The use of particle monitoring in the performance optimisation of conventional clarification and filtration processes

~

Pergamon PH: S0273-1223(97)00443-5

War. Sci. Tech. Vol. 36. No.4. pp. 191-198. 1997. CCI 1997 IA WQ. Published by Elsevier Science Ltd Printed in Great Bntaln 0273-1223/97 $17'00 + 0-00

THE USE OF PARTICLE MONITORING IN THE PERFORMANCE OPTIMISATION OF CONVENTIONAL CLARIFICATION AND FILTRATION PROCESSES G. Standen*, P. J. Insole*, K. J. Shek** and R. A. Irwin* '" Southern Water Services Ltd. Southern House. Sparrowgrove. Otterbourne. Hampshire. S0212SW. UK .. Cranfield University. School of Water Sciences. Cranfield. Bedfordshire. MK430AL. UK

ABSTRACT The application of laser diffraction particle monitoring to the performance optimisation of a pilot clarifier and full-scale rapid gravity filters (RGF). operating on water supply works in Hampshire. IS described. Furthermore the dosing of powdered activated carbon (PAC) into the works' clarifiers has been evaluated in terms of RGF performance. A costly proposal to install a third filter medium was subsequently abandoned when It was found that particle numbers in the filtered water were consistently below I x IOZJml. Various combinations and doses of coagulants and flocculant aids. shown to give optimum particulates removal during intensive jar testing trials. were transferred to the pilot clarifier. Particle monitonng enabled a more accurate denvation of suitable blanket chemistry and optimum blanket heights than turbidity changes Raw water turbidities were 10-15 NTU at start-up wnh corresponding counts beyond the upper limit of the particle monitor. An on-line dilution system was developed to overcome this problem. Latex bead (4.33 ~m) and Lycopodium spore (4-5 ~m) suspensions (about I x 109 particles) were injected into the pilot clarifier to assess the removal efficiency of Cryptosporidium-sized particles. Reductions of about 1.7 log and 2.6 log were achieved for the beads and spores. respectively. Particle distributions of various PAC's and a bentonite were obtained in order to assess their potential effects on the coagulation process during clarification. Bentonite was also beneficial as an on-line means of checking particle monitor response and calibration. The works' filters achieved 1.5 to 2.0 log removals of 2-5 ~m particles without media addition or operational changes. Combined clarification and filtration gave better particulates removal than two-stage microfiltration. © 1997 IA WQ. Published by Elsevier Sctence Ltd

KEYWORDS Clarification; cost savings; Cryptosporidium; filtration; particle monitoring; seeding; turbidity.

INTRODUCTION Back~round

Testwood Water Supply Works (WSW) in Hampshire is supplied by the River Test, which receives substantial wastewater discharges from Fullerton and Romsey Wastewater Treatment Works and a minor 191

