OSMO basic module as a compact system including booster pump and 1000 liters process water storage tank. Can be expanded at any time by further 1,000 liters reservoirs with the following system functionality: Reverse osmosis system for mixing water production from softened or hardness stabilized drinking water intended for pressure less supply into the available reservoir with filling level control via floating switch or pressure sensor. The system has a functional industrial standard type of design and is equipped with easily accessible components.