complex because of various foulants. A contributing factor to its complexity is the complex nature of the fouling problem that initiated the cleaning in the first place. The best cleaning solution is found by trial and error but however this can be minimized by by a basic understanding of fouling problems and the general types of cleaning solutions used for these problemsEach membrane manufacturer publishes specific instructions for cleaning and storing membranes when necessary. Cleaning and storage are critical operations that can extend or shorten the life of membranes.

43

Cleaning RegimesThere are two basic types of cleaning regimes i.e. at low pH and high pH. Organic and biological films are best broken down with a high pH solution at the maximum temperature and pH limits for the membrane. Sodium hydroxide works well. Some suggest adding enzymes to help break down cellular matter, surfactants to help penetrate and dissolve the film, and a chelating agent to bind calcium ions. Calcium is an important component in extra-cellular polysaccharides (EPS, or extra-cellular polymeric slime) which are produced by well-established bacteria. EPS protects them from disinfectants and cleaning agents. Depleting the EPS of its calcium building blocks helps the cleaning solution penetrate the biofilm.

EquipmentThe components of Clean in system (CIP) are1. Mix tank or storage tank with some method of mixing (volume should

be twice the capacity of the vessel or stage that is to be cleaned)2. Pump with some method of flow control3. Flexible plumbing connectors (to isolate stages or vessels)4. Temperature control5. Cartridge filter6. Temperature/pH sensor (portable or hand-held model will do)

Generic Cleaning ProcessNo more than one stage should be cleaned at a time. Ideally, only one element would be cleaned at a time, but that would be far too labor intensive. As a compromise, each stage should be cleaned separately so that the cleaning solution needs to go through only one vessel before returning to the mix tank. When two stages are cleaned at once, the foulants from the first stage have to be moved all the way through the second stage. It is difficult enough to get them out of the first stage; the likelihood of getting them all through the second is very low.

Preparation1. Isolate the vessel or stage that is to be cleaned.2. If pump does not have a variable speed drive, be sure that plumbing allows for

flow bypass of pressure vessels so that flow can be controlled without increasing pressure to the membranes.

3. Make sure that the mixing tank is clean and that fresh cartridges have been installed in the filter. Also, make sure that the hoses or piping used to connect the cleaning equipment to the membrane system is clean.

4. Fill the mix tank with an adequate volume of RO product water, at least two times the volume of the piping and pressure vessel that is being cleaned.

5. Begin mixing and warming the RO product water with a heater or 100 percent bypass.

44

6. The operator prepares the cleaning solution as per manufacturers instruction. Adjust pH as recommended by manufacturer. Adjust temperature to maximum possible limit as recommended by manufacturer.

7. Thoroughly mix the cleaning solution and adjust for proper pH and temperature. Allow enough time for the pH and the temperatures stabilize before starting the cleaning process. If you are using a commercial cleaning formulation, make sure to follow the manufacturer’s instructions.

Fill SystemIntroduce the cleaning solution into the vessel or stage at a low flow rate with the concentrate stream going to drain. To determine when the system is completely filled, monitor the pH of the concentrate and divert the concentrate stream back to the mix tank when it matches that of the cleaning solution.

Low Flow PumpingPump the mixed, preheated cleaning solution at a low flow rate with solution recirculating to the mix tank for about 15 minutes or until the return solution looks cleaner. The backpressure valve should be completely open so that no permeate is produced. The objective of this cycle is to dislodge larger particles and remove them from the system. Table 9.1 lists sample cleaning flow rates for different sized elements.

Moderate Flow PumpingIncrease to moderate flow rate with the solution still recirculating for another 15 minutes with the backpressure valve still open. Check solution appearance and make note of any cloudiness or suspended solids accumulation.

SoakReduce flow to lowest possible level for the soak period. Soaking helps to dissolve stubborn films and precipitates. The membranes should soak for anywhere from one to fifteen hours, depending on the degree of fouling. It is important to control the temperature during long soaking periods. The cleaning solution should be kept at the optimum temperature. In most cases, this means keeping the temperature from rising too high; however, if the ambient temperature is low and the optimum temperature is high, heating may be necessary.

High Flow PumpingAfter soaking, slowly increase the flow rate to the maximum allowable for your system. The pressure drop must be monitored during the high flow cycle. Maximum pressure drops for each element type given by the manufacturer. Flow rate should be increased slowly to flush out any large material loosened by soaking. Then the high flow may remove stubborn material and particles stuck in the spacer material.

