This paper describes the application of a multiple step membrane train for the processing of ground and surface waters contaminated with ammonium sulfate. The key to the process is the unique ability of nanofiltration to separate mono and divalent ions such that the concentrated stream can be forwarded to an evaporator. The paper outlines the problems to be solved, the initial separation concept, the plant design, unique design features and the experience gained over a decade of operation. The process utilises microfiltration, nanofiltration and reverse osmosis.