Effects of alum addition on the treatment performance of cloth-media membrane bioreactor were investigated. At Mixed Liquor Suspended Solids (MLSS) of 3.5 g/L and permeate flux of 1 6.3Ll/m(2).h, the addition of alum at 7.5 mg Al-2(SO4)(3)/L (R2) and 15 mg Al2(504)3/L (R3) enhanced phosphorus removal from 45% in control with zero alum (R1) to 58 and 69% in R2 and R3, respectively. Alum did not affect the removal efficiency of other treatment parameters which record an average of 99% for turbidity, TSS and ammonia, an average of 96% for COD and an average of more than 97% for TKN. Alum addition also significantly reduced sludge volume index (SVI) and particle size distribution (PSD) range and inhibited growth of filamentous bacteria. The results show that the best range for the particle size which reflects good settling properties is between 35-55 and 240-290 mu m. In the batch experiments, alum doses above 60 mg/L have toxic effect on the autotrophic bacteria with significant reduction in ammonia oxidation and TKN removal. Under the operating condition of vigorous aeration of sludge biomass the addition of alum did not affect the flocculation process in the aeration tanks. (C) 2012 Elsevier B.V. All rights reserved.