This article is concerned with the water network synthesis problem. We propose a superstructure that consists of three layers similar to a pooling problem: sources for reuse/recycle; regenerators for contaminants removal; and sinks for acceptance of water for direct or regeneration-reuse/recycle. The superstructure accounts for membrane separation-based regenerators such as ultrafiltration and reverse osmosis. Linear models are developed for the membrane regenerators. A MINLP is then formulated based on the superstructure to determine the optimal interconnections in terms of total flowrates and contaminant concentrations. Computational experiments are performed using GAMS/BARON on an industrial case study of a petroleum refinery water system. We include model-tightening linear logical constraints to enhance solution convergence, resulting in a globally optimal water network that promotes 27 savings in freshwater use. © 2012 Elsevier B.V.