APPLICATION OF THE FLOTO FILTER UNIT FOR CONTACT FLOCCULATION FILTRATION OF SURFACE WATERS

Transcription

1 APPLICATION OF THE FLOTO FILTER UNIT FOR CONTACT FLOCCULATION FILTRATION OF 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 Previous Degree Scholarship Donor : Sri Lankan : BSc.(Eng) Hons. University of Peradeniya, Sri Lanka : Swedish International Development Agency (SIDA) Asian Institute of Technology Bangkok, Thailand April 1993

2 ACKNOWLEDGEMENT The author wishes to express his sincere appreciation and gratitude to his advisor Dr.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 Asian Institute of Technology. Finally the author gratefully dedicates this work to his beloved parents for their affection, concern and encouragement throughout his career. -2-

3 ABSTRACT Laboratory scale experiments were carried out in contact flocculation filtration using a dual media 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. -3-

4 TABLE OF CONTENTS CHAPTER Title Page Acknowledgement Abstract Table of Contents Abbreviations iii iv PAGE i ii v 1 INTRODUCTION 1 2 LITERATURE REVIEW Introduction Theory of Filtration Contact Flocculation - Filtration Headloss Development Contact Times for Polymers Rate Control Patterns and Methods Some Parameters Affecting Turbidity Filter Backwashing Floating Bed Filters Mathematical Models for Deep Granular Filters 26 3 EXPERIMENTAL INVESTIGATION Experimental Set Up Experimental Runs Materials Measurements 48 4 PRESENTATION AND CRITICAL DISCUSSION OF RESULTS Introduction Experiments with Floating Media Effect of Physical Parameters on Filter Runs Polymer Dosage and Mixing Time of Polymers Effects of Controls and Processes Experiments with Sand Medium Comparison of Experimental Runs and Concluding Remarks Mathematical Modelling 88 5 CONCLUSIONS 95 6 RECOMMENDATIONS FOR FURTHER STUDY 98 REFERENCES 102 APPENDIX APPENDIX APPENDIX

5 ABBREVIATIONS A c - area of collectors in a unit volume of filter (m 2 ) A p - area of retained particles in a unit volume of filter (m 2 ) C i C o d c d p f f o - concentration in ith time step (mg/l) - influent concentration (mg/l) - diameter of collector (m) - diameter of particles (m) - porosity of bed at time t - porosity of clean bed g - acceleration due to gravity (m/s 2 ) GAC h f H o J JTU K K w L ÄL - Granular Activated Carbon - headloss through a clogged bed (m) - initial headloss (m) - head gradient - Jackson Turbidity Unit - Kozeny's constant - Kuwabara's constant - filter depth (m) - filter depth increment (m) -5-

6 N - number of particles directly attached on the filter grain N p - number of particle collectors in unit volume (m -3 ) n - particle concentration at a given time and depth (m -3 ) n i - particle concentration at i th time step (m -3 ) n o - influent particle concentration (m -3 ) NTU OTV PAC PACEFILT P e PP PS REFIFLOC RSF S 1 S 2 SSF t US EPA V - Nephelometric Turbidity Units - Omnium de Traitments et de Valorization of France - Powdered Activated Carbon - PAC Embedding Filtration - Peclet Number - Polypropylene - Polystyrene - Refiltration Flocculation Process - Rapid Sand Filter - shape factor of suspended particle - shape factor of filter grain - Slow Sand Filter - filtration time (s) - United States Environmental Protection Agency - filtration velocity (m/s) V p - volume of particles deposited (m 3 ) -6-

7 ç ç c ç D ç I ç Lo ç p force - single collector efficiency of a clean collector - combined removal efficiency of a single collector - single collector contact efficiency by diffusion - single collector contact efficiency by interception - single collector contact efficiency by London Van der Waals - contact efficiency of a retained particle ç r ç s á á p â - single collector removal efficiency of a filter grain and its associated retained particles - single collector contact efficiency by sedimentation - particle to filter grain attachment coefficient - particle to particle attachment coefficient - fraction of retained particles acting as particle collectors â' - the fraction of retained particles that contribute to the additional surface area â 1 â 2 å d - fraction of filter grain surface, which is exposed for the particle deposition - detachment coefficient - porosity of deposit ñ - density of water (kg/m 3 ) ñ p - density of particles in suspension (kg/m 3 ) ì - viscosity of suspension (Ns/m 2 ) ó - specific deposit -7-

8 ã - fraction of coarse particles which can act as particle collectors to remove finer particles in the suspension -8-

9 -1- 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 m 3 /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 have focused 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

10 -2- 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.

11 -3- 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.

12 -4- 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 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 Theory of Filtration 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

13 -5- microfloc is agglomerated into macrofloc 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. As the 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).

14 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 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

15 -7- 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 the effluent 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 m 3 /m 2.h. The cationic polyelectrolytes made higher filtration rates (20 m 3 /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 to coarse 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 Advantages and Disadvantages

16 -8- 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 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 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.

17 -9- 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.

18 -10- 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 rates above 10 m/h the headloss development had been very rapid resulting in very short filter runs 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

19 -11- 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 3.0 mg/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.

20 -12- 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.

21 -13- 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 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.

22 -14- 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 in the top layers of the backwashed filter, provides optimum cleaning. Simultaneous air scour (54-90 m/h) and subfluidization water wash ( 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

23 -15- (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 The Biostyr The Biostyr or the Upflow Floating Aerated Biofilter 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 nozzles 4 Filter media 10 Treated water storage & discharge 5 Backwash air 11 Recirculating pump 6 Non aerated zone

24 -16- 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 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 kg/m 3 Effective Size 0.57 mm Uniformity Coefficient 1.66

25 -17- 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 m in diameter and meters in height. The height of the bed was 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.

26 -18- 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) 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 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 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 2 minutes. 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.

27 -19- 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 m 3 /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 Haberer PAC 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

28 -20- 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

29 -21- 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 & SCHMIDT, 1991) 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.

30 -22- 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 m. They did not use coagulant aid. Their experiments on a pilot filter with PAC coated polystyrene media showed that the smaller beads (size mm) gave consistently better organics removal than the larger beads (size mm). TABLE 2.2 MAXIMUM OPERATING PRESSURE IN RELATION TO FOAMED POLYSTYRENE BEAD DENSITY (HABERER & SCHMIDT, 1991) Density(kg/m 3 ) Permitted Pressure(kPa/cm 2 ) The effect of coagulant aid on iron content of a REFIFLOC test filter after coagulation with 30 g FeCl 3 /m 3 is shown in figure In this case the bed depth was 1.35 m and the filter diameter was 0.19 m. Size of the polystyrene beads was mm. Filter velocity was 6 m/h and the maximum filter run was 8 h.