ELSEVIER Desalination 167 (2004) 23-26

DESALINATION

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Study of reservoir water treatment by ultrafiltration for drinking water production

Shengji Xia a*, Xing Li b, Ruiping Liu a, Guibai Li ~ aSchool of Municipal and Environmental Engineering, Harbin Institute of Technology, P.O. 2 79, Huanghe Road,

150090 Harbin, China Tel. + 86 (451) 86842032; Fax + 86 (451) 82306643; e-mail: xiashengji@yahoo.com

bSchool of Architecture and Civil Engineering, Beijing University of Technology, Beijing, China

Received 12 January 2004; accepted 25 January 2004

Abstract

Ultrafiltration (UF) of reservoir water for drinking proposes was performed in a crossflow filtration with frequent membrane backwash. The effects of operating condition and raw water quality on UF behavior were investigated. The membrane flux decreases with increasing turbidity and there is a somehow linear relationship between membrane flux and the log of turbidity. Permeate flux increase with increasing the transmembrane pressure (TMP) and the permeate flux increase slightly when the TMP is added to a certain level. Chemical analyses of the permeate water showed that drinking water regulations were fulfilled. UF treatment provided effective turbidity removal. UF membrane also perfectly removes all coliform bacteria. With the exception of one weak positive effect in raw water, the genetoxic activities of raw water and ultrafiltrated water samples are negative.

Keywords: Ultrafiltration (UF); Drinking water production; Reservoir water; Ames test

1. Introduct ion

In recent years, membrane ultraffltration 0.IF) of surface water for drinking water treatment has become a more attractive technology world- wide as a possible alternative treatment to con- ventional clarification. In China, surface water is

*Corresponding author

seriously polluted in some regions in which the advanced treatment facility should be construc- ted to produce safe drinking water. Compared to the conventional treatment, membrane filtra- tion offers several advantages such as no need of chemical agents, good quality of produced water, less production of sludge, compact pro- cess and easy automation [1,2]. In this paper we

Presented at the EuroMed 2004 conference on Desalination Strategies in South Mediterranean Countries: Cooperation between Mediterranean Countries of Europe and the Southern Rim of the Mediterranean. Sponsored by the European Desalination Society and Office National de l'Eau Potable, Marrakech, Morocco, 30 May-2 June, 2004.

0011-9164/04/$- See front matter © 2004 Elsevier B.V. All fights reserved

doi;10.1016/j.desal.2004.06.109

24 s. Xia et al. / Desalination 167 (2004) 23-26

report the results of UF experiments with poly- acrylonitrile (PAN) membranes and fed with raw water drawn from Binxian Reservoir located near Harbin in China. The UF performance was evaluated in terms of flux and quality of treated water, specific tests were conducted in order to investigate the behavior of permeate flux as a function of TMP pressure and feed water quality.

2. Experimental

2. 1. Raw water characteristics

Binxian reservoir water was used as raw water and the qualities of raw water are given in Table 1.

Table 1 Average characteristics of Binxian reservoir water

Parameter Raw water

Temperature, °C 15 pH 7.31 Turbidity NTU 23 COD mg/1 5.3

2.2. The process

Fig. 1 shows the schematic diagram of file UF facility used for the treatment of reservoir water. Two pumps are used for feed and back- wash, respectively. Reservoir water was prefil- trated to 200um and injected in the raw water tank for UF. Cross flow filtration inside the hol- low fiber module divides feed into permeate and retentive which were recycled to the raw water tank.

n n v w ~ f~lptunp ~ g st~'age tsnk pump lank

Fig. 1. Flow diagram of the apparatus for UF and back- wash.

The test apparatus had flow, pressure and temperature indicators and had the capabilities of automatic backwash. Backwashing was per- formed by pumping the permeated water from the storage tank to the shell side of the hollow fiber membranes in the module. After washing the fouling from the membrane, the water was discharged from the recirculation loop.

2.3. Membrane characteristics

The membranes used in this study were hol- low fiber UF membrane. They were made of polyacrynitrile (PAN) with nominal cut-off values of 50000 Dalton. The flow of the feed water was from inside of the fiber to the outside of the fiber. The capillaries could operate over a pH-range of 4-9 and had internal and external diam. of 0.9mm and 1.5mm, respectively.

3. Results and discussion

3.1. Effect o f raw water on permeate f lux

The behavior of the permeate flux as a func- tion of feed water turbidity can be described through the classical mass transfer model [3]. The effect of raw water quality on UF behavior is shown in Fig. 2, where the measured flux is plotted against the turbidities in feed water. We can see from Fig. 2 that there is a straight- forward relationship between permeate flux and log (turbidity) for both runs. The different slopes of the two straight lines are related to different

20~

180

14o

120

100

gO 10

a'larl~Uty O¢IX~ 100

Fig. 2. Effect of the feed water turbidity on permeate f lux.

