A novel method for on-line evaluation of floc size in coagulation process

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A novel method for on-line evaluation of floc size in coagulation process Wen Po Cheng, Yu Pin Kao, Ruey Fang Yu Department of Safety, Health and Environmental Engineering, National United University, No. 1, Lien Da, Kung Ching Li, Miaoli 360, Taiwan

art i cle info

ab st rac t

Article history:

Chemical coagulation is a simple and widely used water treatment process. A jar test based

Received 25 September 2007

on the residual turbidity in the treated water was used to evaluate the optimal conditions

Received in revised form

for floc formation. However, the final residual turbidity does not show up variation of

15 January 2008

turbidity and floc formation during the flocculation process. Hence, a nephelometric

Accepted 22 January 2008

turbidimeter method based on on-line monitoring was devised to determine the floc size

Available online 14 February 2008

variance during flocculation. A nephelometric turbidimeter coupled with a data acquisition

Keywords: Nephelometric turbidimeter Flocculation Floc size On-line monitoring Standard deviation

unit was used to measure turbidity every second at 3 cm below the water surface during the coagulation process. Laboratory results indicated that this new instrument was capable of recording floc agglomeration during slow mixing very accurately. The standard deviation (SD) of the measured turbidity was proportional to the square root of the floc size; a greater SD indicated larger floc sizes. Hence, in addition to monitoring turbidity, the nephelometric turbidity meter is also a valuable tool to study the floc agglomeration process and variations in the resulting floc size. This method is simple and effective; it contributes significantly to the selection of coagulant and optimal flocculation conditions to improve water treatment. & 2008 Elsevier Ltd. All rights reserved.

1.

Introduction

Chemical flocculation is an important water treatment process; its efficiency seriously affects the operation and efficiency of subsequent sedimentation and filtration. However, in water treatment plant not much research has yet been conducted on the relationship between flocculation conditions and floc formation. Furthermore, the selection of an effective coagulant and dosage in most water treatment plants has up to now depended on the operator’s experience based on the results of jar tests. This usually leads to wasteful over-dosage of chemical coagulants, higher operation costs and increased chemical sludge. With jar tests, the supernatant of the treated sample is collected to measure the residual turbidity for evaluating the coagulation efficiency (Jiang and Graham, 1998; Rossini et al., 1999; Franceschi et al., Corresponding author. Tel.: +886 3 7381764; fax: +886 3 7333187.

E-mail address: [email protected] (W.P. Cheng). 0043-1354/$ - see front matter & 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2008.01.032

2002; Byun et al., 2005; Yu et al., 2007). But, the physical characteristics of the floc may be more influential in determining turbidity removal efficiency. For example, large compact flocs have a high settling rate that results in treated water of low turbidity (Wilen et al., 2003), whilst large and porous flocs aid filtration due to high permeability (Bushell et al., 2002). Thus, the average floc diameter measured using floc monitoring was studied to index the real coagulation efficiency. Some of the current floc monitoring techniques, such as microscopy, microscopy combined with photography or image analysis, and light scattering can be adapted to continuous measurement of qualitative changes in floc size (Mas and Ghommidh, 2001; Govoreanu et al., 2004; Chang et al., 2005; Huang, 2005; Rattanakawin, 2005; Han et al., 2006). The monitoring technique of microscopy, however, is time consuming requiring large sample sizes and considerable