192

G. STANDEN etal.

industrial discharge from a paper mill at Overton. Agriculture is the main catchment industry. The Test has been given the Environmental Agency's highest river quality objective (IA-NWC), constituting a clean water, free from pollution and with diverse flora and fauna. Its water is generally of low turbidity, often less than 5 NTU. In 1992 Cryptosporidial oocysts were detected in the treated water at Testwood WSW resulting in a boil notice being issued to customers. Consequently, the options of microfiltration for Cryprosporidium removal, and ozone/granular activated carbon (GAC) for pesticides and Cryptospridium removal were considered. However, an assessment of the associated capital and operational costs led to a review of needs. Clarification and filtration optimisation was then progressed as the cost-effective alternative to microfiltration or ozonefGAC. A pilot clarifier was constructed, installed, modified and operated for 18 months. Particle monitoring was incorporated after about 6 months. Finally, the recommended operational and chemical changes, particularly PAC dosing for pesticide removal, were transferred to full-scale after completion of the pilot trials. Post• filter particle monitoring was installed at the same time to monitor the effects of changes in clarifier blanket chemistry on filtration performance. Testwood WSW has three stages of clarifiers. Stage one consists of two Accentrifloc (16 Mid each) and two circular (8 Mid each) recirculating clarifiers. Stages two and three consist of a single flat-bottomed clarifier (42 Mid) and two flat-bottomed clarifiers (23 Mid each), respectively. Standen et al. (1995) details the rise rates, retention times and blanket depths for the clarifiers. Stage two was known to be hydraulically unstable, with excessive floc carry-over, calling into question its suitability as a barrier to Cryprosporidium. Testwood also operates twelve dual-media (anthracite/sand - 10 Mid maximum rating) rapid gravity filters, supplied by stage one and stage two clarifiers. Stage three clarifiers feed two Enelco R dual-media (anthracite/sand - 2S Mid maximum rating) multi-cell filters. A backwash (post-scour) interval of 72 hours is operated automatically with water quality sometimes dictating intervals as low as 48 hours and as high as 90 hours. An overview of particle monitoring' Clarification and filtratjon Optimisation of water treatment processes has received much attention. Kawamura (1996) gives an overview of the main considerations, including particle monitoring. Applications of particle monitoring to the optimisation of clarification have been restricted to areas such as optimum coagulant dose assessment (Reed and Mery 1986, Lartiges et al., 1995) and pre-ozonation coupling effects (Colin et al., 1982, Wilczak er al., 1992). On-line coagulant dose optimisation using particle monitoring has been shown to offer greater sensitivity and reliabilIty than feed-forward control techniques such as streaming current detection (Dentel and Kingery, 1989) or zeta-potential monitoring. Conventional rapid gravity filtration has been extensively studied using particle monitoring (Hargesheimer et al., 1992), with many workers concentrating on the removal of Cryptosporidium (Bellamy et al., 1993, Leland et al., 1993, Gregory, 1994, Nieminski and Ongerth, 1995, Ongerth and Pecoraro, 1995) or surrogates. The advantages of particle monitoring over turbidity monitoring during filter operation have been noted by a number of authors (Treweek and Morgan, 1978, Hutchinson, 1985). However the search for a satisfactory relationship between turbidity and particle count has not been so fruitful (Treweek and Morgan, 1978). Nevertheless, LeChavallier and Norton (1992) developed an approximation between particles larger than 5 Ilm and turbidity, and Cryptosporidium and Giardia. The effect of particle size in relation to filter efficiency and headloss has also been studied (Darby and Lawler, 1990, Boller and Kavanaugh, 1995).

Particle monitoring

193

METHODS Pilot clarifier - installatjon The pilot clarifier (485 I) was constructed to be a design compromise between circular and Accentrifloc recirculating types, but without sludge concentrators. Coagulants and flocculant aids were dosed, by peristaltic pumps, into a 9-point line containing a Sulzer SMF-25-R (105 mm) static mixer; polyelectrolyte dosing was performed under the cone. A programmable logic controller (pic) was incorporated to effect blanket desludge on time and/or height; timed de sludge was selected for its greater reliability. A raw water flow rate of 9 Vmin was used as a balance between the mean rise rate (2.2 mlhr) and mean retention time (165 mins) seen on the works' recirculating clarifiers, its suitability being confirmed by hydraulic testing. Low raw water turbidity «5 NTU) at the time of pilot commissioning impeded blanket development. Consequently, an intensive jar-testing (Standen et al., 1995) programme was initiated to rapidly evaluate a range of coagulants and flocculant aids, in various combinations and doses, with about 20 sets of conditions being transferred to the pilot. Particle monitoring sensors (PMS Liquilaz E20 units) were installed on the raw water inlet and clarified water outlet. Routine cleaning and an on-line dilution system ensured that accurate particle detection was maintained throughout the trials. Effects of chemical changes on clarifier blanket performance, determined by particle and turbidity removal, were monitored from clarifier re-instatement until the blanket flowed into the clarifier launders at which time rapid desludging was activated. In this wayan optimum height was gained. i.e. the point of greatest particulates removal, whilst the blanket was being both formed and removed. Small adjustments of the PLC de sludge timers provided effective depth control. Pilot clarifier - seedin~ materials A sodium-exchanged calcium montmorillonite clay (Redland Minerals Berkbent CE bentonite) was employed to accelerate blanket formation during low raw water turbidity conditions. Batch additions of material (10, 7.5, 5 g) were made for 2 hours during three separate clarifier start-up periods to determine the optimum concentration i.e. giving the lowest particle removal. A recirculating mixer system. incorporating drive-water, was developed for suspending and dosing the bentonite. Powdered activated carbon (PAC) dosing was initiated prior to pesticide spiking trials. Continuous or intermittent doses of 10 and 20 mgll were made. for two coal- based PAC's - CPL Carbons WP9 and Sutcliffe Speakman 207AP. Particle distributions were obtained for the bentonite (Table I) and PAC (Table 2) at various concentrations. Pilot clarifier - blanket performance testin~ Latex beads (4.33 ~m, 2.5% suspension, 2 ml to 20 ml distilled water, 0.1 ml Tween 80 surfactant) or Lycopodium spores (4-5 ~m, dry, 20 ml distilled water, 0.1 ml Tween 80 surfactant) were injected rapidly into the primary mixing zone of the clarifier; giving 1.12 x 109 beads and 9.98 x 108 spores in 485 I. The areas of peaks above the baseline, i.e. peaks within the time expected. were measured and converted to numbers of particles breaking-through per litre or minute. An approximate log-removal was obtained by comparing the break-through particle numbers to the theoretical numbers of particles injected. Particle monitor on-line dilution system Hargesheimer et al. (1992) and Lewis et al. (1992) showed that a linear relationship between dilution and particle numbers existed. Note that no commercially procurable equipment is available, to date. On-line dilution was achieved using a peristaltic pump with twin heads and silicone tubing with diameters of 4.8 mm and 1.6 mm for sample water and diluent water, respectively. Tap water was used as the diluent because it contained fewer particles than distilled water and was more readily available. Mains water was drawn-off the side of a 30 I, overflowing, reservoir and mixed with sample water post-peristaltic pump,

