Characterization of membrane foulants in drinking water treatment

Membranes are very effective in removing a wide variety of water contaminants. Therefore, the use of these processes in water purification to replace or to improve conventional treatment has increased. An inherent problem of membranes is fouling, the accumulation of materials (foulants) near, on, or within the membrane that causes a reduction in the amount of product water over time. As a result of fouling, capital and operating costs of membrane systems are higher, making them less attractive. In this project the effect that several operating conditions have on the characteristics of the foulant layer is evaluated. The tested operating conditions included biofiltration pretreatment and permeate flux. Commercial low-pressure membranes were employed and characterized according to their physicochemical properties before and after being fouled with organic matter–rich feed solutions. The foulant layer was analyzed in terms of its chemical and microbiological characteristics, aided by microscopic images (scanning electron microscopy, SEM and atomic force microscopy, AFM). Since the commercial membrane employed in this study was a loose UF membrane, most of the material encountered in the foulant layer was in particulate form. Pretreatment decreased the amount of organics and microorganisms deposited on the membrane. However, similar microbial species were found in the foulant layer of membranes with and without pretreatment.

The following conclusions can be drawn from this bench scale study involving the effect of operating conditions on membrane fouling: C The commercial UF membrane alone mainly removed organics in particulate form in agreement with the sieving curve. C While the biofiltration pretreatment had a significant effect on the amount of organics deposited on the membrane, the permeate flux did not. C Microbial species that commonly form biofilms in water distribution systems were found in both the feed solution and the membrane foulant layer. Acinetobacter was the most common of these species. C Heterotrophic plate counts in the feed solution and the foulant layer of the membranes with biofiltration pretreatment were lower than those on membranes without pretreatment. No effect of the permeate flux was observed in the microbial counts. C SEM images showed substantially more debris on top of and potentially within the foulant layer for the membrane without pretreatment than was the case for the membrane with pretreatment. Acknowledgments Funding for this project was provided by the Natural Sciences and Engineering Research Council of Canada (NSERC) in the form an Industrial Research Chair at the University of Waterloo and a Postdoctoral Fellowship to the first author. The current Chair partners include: AWS Engineers and Planners Corp., Brightwell Technologies Inc., the City of Brantford, the City of Guelph, the City of Ottawa, the City of Toronto, Conestoga-Rovers & Associates Limited, EPCOR Water Services, the Ontario Clean Water Agency (OCWA), PICA USA Inc., RAL Engineering Ltd., the Regional Municipality of Niagara, the Regional Municipality of Waterloo, Stantec Consulting Ltd., and Zenon Environmental Inc. The authors would like to thank Zenon Environmental Inc. for providing the membrane modules used in this research. Special thanks to Dr. Onita Basu, Dr. Michele Van Dyke, and Dr. Kirsten Exall for their valuable support to this study. We would also like to thank Dr. Shijie Wu for his help with the AFM images.