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preparation but provides good data regarding floc shape and form. Light scattering and transmitted light techniques have been used to good effect in measuring floc size on-line whilst individual particle sensors have limited the applicability of measuring floc size (Jarvis et al., 2005). Additionally, the floc agglomeration kinetics and floc characteristics were further analyzed to confirm floc growth using a simple but sensitive photometric dispersion analyzer (PDA) developed by Gregory (1985). This method is applicable in a wider range of floc concentrations without understanding the photometric characteristics of the flocs (Gregory and Nelson, 1986; Huang and Chen, 1996; Kan and Huang, 1998). The PDA also has been widely used in closed loop systems to measure dynamic floc size (Burgess and Phipps, 2000; Fitzpatrick et al., 2003; Gregory, 2004; McCurdy et al., 2004; Yukselen and Gregory, 2004; Wang et al., 2005). However, the solids passing through the PDA measuring cell must be at a high enough concentration to provide a reliable signal (Jarvis et al., 2005). The mixing intensity and instrument calibration may also cause some problems in on-line measurement (Burgess et al., 2002a, b). Beside, the PDA or other instruments for the continuous evaluation of the floc size change during the flocculation process almost is not a commonly used instruments in water treatment plants (Biggs and Lant, 2000; Govoreanu et al., 2002; Swift et al., 2004; Rattanakawin, 2005). In this study, suspensions of Chlorella sp. algae cultivated in a laboratory were used as low turbidity samples (10 NTU) to test a common nephelometric turbidimeter for the on-line monitoring of the turbidity during coagulation. The turbidity fluctuations were then analyzed as basis for the evaluation of floc size growth.

2.

Experimental equipment and methods

2.1.

Cultivating the algal suspension

2.1.1.

Algal species

The freshwater Chlorella sp., which is a round single-celled green alga with an average diameter of 4 mm (varying from 2 to 8 mm) was used in this study. It is a predominant species found in domestic eutrophic lakes. The algal sample was obtained from the Taiwan Fishery Research Institute, Tung-Kong Branch.

2.1.2.

Algal cultivation method

The cultivation solution used an inorganic medium prepared according to the formula proposed by Walne (1974). Initially, the volume ratio of algae (obtained from Taiwan Fishery Research Institute) to cultivation solution was maintained at 4–6. Under aseptic conditions, the Chlorella sp. sample was cultivated in a l-L glass container until the concentration reached a certain level. During the initial cultivating period, a higher alga to nutrient ratio was maintained and the aeration was kept mild. After 7 days, the algal concentration became high enough for the algal mass to be isolated and transferred to another 5 L cylindrical container for mass production. The glass container was sterilized beforehand at 121 1C and 1.3–1.5 kg/cm2 for 30 min. The 5 L cylindrical container was aerated with aseptic air at 1 L/min. Fluorescence tubes placed along the side of the reactor provided 3000–4000 lx for 16 h per day. Quantitative analysis of the algal growth was

done by measuring the light absorption at 684 nm using a spectrophotometer.

2.2. Preparation of poly-aluminum–chloride (PACl) and poly-aluminum–silicate–chloride (PASiC) coagulants The PASiC coagulants used in this study were prepared in the laboratory using the method developed by Gao et al. (2002) and Yang et al. (2004). The final PASiC coagulants had a Al/Si ratio of 5 and B value of 1.5 (B ¼ [OH]/[Al]). Polysilicic acid (Psi) solution was prepared by adding 23.5 ml 1.5 M HCl into 50 ml 0.5 mol/L SiO2 solution while mixing rapidly. The mixture pH was adjusted to 2 to yield a 0.329 mol/L Psi. Then, 23.92 mL de-ionized water was added to 40 ml 0.25 M AlCl3 for PASiC preparation. Depending on the required Al/Si ratio of 5, 6.08 mL Psi solution was added to the Al solution and subsequently 30 mL 0.5 M NaOH was added slowly (speed of titration was 0.05 ml/min) to reach the specified B value allowing for metal–silicate polymerization. At the completion of the NaOH addition, the final volume of the solution was 100 mL. For the preparation of a PACl coagulant with a basicity value 1.5 (B, B ¼ OH/Al3+), 50 mL 0.3 M NaOH solution was added using a peristaltic pump with a flow rate of 0.05 ml/min into 50 mL 0.2 M AlCl3 solution.

2.3.

