At the Heemskerk water treatment plant of N.V. PWN Water Supply Company North Holland the first full scale integrated membrane system in the Netherlands is realized. The 20 Mm(3)/y (15 MGD) ultrafiltration (UF)/reverse osmosis (RO) plant servicing IJssel Lake water is in operation since the second half of 1999. Initially coagulation, sedimentation and filtration (CSF) were studied as RO pretreatment. Membranes had to be cleaned every 400 h. By CSF/UF pretreatment a much better RO feed quality was obtained. With CSF-pretreated feed water initially UF performance was very good. In periods of algae bloom enhanced backwash with hydrogen peroxide was insufficient and had to be replaced by sodium hypochlorite backwash. On long term adsorption of macromolecular organics, i.e. coagulant aids may cause permanent fouling. Therefore in the full-scale application GAC filtration is applied to the UF-feed. The full scale plant run smoothly for the first 200 days. During the first 2000 h operation of the RO pilot plant MTC was irreversibly decreased by 25%. During the following 5000 h the MTC of stage 1 was stable, in stage 2 a MTC decrease together with a NPD increase took place caused by a combination of BaSO4 scaling and biofouling. After optimization of the CSF/UF system and an antiscalant dosage the pilot plant run very smoothly. The first 200 d of full-scale operation confirms the pilot plant data. System performance is excellent, quality objectives are achieved and membrane integrity is very satisfying.