45

Flush and SanitizeTake samples of the spent cleaning solution for chemical analysis. Drain tank and piping. Rinse the tank, and refill it with RO permeate. Flush out the system at a moderate flow rate with the concentrate stream diverted to drain. After flushing, refill the tank and add a sanitizer recommended by the membrane manufacturer. Sodium bisulfite or chlorine can be used with CA membranes. Formaldehyde or hydrogen peroxide can be used with thin film composites. Use caution with these chemicals, though; high concentrations may cause damage.After the cleaning process, some cleaning solution will still be on the permeate side of the membrane; so, after putting the clean stage back on line, divert product to drain until quality is acceptable.

High pH Cleaning It is often possible to determine what has happened to a membrane system by noting changes in the NPF, AP, and rejection for each stage over time. Table 9.2 describes the symptoms of major types of fouling and which cleaning regime is called for. Generally, a decrease in NPF in the first stage means particulate fouling. Particulates could be mineral, vegetable, or animal in nature. Spiral wound and HFF membranes serve as excellent cartridge filters - most particulates are trapped in the first couple of elements. The cleaning strategy indicated for front end fouling of this sort is a high pH /high temperature regime.

Low pH CleaningPrecipitative fouling, or scaling, occurs at the tail end of the system where the feed stream is at its highest concentration. Effects are a decrease in rejection and NPF and an increase in AP. A low temperature/low pH-cleaning regime is needed for scaling. Low temperature means normal operating temperature. Since calcium carbonate is less soluble at higher temperatures, it is best to use a low pH /low temperature cleaning solution first if more than one type of cleaning will be used.

Passive CleaningPassive cleaning is a milder form of cleaning that does not use chemicals or even a separate pump. If, say at 5 percent change in AP, NPF, or rejection, passive cleaning is performed, it may be possible to extend the time between chemical cleanings. The following are some procedures that can have a beneficial effect without avoiding the manufacturers warranty.1. Turn off the system for an hour. Osmotic pressure will draw product water

through the membrane to the concentrate side. This can help lift foulants off the surface. If live bacteria are present, stopping flow could encourage a growth spurt, so this method should be used only when disinfectants are present.

2. Reduce backpressure for a short time. Permeation rate should drop, and flow rates through the system should increase. This changes flow patterns through the system and can disrupt films that may be in process of taking root. It also

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washes away the high concentration layer right at the membrane surface and, in doing so, could slow the scaling process.

3. Perform a high flow, low pressure flush with product water. RO product water is very aggressive in dissolving precipitates. The change in flow pattern will also have the effect described above.

Membrane StorageMembrane systems work best when they operate continuously. It is unavoidable to have occasional shutdowns, though. When a membrane system is to be down for a few days, flush the process water from the system and replace it with RO product water pumped into the system at low pressure. If the concentrated process water and pure product water are left in the membrane vessels, the difference in concentration across the membranes will cause osmotic flow from the product side to the concentrate side. This can be beneficial in lifting foulants from the membrane surface in small quantities, but too much back flow can destroy the glue lines. For thin film composite membranes, the storage water should be oxidant-free. Otherwise, the membranes will degrade over time. CA membranes should be stored in chlorinated water, though. While the danger of biological growth is the same for both types of membrane, damage from oxidation would be worse than biofouling for thin film composites.

Table 1Cleaning Cycle Feed flow rate, L / min (gal/min)Module diameter 64 mm

(2.5”)102 mm (4”)

203 mm (8”)

Soaking 2 (0.5) 3 (1) 15 (4)Low flow 10 (2.6) 19 (5) 75 (20)Moderate flow 15 (4) 30 (8) 100 (26)High flow 20 (5) 38 (10) 150 (40)

Table 2Impurity Causes Effects Prevention RemedySoluble inorganic substances

Over saturationPresence of crystallization centers

Decrease In salt rejection in end stagesIncrease in pressure drop in end stagesDecrease Normalized Permeate flowW’F)Scale formation on membrane surface or In bulk w/subsequent deposition

Formation of ‘salt bridge’ facilitating

SofteningAcidificationUse of chelating agents

Low pH w/ chelateNormal operating temperature“Soak cyclePhysical methods: ultrasound, magnetic,

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proteinAdsorptionHigh concentration at membrane surfacecan cause denaturation of proteins whichthen are more of a fouling problem

Soluble organic substances

Humic and fulvic acids natural tosurface watersLack of adequate pretreatmentOver utilization

Formation of H bonds on contact w/membranePartial diffusion through membrane -dependent on degree of branching

UltrafiltrationCoagulation Sedimentation

High pHHigh temperatureHigh flow rate“Detergent

Colloid materials (water insoluble inorganicCompounds: silica, iron hydroxides, etc.)

