application of the floto filter unit for contact flocculation filtration of surface waters

8/11/2019 Application of the Floto Filter Unit for Contact Flocculation Filtration of Surface Waters

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APPLICATION OF THE FLOTO FILTER UNIT

FOR CONTACT FLOCCULATION FILTRATIONOF SURFACE WATERS

by

D.R.Induka B.Werellagama

A thesis submitted in partial fulfillment of the requirement for the degree of Master of Engineering.

Examination Committee: Dr C.Visvanathan (Chairman)

Dr S.Fujii

Mrs Samorn Muttamara

D.R.Induka B.Werellagama

Nationality : Sri Lankan

Previous Degree : BSc.(Eng) Hons.

University of Peradeniya, Sri Lanka

Scholarship Donor : Swedish International Development Agency (SIDA)

Asian Institute of Technology

Bangkok, Thailand

April 1993


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ACKNOWLEDGEMENT

The author wishes to express his sincere appreciation and gratitude to his advisorDr.C.Visvanathan for all the advice and guidance given throughout the thesis study period. Sincere

thanks are offered to Mrs. Samorn Muttamara and Dr. S. Fujii for sharing their interest in this study

and serving in the examination committee. Special thanks are also due to Professor R.Ben Aim and

Dr. S. Vigneswaran for their valuable suggestions and help during this study.

The Calgon Corporation of U.S.A. which provided the Catfloc T2 used in the experiments,

Prof. R. Ben Aim and HMC Polymers Ltd of Bangkok who provided synthetic media and the US

Environmental Protection Agency which provided technical information are gratefully

acknowledged.

Thanks are also extended to Udeni for helping during endless sieving sessions to isolate the

correct size of media, Deepa and Thayalan for helping by taking readings when the filter runs

extended beyond 40 hours, Mr. Uttam Manandhar, Jayaweera, Gemunu, DN, Jeyaranie and all other

friends for helping in one way or another, Mr. Varine of the Ambient Laboratory who built the set up

and helped with the repairs whenever necessary, and to the laboratory staff of the Environmental

Engineering Division for their support.

The author wishes to express his gratitude to the people of Sweden and The Swedish

International Development Agency (SIDA) for providing him the scholarship for study at the AsianInstitute of Technology.

Finally the author gratefully dedicates this work to his beloved parents for their affection,

concern and encouragement throughout his career.


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ABSTRACT

Laboratory scale experiments were carried out in contact flocculation filtration using a dualmedia filter. The objective of the research was to find an optimum synthetic media combination

which would give acceptable quality water under varying conditions while maintaining a low

headloss. The media was lighter than water hence the bed was floating. Since coarse media remained

at the bottom the flow direction was upflow which had the added advantage that the flow was in the

direction of grain compression. Polypropylene and Polystyrene were selected as the optimum

combination of media which due to their large density difference did not intermix even under severe

agitation. Spherical fine media performed better than angular fine media giving lower headloss and

better effluent quality. The dual media combination and the higher rates of filtration are in line with

the current trends in the water industry. The influent concentrations were kept constant and the flow

velocity and filter media size were varied. The headloss variation along the filter, the influent quality

to the two filter layers and the effluent quality were studied.

The filter performed for over 40 hours producing acceptable quality water at conventional

rapid sand filtration rates, also having low headloss development. Another major advantage was the

ease and economy in backwashing. The filter media did not mix even during or after backwashing,

thereby eliminating the most common problem encountered in conventional multi media filters.

The Floto Filter operation was compared with upflow sand filtration. The headloss

development curves for Floto filter showed a characteristic shape easily identifiable from that for

sand.

The existing mathematical model was able to predict the headloss profile but the

concentration profile was not predicted. The necessity of particle size data for mathematical

modelling of filtration of heterodisperse suspensions is emphasized.


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TABLE OF CONTENTS

CHAPTER PAGE

Title Page i

Acknowledgement ii

Abstract iiiTable of Contents iv

Abbreviations v

1 INTRODUCTION 1

2 LITERATURE REVIEW 4

2.1 Introduction 4

2.2 Theory of Filtration 4

2.3 Contact Flocculation - Filtration 6

2.4 Headloss Development 8

2.5 Contact Times for Polymers 12

2.6 Rate Control Patterns and Methods 12

2.7 Some Parameters Affecting Turbidity 132.8 Filter Backwashing 14

2.9 Floating Bed Filters 16

2.10 Mathematical Models for Deep Granular Filters 26

3 EXPERIMENTAL INVESTIGATION 36

3.1 Experimental Set Up 36

3.2 Experimental Runs 41

3.3 Materials 43

3.4 Measurements 48

4 PRESENTATION AND CRITICAL DISCUSSION OF RESULTS 51

4.1 Introduction 51

4.2 Experiments with Floating Media 51

4.3 Effect of Physical Parameters on Filter Runs 69

4.4 Polymer Dosage and Mixing Time of Polymers 74

4.5 Effects of Controls and Processes 75

4.6 Experiments with Sand Medium 79

4.7 Comparison of Experimental Runs and Concluding Remarks 82

4.8 Mathematical Modelling 88

5 CONCLUSIONS 95

6 RECOMMENDATIONS FOR FURTHER STUDY 98

REFERENCES 102

APPENDIX 1 106

APPENDIX 2 138

APPENDIX 3 140


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ABBREVIATIONS

Ac - area of collectors in a unit volume of filter (m2)

Ap - area of retained particles in a unit volume of filter (m2)

Ci - concentration in ith time step (mg/l)

Co - influent concentration (mg/l)

dc - diameter of collector (m)

dp - diameter of particles (m)

f - porosity of bed at time t

fo - porosity of clean bed

g - acceleration due to gravity (m/s2)

GAC - Granular Activated Carbon

hf - headloss through a clogged bed (m)

Ho - initial headloss (m)

J - head gradient

JTU - Jackson Turbidity Unit

K - Kozeny's constant

Kw - Kuwabara's constant

L - filter depth (m)

L - filter depth increment (m)


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N - number of particles directly attached on the filter grain

Np - number of particle collectors in unit volume (m-3

)

n - particle concentration at a given time and depth (m

-3

)

ni - particle concentration at i thtime step (m-3

)

no - influent particle concentration (m-3

)

NTU - Nephelometric Turbidity Units

OTV - Omnium de Traitments et de Valorization of France

PAC - Powdered Activated Carbon

PACEFILT - PAC Embedding Filtration

Pe - Peclet Number

PP - Polypropylene

PS - Polystyrene

REFIFLOC - Refiltration Flocculation Process

RSF - Rapid Sand Filter

S1 - shape factor of suspended particle

S2 - shape factor of filter grain

SSF - Slow Sand Filter

t - filtration time (s)

US EPA - United States Environmental Protection Agency

V - filtration velocity (m/s)

Vp - volume of particles deposited (m3)


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- single collector efficiency of a clean collector

c - combined removal efficiency of a single collector

D - single collector contact efficiency by diffusion

I - single collector contact efficiency by interception

Lo - single collector contact efficiency by London Van der Waalsforce

p - contact efficiency of a retained particle

r - single collector removal efficiency of a filter grain and itsassociated retained particles

s - single collector contact efficiency by sedimentation

- particle to filter grain attachment coefficient

p - particle to particle attachment coefficient

- fraction of retained particles acting as particle collectors

' - the fraction of retained particles that contribute to theadditional surface area

1 - fraction of filter grain surface, which is exposed for theparticle deposition

2 - detachment coefficient

d - porosity of deposit

- density of water (kg/m3)

p - density of particles in suspension (kg/m3

)

- viscosity of suspension (Ns/m2)

- specific deposit


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- fraction of coarse particles which can act as particlecollectors to remove finer particles in the suspension


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CHAPTER 1

INTRODUCTION

1.1 General

There is an increasing need to treat low quality surface water to produce drinking quality

water complying with consent conditions. Water percolating through a bed of granular media

(filtration) is widely used in municipal water treatment for clarifying dilute suspensions with particles

of a wide range of sizes.

In surface water filtration, slow sand filters(SSF) and rapid sand filters(RSF) are widely used

for removal of solids present in surface waters, precipitated hardness from lime softened water and

precipitated iron and manganese. Both these types (SSF and RSF) are deep granular filters and often

the filter media is graded silica sand. Another type of filter is precoat filters, which use diatomaceous

earth, perlite, powdered activated carbon etc; as the filter media.

Rapid sand filters are more popular for municipal applications due to their lower space

requirement, higher production capacity, and higher flexibility of treating waters of different

turbidities. Lower space requirement means lower capital cost to achieve the water of desired

quality. The flow rate in a conventional rapid filter is in the range of 2.4 - 10 m3/m

2.h. The rapid

filters conventionally treat water that are passed through several pretreatment steps like screening,

primary sedimentation, rapid mixing, coagulation and flocculation and secondary sedimentation.

