Journal of Membrane Science 216 (2003) 217–227 Sludge characteristics and their contribution to microfiltration in submerged membrane bioreactors Wontae Lee a , Seoktae Kang b , Hangsik Shin c,∗ a Department of Civil and Environmental Engineering, Arizona State University, Tempe, AZ 85287-5306, USA b Department of Chemical and Environmental Engineering, University of California, Riverside, CA 92521, USA c Department of Civil and Environmental Engineering, Korea Advanced Institute of Science and Technology, Daejon 305-701, South Korea Received 12 December 2002; received in revised form 16 December 2002; accepted 10 February 2003 Abstract This study focused on the physicochemical and biological characteristics of sludge in submerged membrane bioreactors (MBRs) at various sludge retention times (SRT) and their effect on microfiltration and membrane fouling. Three lab-scale submerged MBRs at SRT of 20, 40, and 60 days were performed at a constant permeate flux of 9l/(m 2 h) with hollow fiber microfiltration membranes (hydrophilized polypropylene; nominal pore size = 0.4 m). In order to evaluate the rela- tive contribution of microbial floc and supernatant to the membrane fouling, supernatant was separated from the sludge by centrifugation at 366 rad/s for 5 min. Batch filtration experiments with the same configuration showed that the relative contri- bution of supernatant to overall membrane fouling was higher at SRT of 20 days (37%) than at SRT of 40 (28%) and 60 days (29%), whereas the overall fouling resistance increased as SRT prolonged. Hydrophobicity (correlation coefficient, r = 0.86; significant value, P< 0.05), surface charge (r = 0.87, P< 0.05) and microbial activity (r =-0.87, P< 0.05), which were related to composition and properties of extracellular polymeric substances (EPS), appeared key parameters relating to fouling by microbial floc. However, no remarkable factor was found in fouling caused by supernatant. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Membrane bioreactor; Microfiltration; Membrane fouling; SRT; Sludge characteristics 1. Introduction By replacing a secondary clarifier by a membrane separation unit in an activated sludge process, mem- brane bioreactor (MBR) technologies have emerged as one of the innovative and promising solutions for wastewater treatment and reclamation. MBRs use ultrafiltration and/or microfiltration membranes for the complete retention of sludge. This leads to an increased microbial concentration in the reactor ∗ Corresponding author. Tel.: +82-42-869-3613; fax: +82-42-869-3610. E-mail address: hangshin@kaist.ac.kr (H. Shin). and an improved biological reactor operation with reduced sludge production, persistence to high or shock loadings, and the effective separation of bac- teria and viruses when ultrafiltration membranes are used [1,2]. However, membrane fouling is a main ob- stacle to the wide application of MBRs, which causes declining permeate flux and increasing operation costs [3]. Some previous investigators reported that the mem- branes in MBR were more fouled at higher sludge concentration [4,5], while others suggested that higher sludge concentration resulted in less fouling under cer- tain conditions [6,7]. This conflict implies that mem- brane fouling is related to not only sludge quantity 0376-7388/03/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0376-7388(03)00073-5