Jar test

Fig. 1 shows the schematic diagram of the jar test apparatus and the nephelometric turbidimeter (WTW model MIQ/C1184) attachment. A hole was made in the wall of the jar test vessel to enable the probe of the nephelometric turbidimeter to reach into the water. The turbidity was measured once every second. A light-emitting diode (LED) on the probe of nephelometric turbidimeter was used to illuminate the solution at 901 to the path of light entering the turbidimeter. The intensity of the scattered light, which was measured with the detector at 860 nm red light, was converted to turbidity value. This method can be used in a range of 0–4000 NTU turbidity. The measured turbidity was recorded with a data acquisition unit (YOKOGAWA model FX-106-0-2) and then uploaded to a computer. The on-line monitoring setups greatly reduce human errors and save sampling time. The jar test was done with covered black opaque acrylic containers instead of traditional jar test flasks. A hole was drilled on the cover for adding chemicals. One liter of the prepared 10 NTU synthetic algae sample was placed in the acrylic flask. After addition of chemicals, the content was rapidly mixed at 100 rpm for 1 min followed by slow mixing at 20 rpm for 10 min, and followed by a settling period of 15 min. Instantaneous variations of the turbidity were monitored and shown on the computer monitor. The data were processed using the Microsoft EXCEL to plot the curve of turbidity vs. time. Each jar test was done by twice in this study and there was no obvious difference between the repeated tests.

2.4. The relationship between particle size and the standard deviation (SD) of the measured turbidity fluctuations After drying and grinding the organic corn cob and inorganic alumina oxide individually, different size particles of the corn

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Variable Speed Mixer

Detector

3 cm

LED

Probe of Turbidity

Nephelopmetric Turbidity

Tc-Based Data Acquisition Unit

Computer

Fig. 1 – Schematic diagram of the nephelometric turbidimeter set-up.

Before Floc. Formation

After Floc. Formation

Detector

Detector

Path 1

Path 2

LED

LED

NTU

NTU

Path 1

Path 2 Time(sec)

Time(sec)

Fig. 2 – Schematic drawing illustrating the relationship between NTU amplitude and floc size before and after flocculation.

cob and the alumina oxide were separated by screening them with a series of different mesh size sieves. The median value of the particle size range in every sieve number was taken as the representative average particle size for each sample. These different size particles were used to simulate the flocculation floc. Nine sets of different average particle size test water were prepared. For each size of 40.5, 115, 137, 163, 335, 460, 670, 920 and 2180 mm in particle diameter, 0.4 g of corn cob particle was added into 1 L de-ionized water. Similarly, for each alumina oxide with particle size of 68, 89, 104.5, 194.5, 335 and 630 mm, half gram of the aluminum oxide was suspended in 1 L de-ionized water to prepare the test water. Then, all the test water were stirred at a speed of 20 rpm. The turbidity measurement from nephelometric turbidimeter (Fig. 1) was done in every second for 10 min to

yield 600 data. Thus, the SD of the turbidity data for each particle size was found.

5.

Results and discussion

Fig. 2 shows a schematic drawing illustrating the relationship between the turbidity fluctuate and floc size. Before flocs are formed, the colloidal suspension in the water was uniform and the reflected light intensity did not show much variation versus time. The resulting turbidity curve was flat and had a relatively small fluctuate. Agglomeration of colloidal particles greatly reduced the number and the total specific surface area of the suspended particles. When large flocs were illuminated directly as shown by path 1 in Fig. 2, a higher intensity of