After some period of operation the filter media gets clogged and the production rate declines.

When this happens the filter has to be taken out of operation and its flow direction is reversed in

order to remove the clogging particles. This process is called backwashing. As the production

capacity of a rapid sand filter is increased, the clogging rate also increases, resulting in more frequent

backwashing. In addition to the loss of production time, product water has to be utilized for the

backwashing operation. Rapid gravity sand filtration would typically consume 2 - 5 % of throughput

for backwashing. In view of saving this water as well as saving operator time required for frequent

backwashing operation (which entails valve operation), researchers in the past three decades havefocused their attention on a wide range of process modification such as upflow/ biflow filtration and

mobile bed filtration for enhancing the filter performance.

One notable research work on modifying the filter media itself was the application of the dual

or multi media filtration which utilized various types of sand, crushed anthracite coal, diatomaceous

earth, perlite and powdered or granular activated carbon etc; as the filter media. Even these modified

arrangements had the major drawback of frequent intermixing of filter media after backwashing in


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the conventional manner.

In this research work, the focus was on the modification of filter media. Here the classic sand

media was replaced by synthetic polypropylene and polystyrene beads, whose density is less than

that of water. Therefore these beads floated in water. During filtration the artificially made turbid

water was dosed with the flocculent just before entering the bed, effecting contact flocculation. As

the turbid water moved up the floating bed, suspended solids in the form of floc were captured within

the filter media. The purified water was collected at the top. As the media itself was always kept in

suspension, it facilitated easy backwashing. The energy required to agitate the floating bed to

resuspend floc was much less than for a conventional sand bed.

1.2 Objectives of the Study

The objectives of this research were

(1) Finding a combination of floating filter media able to produce an acceptable effluent under a wide

variety of conditions and also having a uniform floc and headloss distribution over the depth of bed.

(2) Determination of the effectiveness of contact flocculation filtration on deep bed dual media filters

operating in the upflow mode.

(3) Optimization of the filter performance for different filtration rates, different influent turbidities

and for different media types and sizes. The selected operating conditions reflected the current trend

in water treatment plants towards higher filtration rates and dual media. The analysis was by studying

the resulting filter effluent quality patterns and the head variation along the filter.

(4) Comparison of the Floto filter performance with an upflow sand filter and identifying the

advantages/ disadvantages of floating bed filtration over conventional filtration.

(5) Verifying an existing mathematical model using the pilot plant results.

1.3 Scope of the Study

The study was basically a series of experiments utilizing a dual media filter with floating

media. These were followed by analysis, interpretation and limited computer modelling.

The filter operation was studied for the constant rate flow mode under a constant head. The

experiments were carried out in upflow direction. The usage of floating filter media in dual

arrangement with coarse to fine arrangement was verified as the best option by initial hydraulic

studies.


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The final experimental results were headloss variations along the two media layers as the

filtration progressed more than 40 hours, headloss and filtrate quality relations with time, water

quality variation for two media layers, and the breakthrough behavior at different filtration velocities.

Since only a limited selection of floating media were available, the available media was

crushed, sieved and heat treated in order to prepare smaller sized media fractions.

O'MELIA and ALI's (1978) filtration model which had been modified by VIGNESWARAN

and BEN AIM (1985), VIGNESWARAN and CHANG (1986) and MANANDHAR (1990) was

used for simulation and verification of Floto - Filter results.

1.4 Limitations of the Study

The modelling was limited to O'MELIA and ALI's model only. i.e Attention was given only

to microscopic parameters of filtration.

Artificial suspension of Kaolin clay, the turbidity of which was kept constant was the

influent.

Only contact flocculation - filtration mode was studied which, by definition, allows very

small contact times for coagulation and flocculation.

Due to the limitations of the available dosing pumps very dilute stock solutions had to be

used for dosing of flocculents.

The inability to get smaller sized floating media necessitated preparation of them by methods

available at hand. Some of the resulting media therefore had a density variation which might have

had an effect on experimental results.

The absence of particle size data necessitated the use of simplifying assumptions during the

mathematical modelling.


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CHAPTER 2

LITERATURE REVIEW

2.1 Introduction

The design of a granular medium filter system involves the consideration and specification of

* The type of medium, size and depth

* Filtration Rate

* Pressure available and driving force

* Method of filter operation (including cleaning)

The deep granular filters presently in operation (particularly Rapid Sand Filters and Slow

Sand Filters) generally consist of 0.45-0.75 m of filter medium supported on an underdrain system.

The filter may be open to atmosphere (gravity filter) or enclosed completely in a pressure tank

(pressure filter). Filtered water collected in the underdrain is discharged to a reservoir or to the

distribution system. The underdrain system is also used to reverse the flow to backwash the filter.

Most of the results presented here in the literature review were derived from filtration

experiments utilizing graded silica sand as the principal media. Some researchers have utilized other

media like garnet sand, anthracite coal, perlite, and diatomaceous earth. All these media are of higher

density than water. Experiments utilizing filter media of density less than water (e.g. polystyrene,

polyethylene, polypropylene, filter ag and paraffin) are described in section 2.9.

2.2 Theory of Filtration

2.2.1 Coagulation and Flocculation

Coagulation is the destabilization and initial aggregation of colloidal and finely divided

suspended matter by the addition of a floc forming chemical or by biological processes. Coagulation

involves the charge neutralization and destabilization of colloids and formation of microflocs.

Flocculation is the agglomeration of colloidal and finely divided suspended matter after

coagulation by gently stirring using either mechanical or hydraulic means. During flocculation the


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microfloc is agglomerated into macrofloc.

2.2.2 Principal Mechanisms of Filtration

According to the filtration model formulated by O'MELIA and STUMM (1967), the

dominant mechanisms of removal of suspended solids in a filter depend on the physical and chemical

characteristics of the suspension and the medium, the rate of filtration and the chemical

characteristics of the water.

In deep granular filters of coarse material, the removal is primarily within the filter bed

(Depth Filtration). The removal efficiency depends on a number of mechanisms. Some solids are

removed by the simple mechanical process of interstitial straining. Removal of other solids

(particularly micro particles) depend on two types of mechanisms.

(i). A transport mechanism brings the micro particle from the bulk of the fluid within the

interstices, close to the surfaces of the media. Transport mechanisms for depth filtration may include

gravitational settling, diffusion, interception and hydrodynamic forces. These are affected by

physical characteristics such as size of the filter medium, the size of the suspended particles and the

ratio of suspended particle size to media size, filtration rate, fluid temperature (viscosity) and the

density .

ii). As the particle approaches the surface of the medium, or previously deposited solids on

the filter medium, an attachment mechanism is required to retain the particle. The attachment

mechanism may involve :

* Electrostatic interactions

* Chemical bridging or

* Specific adsorption

The efficiency of the filtration process for a given set of hydraulic conditions depend on the

attachment forces. The pores get clogged due to accumulation of material as the run progresses. Asthe approach velocity is kept constant in the high rate filtration, the hydraulic gradients increase due

to this accumulation of material. Increase in hydraulic gradient increases the shear forces. The

filtration efficiency is effectively set by the relationship between the attachment and shear forces.

Breakthrough occurs when the hydrodynamic shear forces become greater than attachment forces. A

flocculent causing strong attachment forces in accumulating material will prolong the filtration cycle

but also cause high head loss. To decrease the head loss the diameter of the grains has to be

increased. The other solution of decreasing the flow is not practical (ADIN & REBHUN, 1974).


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2.2.3 Water Pretreatment

Water can be pretreated to improve the performance of a filter. The pretreatment may:

1) Decrease the filtration resistance of the suspended solids

2) Increase the ability of the filter to remove and retain suspended solids that are too

small to be removed solely by straining.

When the suspended solids concentration of a water is not high, small doses of coagulants

(alum, ferric chloride, polyelectrolytes) addition before filtration helps to increase the permeability of

the solids that deposit in the filter. Generally this pretreatment reduces the resistance by no more than

50 %. This type of pretreatment changes the filterability of the solids. It does not decrease the total

amount of solids delivered to the filter. In fact, the total amount of solids collected on the filter

increases. Such pretreatment can reduce the breakthrough tendency of the solids by improving the

ability of the filter to retain them.

ADIN & REBHUN (1974) found out that the alum floc are too weak to withstand high shear

forces while the polyelectrolytes formed strong floc.

