The removal mechanism of E. coli from UASB effluent using a Rotating Biological Contractor (RBC) has been investigated. Preliminary batch experiments in a RBC indicate a first-order removal kinetics. Variation in the dissolved oxygen concentration and E. coli counts over the depth of the RBC has been recorded and indicates that the RBC is not a completely mixed reactor. Therefore batch experiments were carried out in a beaker where the different operating conditions can be controlled. Factors affecting the removal of E. coli via a biofilm system as stirring, dissolved oxygen concentration, pH, and addition of cationic polymer were investigated. The results obtained indicated that the most important removal mechanism of E. coli in the biofilm is the adsorption process, followed by sedimentation. Die-off is a relatively minor removal mechanism in an RBC system. Higher removal rate of E. coli was observed in an aerobic compared to an anaerobic biofilm system. Variation of dissolved oxygen concentration (3.3-8.7 mgl(-1)) and pH-values between 6.5 and 9.3 did not exert any significant effect on the removal rate of the E. coli by the heterotrophic biofilm. A rapid adsorption of E. coli to the biofilm occurred during the first days after adding the cationic polymer, after which the adsorption slowed down.