The water quality of effluent from municipal wastewater treatment plants (WWTP effluent) has improved during the last decades. Due to more stringent EU regulations, WWTP effluent will improve even more in the coming years. Since 1997, extensive pilot-plant tests have been performed in the Netherlands in order to enhance the technological possibilities for further improvement of the effluent water quality. This paper deals with the results of the experiments on dead-end ultrafiltration (UF) of WWTP effluent. Much is unclear about the mechanisms that cause the membranes to foul heavily, and most studies seem to focus on fouling problems. Most problems in UF systems start with filtration problems, then leading to fouling problems. The filtration phenomena have been examined both in pilot and laboratory-scale studies. The filterability of the WWTP effluent was investigated in well-defined pilot-scale experiments, and the influence of pre-treatment and cleaning strategies was evaluated. It was found that the filtration characteristics could be measured by a new filtration parameter, specific ultrafiltration resistance (SUR). The SUR proved to be a good parameter for prediction of filtration characteristics. The most important filtration mechanism was found to be cake filtration. The particles of major influence are about ten times larger (between 0.1 and 0.2 mu m) than the pore diameter (10-30 nm), for WWTP effluent both with and without pre-treatment. This presentation focuses on a general description of the SUR test and on the experience in using the SUR test as a tool for optimisation of UF installations.