Currently the IWW-Rheinisch-Westfaelisches Institut fuer Wasserforschung gemeinnuetzige GmbH, an Institute at the University of Duisburg, the Institute of Hygiene of the University of Bonn and the Wasserwerk des Kreises Aachen (WdKA) are realising a project supported by the German Federal Ministry for Education, Science, Research and Technology (BMBF). This project is to investigate whether membrane filtration (micro- or ultrafiltration) is able to enhance the particle removal efficiency of an existing drinking water treatment plant. The first step of the total investigation programme was to operate three different pilot plants in parallel with a capacity of about 10 m(3)/h each. This paper presents results concerning the pilot plant investigations.