G STANDEN .ral.

194

before entering a 100 mI constant-head device of novel design. Blended water was passed through the particle monitor. A 4.2 I volume of diluent water (one hours supply at 0.07 Vmin) was analysed before and after each dilution to gain a mean 'background' particle count for the tap water. The same detector was employed throughout the trials to maintain consistency of results. Furthermore, the flow rate through sample and diluent lines was measured, both independently and in combination to determine the degree of dilution and hence the particle numbers in the original undiluted sample. A theoretical dilution ratio of 9: 1 using the equipment corresponded to one of 6.3: 1 in practice. In another study, using two separate peristaltic pumps, a 10:1 sample to diluent water ratio permitted about 105 particleslml (about 104/m1 as detected) to be counted accurately without losing resolution. Nonetheless, a single pump with twin heads was found to be more than adequate. Works' clarifiers and filters Six months of PAC dosing, to derive a dose for pesticide removal and assess blanket stability, was initiated in June, 1995. Doses of 5, 10, 15 and 20 mgll of PAC were applied for between 1 and 8 weeks; 10 mg/l being chosen as the works' dose. Caustic dosing was also implemented as part of the transfer from pilot to full- scale. The effect of PAC and caustic dosing on filter performance was monitored by connecting a laser sensor to the outlets of rapid gravity filters 4 and 5, each filter being supplied exclusively by stage two and stage one clarifiers, respectively. Filter inlet turbidities were monitored for two days i.e. until the sensors became fouled by floc, thereby causing the laser light level to drop to an unservicable limit. Table I. Particle distribution for bentonite clay suspensions (No on-line dilution); from Shek (1994) Average particle count (per mI) Weight (mgll- dry)

2-4 I'm

4-6 I'm

6-10 I'm

10-15 I'm

>15 I'm

(percentage of total)

TOTAL

(total/weight)

0.25

307 (66.6)

62 (13.4)

53 (11.5)

24 (5.2)

IS (3.3)

461 (1844)

0.50

486 (65.9)

101 (13.7)

88 (11.9)

38 (5.1)

25 (3.4)

738 (1476)

1.25

1258 (67.0)

253 (13.5)

216 (11.5)

97 (5.2)

(2.9)

54

1878 (1502)

2.50

2407 (64.6)

533 (14.3)

466 (12.5)

209 (5.6)

112 (3.0)

3727 (1439)

5.00

5102 (66.0)

1089 (14.1)

(11.8)

906

408 (5.3)

220 (2.8)

7725 (1545)

7.50

7295 (65.8)

1708 (15.4)

1298 (11.7)

513 (4.6)

275 (2.5)

11089 (1479)

10.00

7838 (61.3)

2058 (16.1)

1739 (13.6)

754 (5.9)

(3.1)

396

12785 (1279)

25.0

8886 (48.7)

3448 (18.9)

3315 (18.2)

1621 (8.9)

964 (5.3)

18234 (729)

Particle monitoring

195

RESULTS AND DISCUSSION Pilot-scale clarifier - ~eeding materials Table 1 shows the particle distribution of the bentonite clay used in the initiation and conditiOning of pilot blankets, after wetting for 2-48 hours, addition of surfactant (Tween 80) and ultrasonication. A limit of about II x 103 particleslml of bentonite is indicated, coinCiding with the manufacturers specifications. Table 2 displays an abridged version of Table I but for PAC. The data in Tables 1 and 2 indicate that bentonite and PAC have similar distributions, suggesting that surface charge may account for differences in coagulation effects; later confirmed by zeta potential measurements. Table 2. Particle distribution for PAC suspensions (No on-line dilution); from Shek (1994) Average particle count (per ml) Weight (mg/l- dry)