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scattered light and turbidity was detected. On the other hand, the agglomerated flocs with lower total specific surface reduced the opportunity of LED light illumination thus leading to a lower intensity of scattered light and turbidity. Because the intensity of the scattered light depends on whether the incoming light illuminates a floc, during flocculation the suspensions with larger but fewer flocs caused the illuminated flocs to become less uniform and the turbidity value to fluctuate with greater fluctuate. Thus, the size of the flocs was proportional to the amplitude of turbidity fluctuations; the SD of the turbidity data was found to be directly related to the size of the flocs. This theory is similar to Gregory and Nelson (1986) indicating that a given suspension can be assumed to be root mean square (VRMS) of the turbidity fluctuating signal is roughly proportional to the size. But, the VRMS value does not provide quantitative information on aggregate size. So, for a given suspension, it can be only assumed that larger VRMS values imply larger aggregate size (Burgess et al., 2002a). In this study, after the comparison of linear regression, it was found that in a different particle size range the linear relationship between the square root of floc size and the SD of the measured turbidity fluctuations has the best R2 value. This was further confirmed by the following tests. The first study was done by using various particle size of corn cob water for the test. The corn cob particle sizes used for preparing test water are 40.5, 115, 137, 163, 335, 460, 670, 920 and 2180 mm. The concentration of the test water is maintaining in 0.4 g/L. Then, using the equipment shown in Fig. 1, each corn cob test water was stirred at a speed of 20 rpm, the turbidity measurement was done in every second for 10 min to yield 600 data. The square root of the average particle diameter was plotted versus the SD based on the 600 turbidity data (Fig. 3) to demonstrate the linear relationship between these two parameters. The second test was carried out with 0.5 g inorganic aluminum oxide suspension with average particle sizes varying from 68 to 630 mm. Fig. 4 also shows the linear relationship between these two parameters for inorganic aluminum oxide suspen-

30.0

Turbidity SD

25.0 R2=0.9937

20.0 15.0 10.0 5.0 0.0

0

10 20 30 40 Square Root of Average Particle Diameter (µm1/2)

50

Fig. 3 – Linear relationship between NTU and the square root of average particle diameter for corn cob suspensions.

3.5

3

Turbidity SD

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R2=0.9955

2.5

2

1.5

1 0

5 10 15 20 25 Square Root of Average Particle Diameter (µm1/2)

30

Fig. 4 – Linear relationship between NTU and the square root of average particle diameter for aluminum oxide suspensions.

sions. The perfect linear relationship between the SD of NTU data and the square root of floc diameter shown in Figs. 3 and 4 indicate that this linear relationship was valid for both organic and inorganic particles as well as a large particle size range. The coagulants PASiC, particularly, proved themselves to be superior to the PACl in the treatment of low turbidity water (Cheng et al., 2008). Thus, the floc size monitoring system was further tested on low turbidity algae samples (10 NTU) and flocculated using 1.1 mg/L of PACl or 1.1 mg/L of PASiC. After 60 s rapid mixing at 100 rpm, samples were flocculated at 20 rpm for 10 min and followed by 15 min settling. The turbidity monitoring results plotted in Fig. 5 show that there was no obvious turbidity fluctuations during the initial rapid mixing period for PACl or PASiC coagulation. During the flocculation period, the amplitude of turbidity data became obvious indicating the agglomeration of particles into flocs. Samples flocculated using PACl had relatively slower changes of turbidity and smaller amplitude than the PASiC-flocculated samples. The magnitude of turbidity amplitude shows that the use of PASiC led to relatively larger flocs that settled faster with better sedimentation in a shorter time during the quiescent period than PACl. All turbidity data collected during the flocculation (slow mixing) period were grouped for every 2 min. The standard deviation for each group that consisted of 120 data points was plotted versus time. As shown by Fig. 6, the turbidity SD increased initially and then became steady indicating that the flocs gradually grew in size during the flocculation period. The steady-state region shows the balance between floc growth and breakage where the floc size distribution no longer changed with time (Hopkins and Ducost, 2003). These results also indicated that the floc formation or SD values appeared to be affected by the slow mixing time; short slow mixing did not favor the formation of large flocs. Therefore, as shown in Fig. 7, when the slow mixing time was reduced from 10 to 5 min, the PACl-treated samples did not show much turbidity fluctuations and the resulting tiny flocs did not

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Rapidly Mixing 20.00 Slow Mixing 18.00

Rapidly Mixing 20.00 Slow 18.00 Mixing

Settling Period

Settling Period PACl PASic

16.00 PACl

14.00

Turbidity (NTU)

Turbidity (NTU)

16.00 PASic

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14.00 12.00 10.00 8.00 6.00

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2.00

2.00

0.00

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0

200

400

600 800 Time (sec)

1000

1200

0.00

1400

Fig. 5 – Variations in turbidity of the samples coagulated with 1.1 mg/L PACl and 1.1 mg/L PASiC with 60 s rapid mixing at 100 rpm and 10 min slow mixing at 20 rpm, and followed by a settling period of 15 min. (The initial turbidity of algae solution is 10 NTU.)