2.3 Contact Flocculation - Filtration

2.3.1 Introduction

Sometimes when the turbidity of the raw water is low some of the pretreatment steps of the

conventional filtration process can be taken off. In direct filtration the water is applied directly to the

filter after only screening, coagulant addition, rapid mixing and flocculation. Contact flocculation

filtration is a further development of direct filtration. Only pretreatment is chemical (coagulant &

flocculent) addition (clarification steps by flocculation and sedimentation are omitted). To combine

flocculation and coagulation in a single rapid process, a porous bed is required. The flocculation

occurs during the contact of raw water and flocculent within the filter media and the whole solids

separation process occurs in the filter bed. The process of Contact Flocculation Filtration differs from

the volume flocculation due to the high rate of the flocculation.

Raw Water Flocculent Filter Bed


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Filtered Water

FIG 2.1 Contact Flocculation Filtration

According to ADIN and REBHUN (1974) who experimented using both hydrolytic and

polyelectrolytic flocculents, the removal mechanism of the contact flocculation filtration process has

three stages. A working in stage, a working stage and a breakthrough stage . The working in stage is

characterized by a rapid decrease in effluent turbidity with time, reaching a low, stable value. The

working stage is the effective (main) stage of filtration giving satisfactory effluent quality. If the run

is not terminated due to head loss, the breakthrough stage can be identified by the deterioration of theeffluent quality. The functioning of the bed can be described by the frontal advancement of the

working layer in which effective filtration is taking place.

ADIN and REBHUN (1974) found experimentally that contact flocculation filtration with

alum alone was not efficient at high rates with coarse media. Upto 0.62 mm grain size, alum alone

gave efficient filtration at filtration rate of 5 - 10 m3/m

2.h. The cationic polyelectrolytes made higher

filtration rates (20 m3/m

2.h) possible. They noted that for higher output per cycle, filtration with

polymer must be done through coarse media.

In contact flocculation filtration, flocculation occurs within the filter bed. If alum is used as

the flocculent it will contribute to a large fraction of the sludge produced due to the aluminum

hydroxide precipitate. (raw water containing 10 mg/L of suspended solids may require an alum dose

of about 25 mg/L ). When polyelectrolytes are used as the sole flocculents, they are applied in

smaller doses (usually in the 1 mg/L range), and the sludge produced is composed almost entirely of

solids which originated in the raw water (SHEA et al. 1971). Therefore, polyelectrolytes are more

suitable flocculents for contact - flocculation and are added just ahead of the filter.

Since the entire solids removal takes place within the filter itself, waters with low turbidity

range are more suited for contact flocculation filtration. This is because in the conventional fine tocoarse arrangement the majority of the particles are removed at the top layer of the filter bed,

resulting in rapid clogging. SHEA et al.(1971) report that among various types of media used for

Contact Flocculation Filtration, the best results were given by coarse and uniform dual media, used

in coarse to fine media arrangement.

2.3.2 Advantages and Disadvantages


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The major advantage in contact flocculation filtration is that it eliminates the sedimentation

and flocculation processes required in conventional filtration. This results in large operational and

capital savings. Another advantage is that the sludge is produced only in the filter backwash process,

hence causing less handling problems. The relatively high cost of polyelectrolytes is offset due to the

low volumes needed, resulting in a net saving in the chemical cost. Operational costs are also

reduced due to lower handling and storage requirements.

The disadvantages are the shorter filter runs resulting from the entire solids removal being

within the filter itself. The shorter filter run would increase the frequency and the degree of

backwashing [VIGNESWARAN et al,1983].

2.4 Head Loss Development

2.4.1 In Contact Flocculation Filtration

ADIN & REBHUN (1974) report that when alum was used as the flocculent, the head loss

increased linearly for a greater part of the cycle. Toward the end of the cycle a nonlinear headloss has

been observed. With the polymer the headloss developed exponentially. Hence for the same

hydraulic and operating conditions the increase in the rate of head loss buildup with polymer was

higher than with alum. They therefore state that the smaller the attachment forces and deeper the

penetration, the slower the development of the head loss.

Head Losses developed at a lower rate when the grain diameter was increased or the flow

velocity was decreased. Grain size strongly affected the headloss, while increasing the filtration rate

(for a given bed) did not. The main effect of flow rate for a specific grain size was related to the

initial head loss only.

SHEA et al. (1971) had observed that the initial headloss was not related linearly to the flow

rate. They report that for the same filter, the initial headloss was 2.3 cm for a flow rate of 7.3 m/h and

14 cm for a flow rate of 22.0 m/h.

2.4.2 Headloss development in upflow filtration

As reported by HAMANN & McKINNY (1968), IVES (1967) found that upflow filter runs

carried to give the same head loss were longer than downflow filter runs. Therefore for identical

lengths of run, head loss through the upflow filter was appreciably lower than through the downflow

filter. The filter used in this case had a sand bed of 1.2 m and the sand was graded from 0.6 mm at

the top to 1.2 mm at the bottom. HAMANN & McKINNY (1968) also report about the work of

MINZ (1962) in Russia. His upflow filters had contained gravel and sand to a depth of 2.3 to 2.6 m.


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Increase in head loss during filtration had been slow. He had attributed this to the removal of a

substantial amount of suspended matter in the coarse portion of the filter where it has less influence

on head loss.

HAMANN & McKINNY (1968) also carried out a series of upflow filtration experiments

for filter beds of depth 60 cm and 120 cm, using alum and polymer as coagulant and flocculent. The

deeper the bed, the initial head loss was higher but the head loss development was lower for the

deeper bed. After 3 to 4 hours the head loss in the 60 cm bed exceeded the head loss in the 120 cm

bed.

They also report that in all cases the filter runs were terminated either by bed lifting or by

fluidization of the finer sand. Both of these phenomena resulted in the escape of the solids

(previously accumulated in the filter bed) with the filtrate. They noted that bed lifting or

breakthrough may occur in an upflow filter when the weight of the bed above a given level becomes

equal to the head loss developed above that level.

ADIN and REBHUN (1974) reported that the main effect of flow rate for a specific grain

size is related only to the initial head loss. Experiments on upflow filtration by PERERA (1982) also

show headloss development similar to ADIN & REBHUN (1974).

DANIEL & GARTON (1969) studied various combinations of sand, coal, glass beads,

walnut shells and pelleted paraffin wax as media for upflow filtration. Their model upflow filter was

10 cm in diameter and 1.83 m high. They added varying amounts of coagulant aid to the waters and

noted that both high turbid and low turbid waters required approximately the same amounts of

coagulant aid. The flow rate through the model filters was 2.44 m/h. Six runs were reported for each

media. Each run was of 23 hours duration. The head values before and after backwash are given in

figure 2.2. This figure also gives the filtrate quality for a particular run.


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Fig 2.2 Effluent and Headloss values for different media

(DANIEL & GARTON 1969)

ODIRA (1985) studied the headloss development patterns in upflow filtration utilizing sand

media in six different filter bed configurations. The coagulant used had been alum. He reports that

The increase of headloss with time varied with each filter design at the same filtration rate and coagulant

dosage. The finer filter media sustained the highest increase in headloss with time while the

effluent quality was substantially the same for all the filter designs. For a particular filter design,

the rate of headloss development and the terminal headloss were also affected by the coagulant

dosage and the influent turbidity; with the low dosage - low filtration rate - low influent

turbidity combination exhibiting the lowest headloss patterns. The normal values for the

headloss at breakthrough also varied for the various filter designs.

Results of ODIRA (1985) show a linear variation of headloss with time. For filter ratesabove 10 m/h the headloss development had been very rapid resulting in very short filter runs.

2.4.3 Head Loss Development in Reverse Graded Filters

Reverse graded dual media and multimedia filters use coarser material on the raw water side

of the filter and finer material on the filtered water side. This accomplishes a much more uniform


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distribution of solids throughout the depth of the filter, with much of the suspended matter being

removed in the coarser material. The head loss in these filters is generated at a much lower rate than

that of a conventional filter. Filtration through reverse graded media provides a filter run that is 2 - 5

times greater than obtained with the conventional filter, other conditions being equal. (SHEA et al.

1971)

FIG 2.3 (WEBER, 1972)

The figure 2.3 shows the typical relationship of head loss to volume of flow for a reverse

graded filter.

Since the water is filtered through media of increasing fineness, this type of filter is less

subject to passage of solids due to filtration rate changes. Some breakthrough of solids occur, but not

to the same extent as with a conventional filter.