Over utilizationInadequate sedimentation period

Gel formation on membrane surfaceDecrease in salt rejectionDecrease in NPFSymptoms most likely to appear in last stage

Softening “Same as Colloids

Biological materials (bacteria, algae, Fungi, etc)

Inadequate pretreatmentInadequate flow through module - dead spacesHydrophobic attraction between cell andmembrane surfacesProduction of extracellular

Decrease in NPFInitial increase in salt rejection Increase in pressure drop “Symptoms most likely to appear in first stageAccumulation of byproducts of metabolismEventual deterioration of the membrane resultingin a decrease in rejectionDecrease in flow at

Pre filtrationUse of surfactants during normal operation hasbeen shown to prevent bacterial attachmentReduce recovery rate

Same as ColloidsUse of enzymes has been shown to help loosenbiofilm

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polymeric substancesCell fimbriae may help attach bacteria tomolecular matrix of the membrane

membrane surface canexacerbate concentration polarizationphenomena

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CHAPTER 11

Troubleshooting

Starting the RO unit1. Any interlock provided to RO unit should be tested before the unit is put into

operation 2. Ensure that all valves in the water supply line to RO are open.3. Ensure that all pre RO devices have been flushed, tested, and are operating

within their specification.4. Turn the feedwater supply on gradually and check for leaks in the inlet

plumbing. No flow should go through the RO membrane while the power is off and the solenoid valve is in closed position.

5. Open your concentrate and recycle flow control valves. Proper adjustment of these two valves is critical to the operation of RO Unit. The concentrate valve determines the amount of rejected water leaving the RO unit and creates operating pressure shown on pressure gauge.

6. With the power source to the motor starter de-energized, switch to on position. Water will begin to flow but the pump will not start. Check for leaks and repair if needed

7. Energize the power source to motor starter. The pump should not start at this point.

8. After the minimum suction pressure is built the high-pressure pump will start immediately switch off and check the direction of motor rotation. The motor should rotate clockwise while looking at the motor end.

9. If the motor is rotating in opposite direction change any two of the three leads Always Turn Power off When Changing wire

10. Initiate the start from the control panel 11. Once all condition like desired suction pressure, pH are met, the high pressure

pump will start automatically.12. Once the high-pressure pump starts the Permeate and Concentrate flow should

be adjusted so as to establish the desired recovery rate.13. Once the flow has been established check the delta P (P) across the RO unit.

(P = feed pressure – Concentrate pressure). When the elements get fouled P usually increases, thus indicating the need for cleaning.

14. Adjust the Concentrate valve to achieve desired flow and the recycle valve to bring the operating pressure upto

15. Once the desired flow rate is achieved at the operating pressure no further valve adjustment is required.

16. The system is now operational.17. The Permeate and concentrate are allowed to flow to drain before the RO unit

is taken to operation.18. This will ensure that all the preservative has been removed from the

membrane element.

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Troubleshooting RO PlantTroubleshooting of any plant is carried when it performance does not meet the specification or when there has been abrupt change in its performance. Similarly RO plants also require troubleshooting if there is unacceptable change in quality and quantity of water produced. The following indicates that RO plants need troubleshooting.1. Normalized production changes by 15 % 2. Normalized salt passage changes by 50%3. Differential pressure changes by 15 %

Troubleshooting steps for RO plant 1. Checking and verifying instrument operation2. Reviewing operating Data3. Checking for component failure4. Checking for chemical upset5. Shutdown

