The cooling water system of a chemical plant of Akzo Nobel is a partly open system. The site is located at the North Sea. The air in contact with the cooling water contains seawater droplets dissolving and increasing the chloride concentration. The cooling water contains chromate to protect the installation for corrosion. Nevertheless, the chloride concentration in the cooling water may not exceed 50 ppm. To maintain a constant chloride concentration level, a part of the cooling water should be drained and replaced by de-mineralized water. However, the drainage of chromate is limited by governmental regulations. This paper describes a systematic approach from problem definition to the membrane process design encompassing membrane module pre-selection, lab-scale proof of principle and on-site pilot demonstration, the latter to characterize the fouling and scaling potential of the feed solution. Finally, the paper discusses various membrane process design options.