2.5 Contact Time for Polymers

According to TREWEEK (1979), who conducted direct filtration experiments utilizing 3.0mg/l alum and 0.25 mg/l Catfloc T for treatment of surface water from a reservoir, a flocculation

time shorter than 7 minutes was not sufficient to produce the floc for removal in the filter media. His

filter column was a 30 cm bed of sieved sand (E.S. = 0.5 mm and U.C. = 1.3) and the flow rate was

11.5 m/h. Flocculation times exceeding 7 minutes produced large visible floc but the effluent quality

did not improve further. ADIN & REBHUN (1974) state that for their experiments in contact

flocculation filtration, the total contact time of the flocculent and the suspension before reaching the

bed was few minutes. LO (1984) added polymer to the influent point of the filter in his experiments.


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The contact time in his case had been less than a minute.

The Calgon Product Bulletin 12-42b (1989) states that organic cationic polymers have a

relatively slow destabilization time (time required for adsorption, charge neutralization and initial

floc formation) as compared to inorganic flocculents like alum. It recommends the use of inorganic

coagulants (about 40% to 60% of the amount previously used) to speed up the total destabilization

time when short mixing times are encountered in practical applications.

It also states that in several water supply systems it was possible to feed the cationic

polymers to the raw water line far enough upstream from the plant to obtain several hours of

additional mixing time in the line. In such cases, as little as 0.5 ppm of the organic polymer has

improved clarification considerably and has eliminated the use of alum, activated silica and lime too.

When cationic polymer is used with alum, it is usually better to feed it into the raw water line ahead

of the rapid mix to obtain a maximum mixing time. Sometimes it is advantageous to premix the

polymer solution with the inorganic coagulant solution.

2.6 Rate Control Patterns and Methods

There are two basic methods of operating filters that differ primarily in the way pressure drop

(driving force) is applied across the filter. These methods are:

1) Constant rate filtration

2) Declining rate filtration

In constant rate filtration, the total operating head on the filter is fixed and the flow through

the filter is controlled at a constant rate by means of a flow control valve. As filtration proceeds, the

filter gets clogged with solids resulting in loss of head and declining flow rate. The flow control

valve is opened slowly to maintain a constant flow rate.

In declining rate filtration the incoming flow is supplied to a group of filters on a free flow

basis to meet their individual operating rates. There are no effluent controllers. The only control is

the effluent overflow level in the clear well.

In water treatment practice, the constant rate filtration is the most popular due to its proven

performance and higher operational control (KAWAMURA, 1991).

In operating pilot plants DANIEL & GARTON (1969) observed that preflocculated water

was difficult to control at a constant rate through the small rotameter flow meters. The unsettled floc

disturbed the rotameter floats.


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According to CLEASBY & BAUMANN (1962) and TUEPKER & BUESCHER (1969), if

the filtration rate on a filter which contains deposited solids is suddenly increased, the hydraulic

shearing forces also suddenly increase. This disturbs the equilibrium existing between the deposited

solids and the water, and some solids will be dislodged to pass out with the effluent. Depending on

the type of solids, and the magnitude of the suddenness of the rate change, the effect can be quite

drastic. All sources of sudden rate change should be avoided in the design of filters.

2.7 Some parameters affecting effluent turbidity

HAMANN & McKINNY (1968) report that the effluent turbidity increased exponentially

with the increasing flow rate. They also noted that the turbidity decreased as the depth of the bed

increased. Increasing the bed depth resulted in better stability of operation and less trouble with bed

fluidization.

In analyzing the results obtained for several types of media DANIEL & GARTON (1969)

suggest that the major influencing factor in finished water turbidity was proper coagulation of the

raw water just prior to filtering. Their turbidity results and corresponding head losses were given in

figure 2.2 in section 2.4.2.

2.8 Filter Backwashing

During filtration, as the water containing suspended matter percolates through the bed, the

material accumulates within the interstices of the granular medium. The head loss builds up beyond

the initial value. Also as the granular medium becomes filled with removed particles, the suspended

matter in the filter effluent starts to increase. When the head loss or the effluent turbidity reaches

some predetermined value, the filter must be cleaned. Ideally, the time required for the head loss

build up to reach the preselected terminal value should correspond to the time when the suspended

matter in the effluent reaches the preselected terminal value for acceptable quality.

Most granular filters are cleaned by reversing the flow through the filter bed. Filtered water is

pumped through the bed at a rate sufficient to expand the bed. The suspended matter arrested within

the filter are removed by the shear forces created by the backwash water as it moves through the bed.

The wash water is drained off in washwater troughs.


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The typical method for backwashing the granular media filters is air scour, water scour and

surface wash. Scour can be achieved by stepping up the velocity or rate of backwash per unit area

sufficiently (high velocity wash). As given by TCHOBANOGLOUS & SCHROEDER (1985) the

typical backwash rates for single medium, dual media and multimedia granular filters are 30 m/h, 48

m/h and 48 m/h respectively.

By directing jets of water into the fluidized bed surface scour is achieved. Surface wash

should start 1 - 2 minutes before backwashing begins. The surface wash is continued while the filter

is being backwashed, until the backwash water begins to clear. Scour of the bed can also be

intensified by stirring the fluidized bed mechanically.

Air scour serves to break up accumulated deposits. Air is blown upward through the bed

before or after fluidization. CLEASBY & BAUMANN (1977) state that the best backwash is

achieved with simultaneous air scour and water wash (as compared to air followed by water

backwash or to surface & subsurface wash). AMIRTHARAJAH (1978) has concluded that

backwashing with water alone is an inherently weak cleaning process due to the limitations in

particle collisions. Air scour and surface wash that promote interparticle abrasions during backwash

are indispensable for effective cleaning.

Dirty filters are commonly backwashed with filtered water. If the filters in water filtration

plants are not cleaned properly, fine material will accumulate in the form of mud balls. Mud balls

should be broken up and washed out or problems will quickly develop. A high pressure jet stream

directed into the expanded bed throughout the wash is required for this.

AMIRTHARAJAH (1988) states that for ordinary solids (with low adhesive forces), wash

water alone, which expands the bed by 30% to 40% to give expanded porosities around 0.65 - 0.70

in the top layers of the backwashed filter, provides optimum cleaning. Simultaneous air scour (54 -

90 m/h) and subfluidization water wash (14.7 - 19.6 m/h) provide the best cleaning for solids with

higher adhesive forces (polyelectrolytes and washwater solids). He also observes that dual media and

multimedia filters have the danger of loss of media with air and surface water wash and that they

need a fluidization wash at the end of backwash cycle to restratify the media.

ADDICKS (1991) reports that the scouring action of the simultaneous backwash is superior

to the water fluidization wash only in the transition zone from the packed bed to the fully three phase

(air,water & filter medium) fluidized bed. He notes that most of the particle abrasion is completed in

the first 1-2 minutes and thereby suggests that a backwash cycle of:

(1) Simultaneous air-water


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(2) Water fluidization

(3) Repeating 1 & 2

will achieve a quicker and better result than prolonged wash cycles.

HAMANN & McKINNY (1968) report that nearly all the early upflow filters made a

"mistake" by sending the backwash water in the reverse (i.e. downward) flow direction. This had

been ineffective as it did not expand the media as occurs in washing the filter by upward flow, hence

suspended matter that had penetrated deep into the media was not completely removed. They report

that the Russian upflow filters of MINZ (1962) were washed with a cocurrent flow of approximately

30 m/h. (normal operating flow rate was kept below 6 m/h to prevent sand expansion).

QUAYE (1991) had used optimal upflow wash rates of 65, 54 and 43 m/h for summer, spring

and autumn, and winter respectively to backwash his dual media filter bed with water only.

SMET and GALVIS (1989) who conducted upflow roughing filtration experiments suggest

shock loading the filter as an efficient method of backwashing, when the backwash is done

downflow. They surged the filter by quickly opening the valves in the underdrain system, keeping it

open for one minute and then rapidly closing and reopening the valves.

2.9 Floating Bed Filters

2.9.1 The Biostyr

The Biostyr or the Upflow Floating AeratedBiofilter incorporates the features of the classical

biological aerated filter with the requirements of the upflow filtration. The filter bed consists of

submerged and floating granular medium (polystyrene).

1 Raw water inlet channel 7 Process air

2 Filter feed and sludge discharge 8 Aerated filtering zone

3 Wash water valve 9 Media retention with nozzles4 Filter media 10 Treated water storage & discharge

5 Backwash air 11 Recirculating pump

6 Non aerated zone


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Fig 2.4 The Biostyr (OTV, 1991)

The Biostyr is a trademarked wastewater treatment process marketed by the Omnium de

Traitments et de Valorisation (OTV) of France. Water circulates up through the floating media. The

media is not fluidized, but filtration is carried out in the direction of grain compression.

