Currently, the WAG German Wassergewinnungs- und aufbereitungsgesellschaft Nordeifel builds an extension of their existing water work (WW) at Roetgen/Aachen. The extension consists of an ultrafiltration (UF) membrane plant with a capacity of 6.000 m(3)/h of drinking water. This makes it the largest membrane plant at present for the purpose of drinking water treatment in Germany. Furthermore, a second UF stage with a capacity of 600 m(3)/h for the treatment of the backwash water of the first stage is being built. The new treatment line is innovative and advanced in using membrane technology for the treatment of surface water, not only because of its size. Numerous intensive preliminary investigations were carried out. Among other things the investigations showed that the membrane process will be most efficient if flocculated raw water is used.