Presented at the conference on Membranes in Drinking and Industrial Water Production, L’Aquila, Italy, 15–17 November 2004. Organized by the European Desalination Society. 0011-9164/05/$– See front matter © 2005 Elsevier B.V. All rights reserved Desalination 179 (2005) 117–124 Membrane fouling caused by extracellular polymeric substances during microfiltration processes Chackrit Nuengjamnong a,b , Ji Hyang Kweon c , Jinwoo Cho b , Chongrak Polprasert a , Kyu-Hong Ahn b* a Asian Institute of Technology, PO Box 4, Klong Luang, Pathumthani 12120, Thailand b Korea Institute of Science and Technology, PO Box 131, Cheongryang, Seoul 130-650, Korea Tel. +82 (2) 958-6838; Fax. +82 (2) 958-6854; email: khahn@kist.re.kr c Department of Environmental Engineering, Konkuk University, 1 Hwayang-dong, Gwangjin-gu, Seoul, 143-701, Korea Received 22 September 2004; accepted 22 November 2004 Abstract The effects of both extractable extracellular polymeric substances (EPS) and the supernatant of sludge flocs on the membrane fouling in submerged membrane bioreactors (SMBRs) were invesetigated. Three laboratory-scale SMBRs were operated at a constant permeate flux (12.5 L/m 2 h) with a flat-sheet microfiltration membrane (hydrophilic polyolefin, pore size of 0.25 µm) at different solids retention times (8, 20 and 80 days, respectively). The concen- trations of extractable EPS (i.e., extracted by the cation-exchange resin method) and dissolved organic carbon (DOC) in the supernatant at the steady-state period were compared. The results showed that as SRT increased, the organic carbon content in extractable EPS decreased, whereas DOC in the supernatant tended to be independent of SRT. Batch filtration tests were conducted to determine the specific cake resistances of the fouling layer using both the raw sludge and the twice-washed sludge with buffer solution. The supernatant contributed approximately 50% to the total specific cake resistance. The organic carbon and protein content in the extractable EPS decreased with increasing SRT, and protein content in the supernatant also had the same tendency. Therefore, the supernatant played a relatively significant role in membrane fouling. Keywords: Submerged membrane bioreactor; Fouling; Extracellular polymeric substances; Specific cake resistance; Cation-exchange resin *Corresponding author.