Polystyrene, the filter medium, is lightweight, and is easy to backwash. The size and the

density of the filter grains is controlled to suit the required treatment objective. The fine and regular

medium gives a large specific surface and efficient filtration.

Retaining the filter media at the top of the unit reverses the classical gravity filter system.

Upflow filtration enables feeding of the influent without obstructing the distribution devices. Flow in

the direction of grain compression favors the retention of the suspended solids. Backwash is

facilitated through a simple gravity flush. The lightweight beads facilitate the backwashing through

counter current flushing, rinsing most intensely the filtration zone in contact with the heavily loaded

influent. Sludge can be removed in the direction of gravity by the shortest way (ROGOLLA et al,

1992).

The backwash water is stored on top of the filter, therefore the Biostyr does not need a

backwash pump or a separate clean water reservoir. The backwash water flow rate is about 50 m/h.

2.9.2 Upflow Filter using Filter-ag medium.

RICE et. al.(1980) report about an upflow filter using Filter-ag as the filter medium. Filter-ag

is a commercially manufactured non hydrous aluminum silicate. The properties of this material are

given in table no 2.1.

Table 2.1 Physical properties of Filter-ag (RICE et al, 1980)

Property Description

Color Light grey to off white

Density 385-417 kg/m3

Effective Size 0.57 mm

Uniformity Coefficient 1.66


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Fig 2.5 Configuration of filter using filter-ag media.

Note: The dimensions are in cm.

They had used two filter units as shown in figure 2.5. The size of the larger unit was

0.914 m in diameter and 2.134 meters in height. The height of the bed was 0.152 m. The flow

rate in this unit was 5.5 m/h. For raw water turbidity of 48 NTU it had produced effluent of 2.0

NTU to 1.5 NTU. When the raw water turbidity was 13 NTU the effluent turbidity had been 1.4

NTU.


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The filter media, located at about the midheight of the tank, was held in place with 50 mesh

(i.e. 0.3 mm) stainless steel screens, above and below the material. The distance from the water entry

pipe to the filter media was 92 cm. A correct alum concentration was added and was needed for the

proper and efficient operation of the units.

At the design flow rate it took about 22 minutes travel time from the water inlet to the filter

media. Jar tests had shown, for low & moderate turbidity water with correct alum addition, this was

sufficient time for the floc to settle out. Downflow backflushing removed the collected sediment out

of the bottom of the unit. The space below the filter media thus served as a settling basin as well as

the mixing basin for coagulation, flocculation & disinfection (with chlorine).

The smaller unit was of 0.3m * 0.3m cross section and 1.52 m tall. Distance from the inlet to

the filter media was 1.07 m. Bed was similar to the large unit. The flow rate was 2.5 m/h. This unit

was not effective at higher flow rates. (5 to 15 m/h).

2.9.3 Filter Using Pelleted Paraffin Wax Media.

DANIEL & GARTON (1969) experimented with various types of media one of which was

pelleted paraffin wax. The results of this experiment are given in section 2.4.2 and figure 2.2. As this

material has a specific gravity less than 1.0, they required a screen both above and below the filter

media. After 25 hours of operation the paraffin media had given turbidity less than 5 ppm.

2.9.4 The Haberer Process

Development of the Haberer process began as a search for an improved upflow filter design

in which backwashing of the filter would be accomplished in a downward direction (downwash)

rather than upward, as practiced in other upflow filter designs. Downwashing allows downward

movement, with the force of gravity, of the dense floc formed in the upflow filter and therefore

ensures rapid removal of solids from the filter bed. Conventional backwashing in an upward

direction must remove the solids from the filter bed against gravity and therefore requires a

considerable amount of time and water to clean the filter. Whereas conventional backwashing of a

filter may take upto 8 minutes, downwashing cleans the Haberer filter to the same degree in about 2minutes.

This filter used 1-3 mm foamed polystyrene beads (Styrofoam) as the filter medium. Since

the specific gravity of the medium was less than 0.1 the filter was fitted with a constraint above the

medium rather than with a conventional filter underdrain. A schematic diagram of the filter is given

in Fig 2.6.


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Fig 2.6 Haberer Process (STUKENBURG & HESBY, 1991)

According to STUKENBURG and HESBY(1991), the filter medium can be of any depth, but

a depth of 1.2m is often used. Typical filtration rate is 10 m/h. Downwash rates for the filter vary

with application. Expansion of the medium occurs at 100 m/h.

This filter is also used as a means to contact water with powdered activated carbon(PAC).

Many types of PAC will adsorb on the polystyrene medium so that, in effect, a carbon column can be

formed with PAC. Variations of this process have been patented as REFIFLOC( Refiltration

Flocculation) and PACEFILT( Powdered Activated Carbon Embedding Filtration). With the Haberer

filter, PAC can be used as efficiently as granular activated carbon(GAC) and can be used

intermittently if desired. A 36,000 m3/day plant in Wiesbaden, Germany employs the Haberer PAC

contactor as a second stage process to remove iron and manganese, to nitrify ammonia, and to

remove organic pollutants in water.

STUKENBERG and HESBY (1991) carried out a series of experiments using a HabererPAC contactor to treat alum coagulated water. They suggest that it is ideal for package plants, for

which the goal is to provide the maximum water treatment capacity possible in a given volume

and otherwise concluded that it has no apparent economic advantage over conventional

treatment. In their study polymer was added as a filter aid only for the conventional dual media

filter and not for the floating filter. They repeated their study for alum coagulated water (45 to 54

mg/L) without using PAC in the column. Even then the Haberer column gave good turbidity removal

comparable with the conventional dual media filter, until the breakthrough occurred after seven


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hours. They also concluded that the presence of carbon had little effect on the rate of head loss build

up in the column.

In their trial run, the chemical floc produced by the alum contact, accumulated in the free

space below the polystyrene medium. The volume of this space was approximately 30% of the

volume occupied by the medium.

HABERER and SCHMIDT (1991), point out that resin beads made of foamed polystyrene

are better suited for an upflow filter than either polyethylene or polypropylene because of their lower

density and substantially greater buoyancy in water. The polystyrene is inert and poses no health

hazard. The beads should be as homogeneous as possible, and the optimal size depends on the

application. The compact filter bed can be changed to a fluidized bed without additional expenditure

of energy by simply directing an intense rinsing stream downward through the bed.

In this upflow filter, the backwash water is stored above the nozzle plate on top of the

floating filter bed. To initiate backwashing, the inlet to the filter is closed and the discharge valve

opened. Thus, the high backwash velocities required to fluidize the bed are obtained without a

backwash water pump, and air rinsing is not used.

Fig 2.7 REFIFLOC process with polishing filter

(HABERER & SCHMIDT, 1991)

The upflow filter has a high capacity for the storage of captured solids and will act as a


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flocculator. The Refiltration Flocculation (REFIFLOC) process for wastewater treatment is based on

this principle. Here the effluent is recycled and refiltered, possibly several times. REFIFLOC process

can be used successfully even for the treatment of highly polluted waters, for which large dosages of

flocculents are needed to remove turbidity, algae and other contaminants. HABERER and

SCHMIDT (1991) recommend a REFIFLOC filter followed by a polishing multimedia filter for this

purpose. This process is illustrated in figure 2.7.

PACEFILT process (Powdered Activated Carbon Embedded Filtration) was developed based

on the REFIFLOC process in order to meet the requirements of the adsorption stages of water

treatment. Here as a special pretreatment step, a slurry of PAC is distributed over the entire filter bed

in a high velocity closed recycle stream by using the refiltration pump of the REFIFLOC unit. This

produces an adsorption layer of PAC on the polystyrene beads. PACEFILT combines the advantages

of the PAC, i.e., high reactivity, with those of the filter, i.e., the efficient utilization of adsorption

capacity. After it becomes exhausted, the carbon is removed by an intense backwash stream that is

directed downwards. This is similar to the floc removal in the REFIFLOC process.

Fig 2.8: The 3 Steps of the PACEFILT process


HABERER and SCHMIDT (1991), give the effect of backwash velocity on bed expansion

for polystyrene beads of two different densities. This is given in figure 2.9.


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Fig 2.9 Bed Expansion due to Backwash Velocity (HABERER & SCHMIDT, 1991)

According to their experiment they specify a backwash velocity of between 70 to 110 m/h.

This value is consistent with that given by STUKENBURG and HESBY.

In their upflow filter HABERER and SCHMIDT used foamed polystyrene beads of 1 - 2 mm

size. The filter bed height was 1.0 m - 1.5 m. They did not use coagulant aid. Their experiments on a

pilot filter with PAC coated polystyrene

media showed that the smaller beads (size 0.9 - 1.3 mm) gave consistently better organics removal

than the larger beads (size 1.6 - 2.5 mm).

