The inventions relate to purifying water and can be used in domestic water purification systems. The claimed filtered water storage unit for the storage device of a water purification system includes a housing, an elastic chamber for filtered water, and a region for pressurizing untreated water. The water purification system includes a reverse osmosis membrane, a clean water discharge pipe, an overflow into a drain, a clean water tap, and a storage device having a filtered water storage unit and a hydro-automatic unit. The outlet of the controlled chamber of a valve is associated with the water pressurizing region in the housing of the water storage unit, and the input of the controlling chamber of a valve is associated with the clean water discharge pipe upstream of the tap, wherein the valve for controlling water pressurization is normally open and the valve for controlling drainage is normally closed.