0

200

400

600 800 Time (sec)

1000

1200

1400

Fig. 7 – Variations in turbidity of the samples coagulated with 1.1 mg/L PACl and 1.1 mg/L PASiC with 60 s rapid mixing at 100 rpm and 5 min slow mixing at 20 rpm, and followed by a settling period of 15 min. (The initial turbidity of algae solution is 10 NTU.)

600

2.5

500

PACl PASiC

2

PACl PASiC

1.5 r / r0

NTU SD

400

1

300 200 100

0.5

0

0

2

4

6 Time (min)

8

10

Fig. 6 – Plot of the SD of NTU at various flocculation times for PACl and PASiC.

settle well. Besides, samples coagulated with PASiC for under 5 min slow mixing required a longer settling period than the results in Fig. 5 (slow mixing time 10 min) and also had higher residual turbidity values. This observation not only confirmed that shorter slow mixing time did not favor the formation of flocs but also verified that SD value really was dependent on the floc size. As mentioned in previous sections, the turbidity SD was proportional to the square root of the average floc diameter, r was proportional to the square of SD, i.e. rpSD2, or r/r0 ¼ SD2/SD20 in which r is the floc diameter after flocculation; r0 is the colloidal particle diameter before flocculation; SD is the standard deviation of the turbidity monitored during the flocculation period; SD0 (0.093) is the SD of the turbidity measured during the rapid mixing period. Fig. 8 shows the plot of r/r0 versus the flocculation time for PACl and

2

4

6 Time (min)

8

10

Fig. 8 – Variation of r/r0 at various flocculation times.

PASiC. The curves show that the floc size multiplies several times with increasing flocculation time and that PASiC floc had a greater relative size increase than PACl floc. Thus, the PACl floc had relatively smaller floc sizes leading to poor settling and hence the inefficient removal of turbidity. Additionally, after the slow mixing, the flocs suspended in the supernatant were removed with a pipette and examined with a 20  4 microscope. As shown by the photomicrographs in Fig. 9, the PACl flocs were clearly smaller than the PASiC flocs. Fig. 9A shows the photomicrograph of the minute flocs that did not settle properly, thus causing the high turbidity (about 2.5 NTU) in the supernatant of the sample flocculated with PACl after the sedimentation period (Fig. 5). Conversely, the PASiC-flocculated sample contained only 0.3 NTU in the supernatant after the settling period; the flocs can be seen as complete particles in Fig. 9B. The results demonstrated that the on-line nephelometric turbidity measurement device was simple but

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Fig. 9 – Photomicrograph of the coagulation flocs: (A) PACl and (B) PASiC.

effective in monitoring floc size variation. This provides more detailed experimental data for rapid mixing, flocculation and sedimentation operations. These results can be used to effectively determine the size increase. Besides, results obtained in this study indicated that this nephelometric turbidimeter monitor system may also be used for low turbidity water. Another advantage is that the probe of the nephelometric turbidimeter can obtain the turbidity fluctuating signal directly from the flocculation vessel. In contrast, in the PDA monitor system, a peristaltic pump with plastic tubing is required to continuously sample the flocculation vessel. The peristaltic pump with smaller tubing size may result in excessive shear by flow and cause breakup of flocculated particles (Gregory, 1981). Therefore, this nephelometric turbidimeter technology appears to be a valuable tool for application in water treatment. Furthermore, the nephelometric turbidimeter monitoring system not only can obtain the turbidity change of raw water but also may be used in an on-line monitoring of the state of coagulation enabling automatic control of coagulation in terms of the chemical additives as well as find the optimum operating conditions in water treatment plant.

