A filtration unit composed of two reverse osmosis membranes in series was evaluated for haemodialysis water production. For this purpose, hard and soft water were formulated based on European drinking water directives. The study showed that the double reverse osmosis process is efficient to produce haemodialysis water with the quality defined by the pharmacopeia requirements from synthetic soft water and hard water. In both cases, nitrate and sodium ions diffuse through the membrane limiting the water treatment. This phenomenon is correlated to the electrostatic interactions, the Donnan effect and the ion diffusivity, which govern the membrane selectivity.