TABLE 2.2 MAXIMUM OPERATING PRESSURE IN RELATION TO FOAMED

POLYSTYRENE BEAD DENSITY (HABERER & SCHMIDT, 1991)

Density(kg/m3) Permitted Pressure(kPa/cm

2)

15-25 0.8-1.8

60 5.5100 8.0

The effect of coagulant aid on iron content of a REFIFLOC test filter after coagulation with

30 g FeCl3/m3is shown in figure 2.10. In this case the bed depth was 1.35 m and the filter diameter

was 0.19 m. Size of the polystyrene beads was 1.5 - 2.2 mm. Filter velocity was 6 m/h and the

maximum filter run was 8 h.


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Fig 2.10 Effect of Coagulant Aid

HABERER and SCHMIDT (1991) have carried out experiments to combine REFIFLOC and

PACEFILT processes. Experience to date shows that cylindrical filters are preferred. The nozzle

plate at the top of the filter bed should be designed to withstand the strong buoyancy of the filter

material and to effect a minor head loss to result in equal distribution of backwash water. They

specify that the height of the cylindrical filter should be at least 1.5 times that of the filter bed. If

contact flocculation is used in the REFIFLOC process the filter inlet chamber should be designed to

allow sufficient hydraulic detention time for floc formation. In order to attain complete removal of

the solids during backwashing the lower part of the cylindrical filter should be conical. This

configuration also helped in distribution of PAC to the filter. They also say that due to the buoyancyof the filter medium, a sieve or screen to prevent its loss through the backwash water outlet is

unnecessary and would impair operation. HABERER and SCHMIDT (1991) also noted that in both

REFIFLOC and PACEFILT processes the smaller grains of the medium remained in the bottom

filtering layer. Therefore the grain composition in the lower reaches of the filter determined the

filtration effect and the required filter bed depth.


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If the removal efficiency of the filter is inadequate, the filtrate quality can be improved by

refiltration. The concentrations of iron manganese and ammonium in the filtrate of the Weisbaden

water utility were considerably reduced in this way. Also the startup period for the plant was

reduced. After maturation of the filter, the refiltration ratio was reduced to the minimum value of 1.6.

This minor but constant refiltration guarantees uniform filtrate quality despite major fluctuations of

flow through the plant. The filtration velocity is kept at a constant level. If the flocculation processes

do not operate optimally, refiltration at a ratio between 2 and 3 may help improve the filtrate quality.

(HABERER and SCHMIDT, 1991). The upper limit of the refiltration rate is determined by the

velocity at which the increased shearing forces begin to destroy the floc and, thus, cause

breakthrough.

Fig 2.11 Organics Removal vs FeCl3dosage


A pilot filter at Biebeshiem, Germany treated the water of the river Rhine to remove turbidity

and organics using a combined REFIFLOC and PACEFILT plant. In this plant the efficiency of

organics removal with various flocculent dosages was studied. Before the filter, the water waspretreated by ozonization (1g/m3) and FeCl3addition (7, 10, 24 g/m

3). The result is shown in figure

2.11.

In this case the filter material was foamed polystyrene with 1.5-2.0 mm diameter. The

optimal filtration velocity was 6 m/h. The filter run was between 6 to 8 hours. The filter bed was

loaded with 700 g Fe(OH)3/m2before the breakthrough occurred.


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HABERER and SCHMIDT (1991), report that about five filter bed volumes of backwash

water are typically required to remove the saturated powdered carbon once the bed was exhausted.

At backwashing velocities between 80 and 100 m/h, the corresponding backwash time was 3 - 5 min.

They also specified that backwash should effect 25 to 30 percent filter bed expansion.

2.9.5 Horizontal Filter Using Plastic Media

TANUMIHARJA (1981) used a bench scale horizontal filter to study the effectiveness of

coarse plastic media (25 mm diameter;and specific gravity 0.26) as a filter media. But this media was

prevented from floating by a 10 cm layer of crushed stone of effective size 20 - 25 mm.

This experiment showed that the coarse plastic media have suspended solids removal

efficiencies in the range of 30% to 60%. The flow rates utilized in this experimental study were from

0.5 - 1.5 m/h and the influent turbidity levels were 50 - 100 NTU.

2.10 Mathematical Models for Deep Granular Filters

To predict the behavior of filtration in deep granular filters, several researchers have

developed mathematical models. These can be categorized into two major groups, viz.

* Macroscopic models

* Microscopic models

The common macroscopic models (IVE's approach) relate filtrate quality and head loss with

time, incorporating measurable macroscopic variables of filtration such as filtration rate, grain size

and water viscosity.

Models based on microscopic parameters (O'MELIA's approach) consider microscopic

parameters such as individual particle and filter grain sizes, number of retained particles etc, in

addition to operating parameters of filtration. In this


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Table 2.3 Contact Efficiencies of Clean Bed Filter ( TULACHAN, 1987)

Model Used Contact Efficiencies for Different Mechanisms

STOKES LEVICH 1962 :D= Pe-2/3; Pe= Peclet number

YAO 1971 : I= 1.5 (dp/dc)2

s= (p-)gdp2/18v

=I+s+ 0.9 Pe-2/3

HAPPEL HAPPEL 1958: AS= Happel's model constant

I= 1.5 AS(dp/dc)2

COOKSON 1970:

D = 0.9 AS1/3 Pe

-2/3

SPIELMAN et al. 1973 :

(1) For small particles (dp< 1m)

= 2.498 As1/3(vdc/40)

-2/3{"/("+1)} S(")

(ii) For big particles (dp> 1m)

=I(4/3) { (9/5) NAd}1/3

NAd= Adhesion group = -4 Hcdc2/9Asdp

4

KUWABARA RAJAGOPALAN and TIEN 1976 :

= 0.72 AsNLo1/8(dp/dc)

15/8+

+ 2.4*10-3

AsNG6/5

(dp/dc)-2/5

+ (4/3) As1/3

Pe-2/3

NG= Sedimentation group

LEE and GIESEKE 1979 :

D= 3.54 { (1 -')/Kw }1/3Pe-2/3;' = 1 - f

I= 1.5 { (1 -')/Kw } (dp/dc)2/ [(dp/dc)+1]

2

HereD,s,I,Lo are the contact efficiencies of single collector by diffusion sedimentation, interception, London-

Vander Waals force respectively.


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research the model utilized had been developed using the microscopic approach. Hence it is

discussed in detail.

In 1978 O'MELIA and ALI presented their model to predict the development of headloss and

removal efficiency during filtration by packed beds. Many equations were developed during the

preceding two decades to calculate the clean bed filter efficiency from the fundamental forces

responsible for particle retention within the filter bed. These were given in the table 2.3 .

2.10.1 O'MELIA - ALI Model

O'Melia and Ali (1978) formulated their model based on the postulate that some retained

particles can act as filter media and thereby improve the filtration efficiency. Since retained particles

modify the performance of a filter as filtration proceeds, the equation governing the clean bed filter

becomes inapplicable. They considered the particle transport and attachment mechanisms within the

filter, analyzed it microscopically and proposed a set of equations (Equations 2.1 to 2.5 and 2.8) to

predict the variation of filter efficiency and head loss during filtration. They defined the single

collector efficiencyas

Rate at which particles strike the collector

= --------------------------------------------------------------- (2.1)

Rate at which particles approach the collector

The actual collector consists of a filter grain and a number of particles attached to it which

also act as collectors. The combined removal efficiencycof a single collector is given by

(Rate at which particles + N * (Rate at which particles strike

a retained particle

strike the filter grain) acting as a collector)

c= ------------------------------------------------------------------------------------------------------- (2.2)Rate at which particles approach the collector

nv (/4) dc2 is rate at which particles approach the collector where n is the bulk particle

concentration and v is the fluid velocity.


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Equation 2.2 can be rearranged as

c=+ Np(dp/dc)2 (2.3)

wherepis the contact efficiency of retained particles which is defined as the rate at which

particles strike a retained particle acting as a collector divided by nv(/4) dp2, where dp is the

diameter of the suspended particles.

O'MELIA and ALI considered the removal in the packed bed is dependent upon the transport

and attachment. Transport is defined by,randp. They introduced two empirical coefficients

andpto define attachment and rewrote the equation 2.3 as

r= + Npp(dp/ dc)2 (2.4)

where, dc = diameter of the filter grain

dp = diameter of particles in the suspension

N = number of particle collectors attached to a filter grain

= particle - filter grain attachment coefficient

p = particle - particle attachment coefficient

= contact efficiency of a single collectorc = combined removal efficiency of a single collector

p = contact efficiency of a retained particle

r = single collector removal efficiency of a filter grain and its

associated retained particles.

