“All in one” unit in a cabinet with lockable door and observation window. All components (pretreatment, water softening, reverse osmosis, electro-deionisation, pure water tank with level control and pressure booster) are integrated and premounted ready to connect. Multilingual microprocessor controller with LCD display for controlling and monitoring pure water systems. Display of permeate and pure water conductivity, fully automatic rinsing cycles, potential-free fault reporting relay. Module-based capability of increasing capacity at a later stage.