Abstract: Water purifying apparatus comprises a purified water storage vessel and a filter, preferably a reverse osmosis filter, for purifying water. An inlet side of the filter is adapted for connecting to a pressurized source of water to be treated and an outlet side of the filter is connected to an inlet of the storage vessel. A purified discharge line is also connected to the vessel inlet. The vessel is configured for being pressurized by a gas, such as air, in a manner that the pressurizing gas is in direct contact with purified water in the tank. A gas check valve is installed at the vessel combination inlet and outlet, the check valve comprising a spherical valve which floats in water and a valve seat against which the valve seals when most of the water has been withdrawn from the vessel, so as to prevent escape of pressurizing gas from the vessel.