6.

Conclusion

Laboratory results show that the turbidity data collected with a nephelometric turbidimeter can help measure the size variation of the resulting flocs. Thus, using the nephelometric turbidimeter to collect turbidity data every second provides automatic on-line control of the water flocculation operation reducing possible human error. The fact that the SD of turbidity data is linearly proportional to the square root of the floc diameter assists in predicting the variation of floc size without delay. Hence, the nephelometric turbidimeter monitoring system may be used as a tool to assist determine dosing, effective selection of the coagulant and control of the flocculation conditions then help saving cost in water treatment plant. In addition, some extended study about the suitability of nephelometric turbidimeter monitor system was completed and will be presented in later publication.

Acknowledgement The authors acknowledge the financial support of National Science Council, Taiwan, R.O.C. for this work (NSC-95-2211-E239-033).

R E F E R E N C E S

Biggs, C.A., Lant, P.A., 2000. Activated sludge flocculation: on-line determination of floc size and the effect of shear. Water Res. 34, 2542–2550. Burgess, M.S., Phipps, J.S., 2000. Flocculation of PCC induced by polymer/microparticle systems: floc characteristics. Nordic Pulp Paper Res. J. 15, 572–578. Burgess, M.S., Curley, J.E., Wiseman, N., Xiao, H., 2002a. On-line optical determination of floc size. Part I: principles and techniques. J. Pulp. Paper Sci. 28 (2), 63–65. Burgess, M.S., Curley, J.E., Wiseman, N., Xiao, H., 2002b. On-line optical determination of floc size. Part II: the effect of shear on floc Size. J. Pulp. Paper Sci. 28 (10), 323–326. Bushell, G.C., Yan, Y.D., Woodfield, D., Raper, J., Amal, R., 2002. On techniques for the measurement of the mass fractal dimension of aggregates. Adv. Colloid Interface Sci. 95, 1–50. Byun, S., Oh, J., Lee, B.Y., Lee, S., 2005. Improvement of coagulation efficiency using instantaneous flash mixer (IFM) for water treatment. Colloid Surf A 268, 104–110. Chang, Y., Liu, Q.J., Zhang, J.S., 2005. Flocculation control study based on fractal theory. J. Zhejiang Univ. Sci. 6B, 1038–1044. Cheng, W.P., Li, C.C., Yu, R.F., Chuang, W.C., 2008. A study on the removal of organic substances from low-turbidity and lowalkalinity water with metal–polysilicate coagulants. Colloids Surf A 312 (2–3), 238–244. Fitzpatrick, C.B., Fradin, E., Gregory, J., 2003. Temperature effects on flocculation using different coagulants. In: Proceedings of the Nano and Micro Particles in Water and Wastewater Treatment Conference, International Water Association, Zurich, September. Franceschi, M., Girou, A., Carro-Diaz, A.M., Maurette, M.T., PuechCoste, E., 2002. Optimisation of the coagulation–flocculation process of raw water by optimal design method. Water Res. 36, 3561–3572. Gao, B.Y., Hahn, H.H., Hoffmann, E., 2002. Evaluation of aluminum–silicate polymer composite as a coagulant for water treatment. Water Res. 36, 3573–3581.