2-4j.1m

6-IOj.lm

4-6j.1ffi (~

10-15 j.lm

>15j.1m

rcentage of total)

TOTAL (total/weight)

0.25

503 (56.4)

202 (22.7)

135

(15.1)

36 (4.0)

16 (1.8)

892 (3568)

2.50

4015 (57.1)

1493 (21.2)

1075 (15.3)

321 (4.6)

(1.8)

125

7029 (2812)

25.0

3395 (20.7)

2884 (17.6)

5534 (33.8)

3314 (20.2)

1265 (7.7)

16392 (656)

Pilot clarifier - blanket performance testin~ Particle monitoring during blanket formation produced an optimum height, based on lowest particle count, of between 0.65 and 0.70 m in a total water depth of 0.95 m. The optimum heights derived during formation and desludge were constant to within 0.02 m, depending on water quality conditions and coagulant. Table 3. Particle distributions for Lycopodium suspensions (with surfactant); from Shek (1994) Average particle count (per ml) Theoretical Particle No.lml

2-4j.1m

4-6j.1m

6-10 j.lID

10-15 j.lID

>15 j.lm

(percentage of total)

TOTAL (total/theoretical)

2000

413 (43.7)

298 (31.6)

120 (12.7)

46 (4.9)

67 (7.1)

944 (0.47)

6000

792 (44.4)

688 (38.6)

162 (9.1)

47 (2.6)

95 (5.3)

1784 (0.30)

10000

1407 (47.9)

1101 (37.6)

233 (8.0)

64 (2.2)

125 (4.3)

2930 (0.29)

15000

2001 (47.0)

1614 (37.9)

358 (8.4)

100 (2.3)

196 (4.6)

4259 (0.28)

20000

2569 (46.6)

2045 (37.1)

505 (9.2)

147 (2.7)

245 (4.4)

5511 (0.28)

196

G. STANDEN et al.

Tables 3 and 5 show the respective particle distributions for Lycopodium spores and latex beads. that were injected on separate occasions into a pre-formed pilot clarifier blanket. The theoretical and total measured particle number ratios are different. with measured concentrations being about 30% and 40% of the calculated values for Lycopodium and latex beads. respectively. A full explanation cannot be offered for the discrepancy. However. comparing the distributions of Lycopodium suspensions minus surfactant presented in Table 4 with those in Table 3 (surfactant omitted) indicates that particle aggregation must occur normally. generating artificially large particles. The data in Table 4 also demonstrate that ultrasonication alone cannot disperse the particles; particle aggregation often resulting at low ultrasound energies. The clustering of particles. especially Cryptosporidium. during water treatment has been investigated (WRc. Swindon - unpublished) and documented. Table 4. Particle distributions for Lycopodium suspensions (No surfactant); from Shek (1994) Average panicle count (per m1) Theoretical Particle No.lml

2-4 I'm

4-6 I'm

6-10 I'm

10--15 I'm

> 15 I'm

(percentage of total)

TOTAL (totalltheoreticaIL

2000

119 (24.5)

135 (27.7)

80 (16.4)

45 (9.2)

109 (22.2)

488 (0.25)

10000

566 (32.9)

573 (33.2)

249 (14.4)

121 (7.0)

216 (12.5)

1725 (0.17)

20000

(32.1)

1077

1124 (33.5)

479 ~4.3)

242 (7.2)

437 (13.0)

3359 (0.17)

Table 5. Particle distributions for Latex Bead suspensions (No on-line dilution); from Shek (1994) Average panicle count (per ml) Theoretical Particle No.lml

2-4 I'm

4-6 I'm

6-10 I'm

10-15 I'm

> 15 I'm

(percental?;e of total)

TOTAL total/theoretical

2000

1691 (84.1)

149 (7.4)

(2.5)

SO

26 (1.3)

98 (4.9)

2014 (1.01)

6000

(89.5)

2776

209 (6.7)

(1.S)

46

19 (0.6)

54 (1.7)

3104 (0.52)

10000

4796 (89.0)

468 (8.8)

(1.4)

73

24 (0.4)

