Organic and inorganic polyelectrolytes are commonly used as coagulants or flocculants in membrane bioreactor to enhance the filtration properties of membranes. The main objective of this study was to determine the effectiveness of organic cationic polyelectrolyte (CPE) addition on filterability of various membranes and to investigate the influences on fouling of membranes. First, batch tests were performed to determine the optimum polyelectrolyte concentration at which the highest soluble microbial products (SMPs) removal was observed. Then, short-term filtration test were conducted. Cellulose acetate, polyethersulfone, mixed ester, polycarbonate (CA, PES, ME, PC) membranes with two different pore sizes (0.45-0.40 and 0.22-0.20 mu m) were used in submerged MBR experiments. Permeate flux decline at constant vacuum was observed with time. Protein and carbohydrate analysis were performed in the permeate samples. Filtration resistances were determined for each membrane. Cake resistance was observed to be the most significant fouling mechanism for all membranes. Polyelectrolyte addition decreased the cake resistance. The effect of polyelectrolyte on pore resistance was relatively insignificant. The results showed that addition of polyelectrolyte had significant impact on sustainable filtration time. (C) 2011 Elsevier By. All rights reserved.