ARTICLE IN PRESS WAT E R R E S E A R C H

42 (2008) 2691 – 2697

Govoreanu, R., Seghers, D., Nopens, I., De Clercq, B., Saveyn, H., Capalozza, C., Van der Meeren, P., Verstraete, W., Top, E., Vanrolleghem, P.A., 2002. Linking floc structure and settling properties to activated sludge population dynamics in an SBR. In: Proceedings of the Third IWA World Water Congress, Melbourne, Australia, 7–12 April. Govoreanu, R., Saveyn, H., Meeren, P.V., Vanrolleghem, P.A., 2004. Simultaneous determination of activated sludge floc size distribution by different techniques. Water Sci. Technol. 50 (12), 39–46. Gregory, J., 1981. Flocculation in laminar tube flow. Chem. Eng. Sci. 36, 1789–1794. Gregory, J., 1985. Turbidity fluctuation in flowing suspension. J. Colloid Interface Sci. 105, 357–372. Gregory, J., 2004. Monitoring floc formation and breakage. Water Sci. Technol. 50 (12), 163–170. Gregory, J., Nelson, W.D., 1986. Monitoring of aggregates in flowing suspension. Colloid Surf A 18, 175–188. Hopkins, C.D., Ducost, J.J., 2003. Characterizing flocculation under heterogeneous turbulence. J. Colloid Interface Sci. 264, 184–198. Jarvis, P., Jefferson, B., Parsons, S.A., 2005. Measuring floc structural characteristics. Environ. Sci. Biotechnol. 4, 1–18. Jiang, J.Q., Graham, N.J.D., 1998. Research note; observations of the comparative hydrolysis/precipitation behavior of polyferric sulfate and ferric sulphate. Water Res. 32, 930–935. Kan, C.C., Huang, C.P., 1998. Coagulation monitoring in surface water treatment facilities. Water Sci. Technol. 38 (3), 237–244. Han, M., Kim, T., Kim, J., 2006. Application of image analysis to evaluate the flocculation process. J. Water Supply Res. Technol. AQUA 55, 453–459. Huang, C.P., Chen, G.S., 1996. Use of the fiber-optical monitor in evaluating the state of flocculation. Water Res. 30, 2723–2727. Huang, Y.X., 2005. Simultaneous measurement on particles in solution with size ranging from nm to sub-mm by microscope

2697

light scattering spectroscopy and image analyzing system. Curr. Appl. Phys. 5, 549–552. Mas, S., Ghommidh, C., 2001. On line size measurement of yeast aggregates using image analysis. Biotechnol. Bioeng. 76 (2), 91–98. McCurdy, K., Carlson, K., Gregory, D., 2004. Floc morphology and cyclic shearing recovery: comparison of alum and polyaluminum chloride coagulants. Water Res. 38, 486–494. Rattanakawin, C., 2005. Aggregate size distributions in sweep flocculation. Songklanakarin J. Sci. Technol. 27, 1095–1101. Rossini, M., Garrido, J.G., Galluzzo, M., 1999. Optimization of the coagulation–flocculation treatment: influence of rapid mix parameters. Water Res. 33, 1817–1826. Swift, J.D., Simic, K., Johnston, R.R.M., Fawell, P.D., Farrow, J.B., 2004. Study of the polymer flocculation reaction in a linear pipe with a focused beam reflectance measurement probe. Int. J. Miner. Process. 73 (2–4), 103–118. Walne, P.R., 1974. Culture of Bivalve Molluscs. 50 Years’ Experience at Conwy. Fishing News (Books) Ltd., Surrey, England. Wilen, B.M., Jin, B., Lant, P., 2003. Impacts of structural characteristics on activated sludge floc stability. Water Res. 37, 3632–3645. Yang, H.Y., Cui, F.Y., Zhao, Q.L., Ma, C., 2004. Study on coagulation property of metal–polysilicate coagulants in low turbidity water treatment. J. Zhejiang Univ. Sci. 5, 721–726. Yu, J.F., Wang, D.S., Yan, M.Q., Ye, C.Q., Yang, M., Ge, X.P., 2007. Optimized coagulation of high alkalinity, low temperature and particle water: pH adjustment and polyelectrolytes as coagulant aids. Environ. Monitor. Assess. 131, 377–386. Yukselen, M.A., Gregory, J., 2004. The reversibility of floc breakage. Inter. J. Min. Process. 73, 251–259. Wang, Y., Gao, B.Y., Yue, Q.Y., Zhou, W.Z., Chu, Y.B., 2005. On-line optical determination of floc size of Fe(III) coagulants. J. Environ. Sci. 17, 921–925.