47 (0.4)

5408 (0.54)

15000

5013 (87.4)

562 (9.8)

82 (1.4)

27 (0.5)

52 (0.9)

5736 (0.38)

20000

6450 (85.7)

877 (11.6)

117 (i.6)

31 (0.4)

-(g.~

52 (70.38 1)

Data within Tables 3-5 emphasise the importance of analysing 'spiking' materials to determine whether they; (a) are of the size range quoted, and (b) require wetting to prevent particle clustering_ Two main factors may have acted to increase the numbers of particles breaking-through during pilot clarifier blanket spiking trials. These were; (a) the appearance of 'welling' zones in the clarifier having a potential to cause flow short-circuiting. and (b) the alteration of particle charge by surfactant which might affect particle coagulation. The addition of I x 109 particles in a concentrated plug. many of which might have risen to the

Particle monitoring

197

surface uncoagulated, may have accentuated both factors. Continuous dosing of particles would have provided a more even distribution of material throughout the clarifier. Nevertheless, both spiking materials were removed well by coagulation, but there is no proof that either 'surrogate' would reflect actual Cryptosporidial oocyst removals under identical conditions. However, it is known that Lycopodium has physical characteristics (size, surface charge, rigidity) more in common with Cryptosporidium than latex beads. One primary feature of difference between Lycopodium and Cryptosporidium is opacity, the latter being semi-opaque. Cryptosporidium is 'down-sized' (about 2-4 Ilm rather than 4-6 Ilm) by most laser particle monitoring instruments. In support of these findings Lewis and Manz (1991) reported that forward angle light scatter (FALS) particle monitoring measured Giardia cysts as spheres of 1-51lm diameter rather than the 8-121J.m determined by optical microscopy. Particle monitorjn~ on-line dilution system A single pump (twin-head) dilution system enabled a sample water containing up to 7 x !O4 particIeslml to be measured. Accuracy and precision were within 6 percent. The numbers of particles in the diluent water was invariably less than 5% of the diluted sample water. However, a Cuno Microwynd (lNm) cartridge filter was subsequently installed in the diluent line to effect simpler, more accurate on-line counting. Full-scale clarifiers and filters The two-day filter inlet study indicated that filter 4 was receiving about 5 times as many particles from clarifier 5 (3 x !03/ml) as filter 5 was from clarifiers 1-4 (6 x !02/ml). Log removals of between 1.5 and 2.0 were being achieved routinely prior to the programme of PAC dosing, agreeing with the findings of Leland et al. (1993) and Ongerth and Pecoraro (1995). Note that turbidity monitoring was performed in this study, although insufficient data was collected to compare with particle monitoring data. Figure 1 shows particle numbers in the filtered water (filter 5) with PAC dosing into the works' clarifier blankets.

'200

1: ~ ~

8

~

rr.

t:;2.Srric:ron

1000 lOG lOG
200

0

• 2-125

.

mcroj

.

~ 0

!. "

'\

20

40

eo

Flter run cycle - blclMalh to blck'Msh I hours

Figure I. Particle monitoring of works' filter 5 with PAC dosing (I0mg/l) into the works' clarifiers.

An investigation carried out by Water Research Centre (WRc) on filters 4 and 5, using a Hiac/Royco (HRLD- 400HC) instrument, produced very similar results to those shown in Figure I; the clarified water turbidity was similar on both occasions. The filtered water was shown to contain up to 50% fewer particles during PAC dosing. This demonstrated that any PAC particles carried-over with the floc did not fully penetrate the filter despite their small (75% less than 5 Jlm) diameter. Backwash intervals were unaffected by PAC. With less than I x !O2 particleslml in the filtered water and log removals (2-5 Ilm) of 1.5-2.0 being achieved adding a third gamet medium (ASG filter) could not be recommended. The capital cost associated with materials and installation was estimated at £150,000. Increased operational costs were also anticipated.

