Abstract: An apparatus for purifying drinking water by a combination of reverses osmosis, deionization by ion exchange resins, resin and final passage through activated carbon. The components for reverse osmosis, de-ionization, and passage through active carbon, and a product water reservoir are housed in a cabinet suitable for placement in a home. Ionic impurities are removed by reverse osmosis through a cellulose acetate membrane which feeds the flux water to a mixed resin bed for deionization. Waste brine is removed from the reverse osmosis module by controlled leakage to maintain concentration at a low level. The product water is filtered through a cartridge of granular activated carbon prior to being stored in a product water reservoir. An attachment is connectable with the tap water faucet to provide tap water to the unit at main water supply pressure and to dispose of waste brine water and excess product water. Components are replaceable and available on the market.