During the last decade of the 20th century an industrial water treatment plant was designed in order to supply the chemical plant of Dow in Terneuzen with high quality process and demin water. The Dutch Zeeland area is rich of water, but fresh water is scarce. In the pre-design of the plant several raw water sources were evaluated, including two sources of fresh water, effluent of an industrial WWTP, effluent of a communal WWTP and seawater. One of the main objectives of the plant's design was a reliable concept, using redundant sources and redundant production lines. Therefore the plant was designed to use different water sources and different treatment processes: seawater (from the Westerschelde estuary) and integrated membrane system (IMS) to produce demin water; fresh water and ion exchange to produce demin water; effluent industrial WWTP and media filtration to produce cooling tower supply water. The construction of the various water treatment lines started in 1998, with the plants being taken into operation in the year 2000. The design and construction were handled in a joint venture. The complete water treatment facilities produce an aggregate of 750 m(3)/h dernineralised water (demi water), 650 m(3)/h cooling tower supply water and 1.050 m(3)/h ultra-pure water (polished water). All water treatment facilities are now 100% owned by Evides. The IMS is equipped with micro strainers and microfiltration as pretreatment and a double pass RO-system to produce demin water. The raw water for the RO-train is taken from the cooling system from Dow. The source water for the cooling system is extracted via an open intake from the tidal Westerschelde estuary and is next to ships and barges pulling in and out of the dock, stirring up the bottom of the sea. The TDS level in the intake can fluctuate widely, depending on the season, the tides and the amount of water from the North Sea and the River Schelde. The conductivity of the water ranges between 30,000 and 45,000 mu S/cm, and changes between these extremes in a matter of days. The temperature of the water is increased approximately 10 degrees C in the cooling system. The main advantages of this water source were: intake and infrastructure already available: reduces costs for feed water intake; high temperature seawater: reduced seawater RO feed pressure; abundant availability: higher reliability. The use of preheated seawater from the tidal estuary as feed water to the IMS has resulted in many unsuspected problems in operation and maintenance of the IMS. Problems like corrosion and biofouling are enhanced by the higher temperature of the seawater. Another problem was the high turbidity loads in the water caused by the ships and barges at the seawater intake. The paper focuses on five years operational experience with the reverse osmosis train. The problems will be elaborated with operational and process trends and the solutions to solve the problems, in some cases extensive alterations of unit operations, will be presented. Based on the experiences in the past five years the choice of seawater as feed water for the RO-train is under reconsideration.