S. Xia et aL / DesaBnation 167 (2004) 23-26 25

specific resistance due to fouling. Such a rela- tionship may allow us to predict the level of flux decline and possibly to switch to a suitable backwashing condition.

3.2, Effect of TMP on permeate flux

UF is a pressure-driven process. Fig. 3 shows the effect of the TMP (TMP = [module inlet pressure + module outlet pressure]/2-permeate pressure) on permeate flux for the PAN mem- brane used in our research. The permeate flux increase with the TMP increasing. As can be seen, at TMP < 0.12MPa, the permeate flux increased linearly with TMP. This is Darcy's law region where the permeate flux is limited by the membrane permeability. The TMP and permeate flux relationship started to deviate from linear when the permeate flux was above 145Lm-2h -1. The permeate flux slightly increases with TMP increasing from 0.12Mpa-0.16Mpa. This is a UF region where the permeate flux is independent of the membrane resistance and limited by the mass transfer condition in the boundary layer.

200

~ 170

14o

~ 110

8o

50 0.05

I I I 1 t

0.07 0.09 0.11 0,13 0,15 0.17 Trcmmembrane pr~sure (MPa)

Fig. 3. Permeate flux as a function of TMP.

3. 3. Permeate quality

Chemical analyses of the permeate water showed that drinking water regulations were ful- filled. As far as microbiological analyses are concerned, the UF membrane appears as a dis- infection step by removing all coliforms. During

1 2 3 4

raw water [] traditional treatment

~ -~i ulwafiltmttion

N N N

5 6 7

Fig. 4, Different treatment on turbidity reduction.

the investigation period, turbidity was used to monitor the membrane separation efficiency. Fig. 4 shows the turbidity of raw water, ultra- filtrated water and the water produced by Binxian Drinking Water Plant, where our test facility was located, using conventional treat- ment: coagulation, sedimentation and sand fil- tration. From Fig. 4 we can see that although the turbidity of raw water varies a lot with seasons, the permeate turbidity was below 0.2NTU 100% of the time. It can be concluded that there is no correlation between the feed and permeate tur- bidity and the reduction in turbidity of UF was satisfactory.

3.4. Ames test

The Ames (Salmonella/microsome) test is widely used as a screening test for the evalu- ation of chemicals for mutagenicity [4,5]. In this study, samples of raw water from the Binxian reservoir, and treated water by UF were concen- trated using XAD resins. Extracts of organic material derived were tested for the induction of gene mutation using the salmonella/mammalian mierosome assay. The Ames test was done without in-vitro mierosomal activation (s9mix) because most of the natural mutagens are direct acting and do not require activation [6]. Table 2 shows the salmonella test date from two water samples extracted for strain TA98 and TA100. In the direct assay, positive activity is observed in the raw water extracts, where extracts of the raw water show a dose-related effect.

26 S. Xia et al. / Desalination 167 (2004) 23-26

Table 2 Analysis of mutagenicity of water samples

Samples Dose, ul/plate TA98 TA 100

PUp + SD a MI b PUp+SD MI

Raw e

Ultrafiltrated d

Solvent control e Positive control r

1 53 +4 1.2 113 + 19 0.98 3 59 + 11 1.35 123 + 19 1.07 5 67 + 9 1.53 123 + 16 1.07 7 97 + 10 2.22 g 125 + 11 1.08 1 42 + 7 0.95 124 + 24 1.08 3 49+7 1.11 118+ 14 1.03 5 64 + 9 1.45 116 + 14 1.01 7 65 + 10 1.47 123 + 21 1.07

44 _+ 5 115 + 21 1462 33.2 1356 11.7

aRevertants/plate + standard deviation; bmutagenesis index, number of revertants obtained in the sample/number of revertants of the negative control; Cextract of the raw water; dextract of the ultrafiltrated water; edimethyl sulfoxide; fdexon, 60ug/plate; gwhen MI > 2 with a dose-response effect is considered to be positive

4. Conclus ion

UF of reservoir water is very useful for drink- ing water treatment. The membrane flux de- creases with increasing turbidity of feed water. A linear relationship between permeates flux and log (turbidity) is observed in our investiga- tion. UF is a pressure-driven process. Permeate flux increase with increasing the TMP. Because o f the fouling on the membrane surface, the permeate flux increased slightly when the TMP was up to a certain pressure.

This study indicated that direct UF of reser- voir water can successfully produce potable water of an acceptable quality. Contrary to con- ventional water treatment, UF technology shows a good property to remove particles and turbi- dity and with permeate quality of <0.2NTU. In the Ames test, positive activity is observed in the raw water extracts, where extracts of the raw water show a dose-related effect and no gene- toxicity was observed for the membranes per- meate water.

Acknowledgements

The authors wish to acknowledge Mr. Zhou Bin for performing part of the experiments.

References

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[5] B.N. Ames and J. McCann, Methods for detecting carcinogens and mutagens with the Salmonella/ mammalian microsome mutagenicity test, Mutat. Res., 31 (1975) 347-363.

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