A reverse osmosis water purification apparatus includes a housing sump, a protective canister disposed within the sump, a filter assembly, an outwater conduit and a manifold disposed within the canister, a valve cap mounted atop the manifold and operably related therewith and including a mounting bracket arranged for securement to a fixed support, a top cap disposed atop the valve cap, an inlet for unfiltered water, an outlet for filtered water together with an outlet for unfiltered water mounted on the valve cap, a rotatable collar threadedly coupled with the housing sump and cooperating with the valve cap to hold the parts in assembled condition or to render the housing sump, canister, filter units, outwater conduit and the manifold separable from the valve cap and parts associated therewith so as to enable the canister and its contents to be removed and replaced with new units without requiring the services of a skilled technician.