The number of retained particles that can act as collectors, N, at a depth L and at time t can

be calculated using the equation 2.5.

where n = local concentration (number of particles per unit volume) = fraction of retained particles which act as particle

collectors

Considering the mass balance of suspended particles,

ACCUMULATION = INPUT - OUTPUT - REMOVAL (2.6)

(2.5)


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whereL = filter depth

t = filtration time

f = porosity of the filter

bed

Simplifying

Equations 2.4, 2.5, and 2.8 comprise the mathematical model to

describe the removal efficiency of a filter bed in time and space.

With time taken discontinuously n/t = 0

O'MELIA assumedr to be constant over some depthL and gave the solution of the

above equation 2.8 for t>L/v for ithtime step as

where nio is the particle concentration entering the media

layer at ithtime step.

For the case of clean bed filter it is given as

Again similarly for ithstep equation 2.5 can be given as

Substituting equations 2.9 and 2.11 in 2.4 we can get

Substituting equation

2.12 in 2.9 we can get the

fraction of remaining particles

at any time t, provided that the values ofandpare known.

(2.7)

(2.8)

(2.9)

(2.10)

(2.11)

(2.12)


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The values ofcan be calculated theoretically from the equations given in table 2.3 or by

using equation 2.10.

This model assumed that the porosity of a filter bed remains constant throughout the filter

ripening stage.

VIGNESWARAN and CHANG (1986) developed a mathematical model based on

O'MELIA and ALI (1978) and considering the particle detachment coefficient2as suggested by

ADIN and REBHUN (1977). They assumed the decrease of filter efficiency during the post ripening

period to be caused by the hydraulic gradient and number of particles already retained on the filter

grain. The final model obtained by them is given as equation 2.13 where detachment is represented

by the last term.

t

ri= [1 +ppV (/4) dp2 ni-1 t exp{(-3/2)(1-f)ri-1(L/dc)}]

i=1

t

- (2/n) Ji-1 rni-1 (2.13)

i=1

where Ji-1= hydraulic gradient at (i-1)thtime step

2.10.2 Headloss Models

KOZENY defined the headloss caused by the clean filter bed in equation 2.14.

where,

(hf/L)o= hydraulic gradient of clean filter bed

f0 = porosity of clean bed filter

k = Kozeny's constant

= density of fluid

= dynamic viscosity

so = specific surface of clean bed

v0 = superficial velocity

(2.14)


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According to CARMAN, k ranges between 4 to 6 for a variety of liquids.

But as the filter run progresses specific surface and porosity change due to deposits of

particles inside the pores. Hence the equation 2.14 is modified to 2.15.

where

hf/L = hydraulic gradient of deposited media

s = specific surface area of the deposited media

f = true porosity after deposition

= (f0-)

There are several models relating headloss, hf, and specific deposit,, as given in Table

2.4 .

Table 2.4 Summary of the Headloss Models (TULACHAN 1987)

Researcher Model

Shekhtman, 1955 H = H0 (1 +/f0)-3

Mackrle, 1961 H = H0(1 + a/f0)

3

(1 -/f0)

-3

Camp, 1964 H = H0(1 +/(1 - f0)

4/3(1 -/f0)

-3

Sakthivadivel, 1969 H = H0(1 - f0+ )2(1 -/f0)

-3(1 - f0)

-3

Herzig, 1970 H = H0(1 + k)

Letterman, 1976 i = k log H/H0

O'MELIA gave the development of headloss based on KOZENY's equation for clean bed

filter as

where

= dynamic viscosity, kg/m.s or g/cm.s

= density of the fluid, kg/m3or g/cm3

k = empirical constant

v = undisturbed superficial velocity above the bed , m/s

(2.15)

(2.16)


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or cm/s

Ac= Surface area of the filter grain, m2or cm

2

Vc= volume of the filter, m3or cm

3

O'MELIA and ALI (1978) considered the change in the surface area as dendrites develop while

neglecting the porosity changes during the progress of the filter run and presented the equation 2.17.

where Vpand Apare volume and area of particle deposited.

The headloss equations presented by O'MELIA and ALI

(1978 ) can be written in the following form.

where

Np= number of retained particles in

the unit volume of bed

Nc = number of filter grains

(collectors) in unit volume

' = fraction of total number of retained particles that contribute to additional

surface area.

Since deposition is non uniform with depth, the bed is subdivided to several layers and hf/L

was calculated for each layer.

VIGNESWARAN and CHANG (1986) developed the headloss equation for clogged bed,

based on Kozeny's equation for clean bed. They give the decrease of porosity during filter run as

f = 1 - (/6) [ Ncdc3

+ (Npdp3

)/(1 -d) ] / { AL} (2.19)

Where Npis the number of deposited particles of size dp. The porosity of the deposit,dis

included in calculating the volume of deposited particles. Shape factors S1and S2of particles and

filter grains respectively have been introduced in the headloss equations.

(2.17)

(2.18)


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Equation 2.20 gives the

modified head loss model.

VIGNESWARAN and BEN

AIM (1985) had shown the influence of

particles of different sizes on the

removal efficiency of a particle of

another size. They showed that the ratio of coarse particles to finer particles influences the removal

of finer particles. A constantis defined by them which is the fraction of coarser particles which act

as particle collectors to remove fine particles in suspension.

After extensive investigations CHANG (1989) and MANANDHAR (1990) had developed

the computer programs to simulate the whole cycle of filtration based on the equations 2.13 and 2.20.

CHANG (1985) cites previous investigators in assigning a value of 8.5 for S1. MANANDHAR

(1990) has simulated the operation of graded bed filters and multi media filters, for varying influent

concentration, varying particle size distribution and for constant and declining rate filtration.

(2.20)


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CHAPTER 3

EXPERIMENTAL INVESTIGATION

3.1 Experimental Set Up

The schematic diagram of the experimental set up is shown in figure 3.1. The main

components of the setup are:

1) Raw water feeder system

2) Chemical dosing system

3) Filter column and the Filter bed

4) Backwash air/water feeder system

The relevant design details of the system are given in the sections 3.1.1 to 3.1.4.

3.1.1 Raw water feeder system

(a) Solution preparation tank

The artificial suspension of Kaoline clay was prepared in a 50 l tank (No. 1 in Fig. 3.1). Once

the required quality was ensured, the suspension was pumped from this tank to the raw water tank

using a small centrifugal pump of 16 l/min capacity.

(b) Raw Water Tank

The artificial suspension of raw water was stored in this tank (No. 3 in Fig. 3.1). The tank

capacity was 290 l which facilitated a 5 hour filter run without replenishment at the maximum flow

velocity used (15 m/h). Since most of the filter runs were much longer than this, the raw water tank

was periodically replenished from the solution preparation tank. The raw water tank had a stirrerarrangement which continuously stirred the suspension to prevent the suspended solids from settling.

c) Constant Head Tank

A centrifugal pump with 45 l/min capacity fed raw water from the raw water tank to the

constant head tank (No. 4 in Fig.3.1). The level was kept constant by an overflow arrangement which


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recycled the excess water from the overhead tank back to the raw water tank.

The constant water level in the overhead tank was 2.05 m above the datum level. (i.e 2.65 m

above the ground level or 2.15 m above the filter bottom)

3.1.2 Chemical Dosing system

The coagulant was Aluminum Sulphate (alum). 20 mg/l of alum was added direct to the stock

solution preparation tank.

The required dosage of Catfloc-T2 (Cationic polymer) was introduced close to the influent

end of the filter directly and continuously to achieve contact flocculation. The Catfloc dosage for 30

NTU influent turbidity was established as 0.5 mg/l in jar test experiments. This confirmed the result

obtained by LIANG (1982). The dosages for 60 and 90 NTU influent turbidity too were found using

Jar Test experiments.

Using a magnetic stirrer the flocculent was thoroughly mixed with water for few minutes,

and kept in a storage bottle of volume 2.7 l (No. 5 in Fig. 3.1) to be fed to the system through a

dosing pump of capacity 2 - 80 ml/min (No.6 in Fig. 3.1). The speed of the dosing pump was kept

constant (at 6.67 ml/min) and the concentration of the flocculent stock solution was varied to suit the

required dosage and the flow rate. The total contact time between the flocculent and suspension

before they reached the filter bed was 1.6 minutes for 15 m/h flow rate and 4.8 minutes for the 5 m/h

flow rate.