198

G. STANDEN etal.

CONCLUSIONS During the pilot clarifier investigations of different coagulants and flocculant aids an optimum blanket height was derived and performance testing was achieved. The latter study produced log removals of about 1.7 and 2.6 for latex beads and Lycopodium, respectively. Some questions remain regarding the use of latex beads or Lycopodium spores as surrogates for Cryptosporidium. Particle distribution analyses of powdered activated carbon and bentonite clay were similar, suggesting particle charge differences. A linear particle distribution relationship (within 6%) was demonstrated up to II x 103 particle simI. An on-line dilution system was developed for water contaimng up to 105 particlesimI. Particle numbers in the filtrate from two works' rapid gravity filters indicated that adding garnet as the third medium would not be cost-beneficial. Further monitoring during PAC dosing into the works' clarifiers showed that PAC carried onto the filters enhanced their performance without reducing standard run times. Particle removals of between 2.5 and 4.6 log were achieved across the clarification and filtration processes at Testwood, exceeding the performance of dual-stage 'dead-end' microfiltration. ACKNOWLEDGEMENTS The authors thank Dr Tom Hall (WRc, Swindon, UK) for carrying-out an independent particle monitoring study of the works' filters, the operators at Testwood for their assistance and John Edwards for installing the monitoring equipment. REFERENCES Bellamy, W. D., Cleasby, J. L., Logsdon, G. S. and Allen. M. J. (1993). Assessing treatment plant performance. J. AWWA. 34-38. Boller. M. A. and Kavanaugh. M. C. (1995). Particle characteristics and head loss increase In granular media filtration. Wal. Res. 29(4).1139·1149 Cohn, J. L.. Bablon, G. and Faucherre, J. (1982). Particle analYSIS apphed to surface water treatment. Trib. Cebedeau. 35(459). 57-70. Darby, 1. L. and Lawler. D. F. (1990). Ripemng in depth filtration: Effect of particle size on removal. Env. Sciellce Tech. 24(7), 1069-1079. Dentel, S. K. and Kingery. K. M. (1989). Using streaming current detectors in water treatment. J.A WWA. 81, 85-89. Gregory, J. (1994). Cr),plospondlum In water treatment and monitoring methods. Fill. and Sep. 31(3), 283-289. Hargesheimer, E. E., Lewis. C. M. and Yentsch, C. M. (1992). EvaluatIon of particle counting as a measure of treatment plant performance. AWWARF. pp. 1-319. Hutchinson. W (1985). On-line panicle counting Improves filter effiCIency. Wat. Eng. Management. 132(7).20-25. Kawamura, S. (1996). Optimisation of basic water-treatment processes - design and operation: sedimentatIon and filtration. J. Wal. SRT· Aqua. 45(3). 130-142. Lartiges, B. S .. Sottero. J. Y., Democrate. C. and Coupe. 1. F. (1995). Optimising flocculant demand by following floc size dIstributIon. J. Wal. SRT· Aqua. 44(5), 219-223. LeChevalher, M. W. and Norton, W. D. (1992). Examining relationships between particle counts and Giardia, Cryptosporidium and Turbidity. JoA WWA. 84(12), 54-60. Leland, D .• McAnulty. J.• Keene, W. and Stevens, G (1993). A Cryptosporidiosis outbreak in a filtered-water supply. J.AWWA. 85(6). 34-42. Lewis, C. M., Hargesheimer, E. E. and Yentsch. C. M. (1992). Selecting panicle counters for process monitoring. J. AWWA. 84(12).46-53. Lewis, C. M. and Manz, D. H. (1991). Light-scatter particle counting: Improving filtered-water quality. J. Env. Eng. 117(2),209223. Nieminski, E. C. and Ongerth, J. E. (1995). Removing Giardia and Cryptosporidium by conventional treatment and direct filtration. J. AWWA. 87(9),96-106. Ongenh, J. E. and Pecoraro, J. P. (1995). Removing Cry'ptosporidium using multImedia filters. J.A WWA. 87(12),83-89. Reed, G. D. and Mery. P. C. (1986). Influence of floc size dIstribution on clarification. J. A WWA. 78(8), 75-80. Shek, K. (1994). Performance evaluation and optimisatIOn of a pilot-scale advanced water treatment plant. MSc Thesis, School of Water Sciences, Cranfield Umverslty. Standen. G., Insole, P J. Shek. K. J and Irwin. R. A. (1995). Optimisation of clarifier performance for pesticide removal. Proc. 3rd Int. Con! Water & Wastewater Treatment, M. White (ed). Harrogate. United Kingdom, November 13-15 pp. 13-33. Treweek. G. P. and Morgan J. J. (1978). PredIction of suspensIOn turbidIties from aggregate SIze distribullons. Presented before the Division of Environmental Chemistry, Am. Chem Soc. March 12-17 pp. 181·184. Wilczak, A., Howe, E. W., Aleta, E. M. and Lee. R. G. (1992). How pre-oxidation affects panicle removal during clarification and filtration. JoA WWA. 84(12), 85·94.