Checking and verifying instrument operationConductivity, pressure and pH play very important role in RO operation. It is therefore very necessary that instrument measuring this should be accurate and functioning properly. Recalibrate all instruments.The recommended calibration schedules are

pH sensors, indicators and recorder 30 days

Conductivity sensors, indicators and recorders 90 days

Flow sensor indicators and recorders 90 days

All other instrument 180 days

Though most companies recommend this, I suggest that Pressure-monitoring

instrument should be recalibrated more often because it is an important

measurement in RO operation. Once it is checked that all instruments are working

properly go the next step

Reviewing operating DataCheck the operating log sheet. These will help you in diagnosing whether there is a system upset or fouling. RO may be working properly but changes in critical operating parameters like TDS, Temperature, recovery or flux can mislead you.The following points will help in identifying the problem1. Increase in feed water TDS increases feed pressure requirement.2. Decrease in feed water temperature increases feed pressure requirement.3. Increase in feed water TDS also increases Product (permeate) conductivity.

(Remember RO rejects a fixed percentage of salts)

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4. Increase in percent recovery of the system increases the reject TDS (This boosts permeate conductivity)

5. We had mentioned earlier that passage of salt is independent of passage of water through the membrane. Therefore if recovery is constant, a reduction in permeate flow results in a lower water flux, which will increase conductivity.

RO operation can be done by examining operating logs and by normalizing the logged operating data. Normalizing helps in predicting system performance and also helps in scheduling the cleaning frequency.Normalization is the mathematical procedure for correcting actual production and salt rejection value to standard condition. Most membrane suppliers today have developed computer software for projecting “Normalization”. The program graphically charts normalized permeate flow, percent salt rejection and feed to reject pressure drop called corrected p.

Checking for component failureThe system upset can also be due to mechanical problem

The mechanical failures can be

1. Damaged O rings

2. Damaged or missing feed tubes in hollow fiber permeators.

3. Damaged brine seals

4. Failure of high pressure pump

5. Failure of dosing pumps

6. Valve leakage

7. Piping failure

8. Cartridge filter

Checking for chemical upsetThe chemical upset can be

1. Improper pretreatment

2. Improper acid addition: More acid can lead to membrane damage and sulfate

scaling if H2SO4 is used. Lower dosage can lead to metal oxide formation or

carbonate scaling.

3. Improper scale inhibitor dosing- high dosing leads to fouling and low leads to

scaling.

4. Higher than recommended dosing of coagulants and polymer can lead to

membrane fouling.

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Once all the above problems – Mechanical, chemical upset and water chemistry have been ruled out as the likely cause, the next step is to identify the foulants. This is done by studying the water analysis of feed, brine and product stream over a period of time, which will help in verifying if any extraordinary changes have taken place. This will help in identifying the problem and the likely foulant, which caused the problem.

ShutdownEvery time the RO shuts down or trips for any abnormal conditions, the unit automatically goes into the flushing mode. If the RO plant is run under manual condition, the RO must be flushed for 5 minutes with DI water before shutting down the system.If the unit is going to be shut down for long, carry out the recommended procedure of storing (see previous Chapter)

A troubleshooting chart is always helpful in fault diagnosis

CHECK VERIFY EFFECT

Pressure drop between

feed and reject.

Has not increased by more

than 15%.

More than 15% indicates

fouling of feed path and

membrane surface.

Requires cleaning

Pressure drop between

feed and permeate

Has not increased by more

than 15%.

More indicates fouling of

membrane surface.

Requires cleaning.

Permeate conductivity Has not increased by more

than 15%.

More indicates fouling of

membrane surface.

Requires cleaning.

Acid dosing Is within recommended

value.

More can cause membrane

damage or sulfate scaling.

Less can cause carbonate

scaling or metal oxide

fouling.

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Instruments Reading Verify by calibration and

carryout of lab check of

the parameters the

instrument is monitoring.

Wrong operation

False sense of security that

everything is OK.

pH meter calibration &

control

The pH controller

generally controls acid

dosing pumps. The pH

controller should be

calibrated periodically and

tripping of dosing pump to

the set point should be

checked.

More or less acid dosing

than required. Effect of

this as already been

mentioned earlier.

O ring Probing with ¼ ‘ plastic

tube and by measuring

how far it has been

inserted.

Failure can lead to

increase salt passage,

increase permeate flow.

Decrease pressure drop.

Brine valve Should not be closed fully. If fully closed, 100%

recovery will result and

cause membrane damage

due to precipitation of

inorganic salt.

For Equipment trouble shooting – See manual on Equipment trouble shooting

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