3.1.3 Filter Column

(a) Filter Column

The filter column was a 1 m high acrylic column of 6.4 cm internal diameter. The transparent

column allowed observation of the media as the filtration process was in progress. Initially there

were a total of 5 ports for head measurement with 3 of them in the top 60 cm where the floating bedwas. After the filter run #18 the column was modified with 5 new ports being introduced. After this

modification the column had 8 piezometer taps in the top 60 cm with 4 piezometer ports for each

media layer. These ports are placed at 7.5 cm intervals. The details of the filter column with ports for

sampling and headloss measurement are shown in figure 3.2. Of the 5 sampling ports only one was

utilized after run # 18.


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The filter column was fixed vertically with raw turbid water entering from the bottom, for the

upflow mode of operation.

(b) Filter Bed

The filter bed was a 0.6 m deep packed bed w granular media. The filter bed had a 30 cm fine

media layer over a 30 cm coarse media layer for experiments with floating media. For the

experiments with sand medium the bed consisted of a 60 cm height of uniform sand. The media sizes

are given in section 3.3.2.

(c) Sampling Arrangement

The sampling and piezometer ports were copper tubes of 6 mm diameter. The mouths of

these tubes were covered with # 50 mesh (i.e. 0.3mm mesh) to prevent loss of media. After the initial

experiments showed that continuous sampling along the filter bed disturbs it, only one sample was

taken from the center (at the port number 4 for the floating bed and at the port number 9 for the sand

bed). This location was selected so that in the dual media filter the center sample represented influent

quality to the fine media layer. The sample flow rate was controlled by the use of a pinchock clamp.

The sample flow rate was adjusted prior to the experiment to give the sample flow velocity equal to

the filtration velocity. This ensured that scouring did not occur and that the sample was

representative. Effluent too was sampled.

(d) Headloss measurement

The piezometer tubes were connected to the pressure taps using flexible plastic tubes of 6

mm internal diameter. The topmost pressure tap was connected to the piezometer No 1, and the other

taps in the descending order of height were connected to piezometers 2 to 9, the bottommost tap

being connected to piezometer no 10. The datum level for head measurements was 60 cm above the

ground level (i.e. 10 cm above the filter bottom).

The piezometer tubes were of 215 cm height. The static head (corresponding to the water

level in the constant head tank ) was 205 cm. This was the maximum value of head loss allowable inthis series of filtration experiments.

(e) Flow Pattern and Control

All experiments were conducted in the upward flow direction with constant rate. A pinchock

clamp (valve 13) was used to control the flow rate. The valve was adjusted frequently to maintain the


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required flow rate. The flow rates were 15, 12.5, 10, 7.5 & 5 m/h. For the lower flow rates (10, 7.5 &

5 m/h) the flow rate was measured using a stop watch and a measuring cylinder.

The flow entered the filter from the bottom and exited from valve 12 (in fig 3.1) and was

wasted to the drain through the rotameter (10-60 l/h).

3.1.4 Backwash System.

The backwash procedure consisted of sending compressed air in upflow direction at 100 m/h

for 2 minutes followed by water wash for 3 minutes at 50 m/h. For floating media the water

backwash was downwards achieving 65% expansion of the filter bed ( 30% expansion in polystyrene

and 100% expansion in polypropylene).There was no intermixing of media during or after the

backwash.For sand medium the water rinse too was upflow. Cocurrent wash for upflow sand filter

had been recommended by HAMANN & McKINNY (1968) too.

The backwash air and water was obtained from the tap water supply in the Ambient

Laboratory. Both backwash air and water flow rates were measured using rate indicators. The flow

rate meter for air had the range 0 - 25 l/min. The backwash water rate was measured from a rotameter

(Metric 14 XS, 2 - 200 l/h). A simple scale fitted along the filter column served as the media

expansion gauge. Visual observation of the bed expansion was complemented in some instances by

photographic recording too.

After a filter run, the valve 7 (see fig 3.1) was closed and remained so until the backwash

operation was complete. The backwash air entered from the valve 8 and exited from valve 12. The

valves 15 and 16 served either as inlet or outlet for the backwash water depending on the direction of

backwash water flow.

3.2 Experimental Runs

The filter runs #1 to #18 were conducted to determine an optimum media arrangement as

well as hydraulic conditions, chemical dosing etc. The data of these runs are presented in Appendix

2. After reviewing the results of these runs the filter column which initially had 5 piezometer ports,was modified by addition of 5 more. Also after run # 18 the center sampling was limited to sampling

the influent to the fine media layer only. Alum was added as primary coagulant from run # 15. The

summary of the filter runs (Runs #19 to #48; different combinations) is shown in the Table 3.1. The

filtration rates were adjusted frequently to the given value by adjusting the control valve.

Table 3.1 Summary of the Filter Runs


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Media NTU Surface Loading m3/m

2.h

15 12.5 10 7.5 5

Dual MediaPP 3.66 mm

PS 1.54 mm spherical

30 #19 #20 #21 #22 #30

60 #23 #24 #25 #26 #27

90 #28

#29

Dual Media

PP 3.66 mm

PS 1.54 mm angular

30 #31 #32 #33 #34 #35

60 #36

90 #37Dual Media

PP 2.57 mm

PS 1.54 mm spherical

30 #38 #39 #40 #41 #42

60 #44

90

Dual Media

PP 2.57 mm

PS 1.54 mm angular

30 #43

60

90

Single MediumSilica Sand 1 mm

30 #45 #46

60 #48

90 #47

# denotes a filter run

* PP = polypropylene, PS = polystyrene

* All runs upflow

* Run #28 with 0.5 mg/l Catfloc T2, Run #29 with 0.2 mg/l Catfloc T2

* Data on Runs #1 to #18 presented in Appendix 2.

3.3 Materials

3.3.1 Artificial suspension

The turbid raw water utilized in this experimental study was an artificial suspension of

kaoline clay dissolved in AIT tap water. The turbidity of the suspension was kept at 30, 60 or 90

NTU. The usage of an artificial suspension facilitated the comparison of various parameters while


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keeping the other conditions uniform.

The mean particle size (D50) of the Kaoline clay was 3.3 microns.

Before pouring into the raw water tank Kaoline clay was thoroughly mixed in a beaker, using

a magnetic stirrer. This prevented the sticking of kaoline clay and made it uniformly distributed.

Continuous stirring arrangement was provided in the raw water tank to prevent the settling of the

suspension and thereby keep the influent water quality constant.

The concentration of Kaoline clay required to get a given turbidity varied linearly with the

NTU value and confirmed the result obtained by LO (1984). This relationship is given in figure 3.3.

Fig 3.3 Turbidity vs Dosage of Kaoline Clay

3.3.2 Filter Media

Figures 3.4 to 3.9 show the 6 types of media utilized in these experiments. The data about

these filter media are summarized in Table 3.2. For dual media runs polystyrene was the fine medium

and polypropylene was the coarse medium.

Table 3.2 Filter Media Characteristics


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Media Size

(mm)

Specific

Gravity

Source / Method of Preparation

Polypropylene 4.36 0.903 Obtained from France

Polypropylene**

3.66 0.903 Commercially available in Bangkok

Polypropylene 2.54 0.903 Crushing the 3.66 mm beads and sieving.

Polystyrene

(spherical)

1.54 0.05 Obtained from France.

Polystyrene

(angular)

1.54 0.24 Crushed the raw material*.

Boiled in water for 1 minute at 1000C. The

resulting polystyrene was dried in the sun and

sieved to obtain the required fractions.

Silica Sand 1.00 2.70 Washed sand, dried and sieved. The sand utilized

was passing the # 16 (1.18mm) sieve and was

retained in the # 20 (0.85mm) sieve.

Geometric Mean Size = 1 mm

* The Raw material was Singlite expandable polystyrene grade 143 S, Manufactured by Thai

Petrochemical Industry Co. Ltd. of Bangkok, Thailand. This is a high strength polystyrene. The

density before expansion is 1040 kg/m3. Composition is 93% - 97% polystyrene and 7% - 3%

blowing agent. This material is recyclable. The cell structure is uniform after expansion. Polystyrene

does not dissolve in hot water, sea water, weak acids or inorganic hydroxides. Hence it's chemicalcomposition is not affected in a drinking water treatment operation.

** Polypropylene commercially available was PRO-FAX 6331 manufactured by the HMC polymers

Ltd. of Bangkok, a licensee of HIMONT Inc. of USA. This polypropylene is a general purpose

homopolymer resin. It meets all requirements of the US Food and Drugs Administration as specified

in the code of federal regulations, Title 21, Section 177, 1500, covering safe use of polyolefin articles

and components of articles intended for food contact uses. The hardness of the media is 97 in the

Rockwell R scale. The deflection temperature at 455 kPa is 960C.

application of the floto filter unit for contact flocculation filtration of surface waters

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