A process and apparatus for treating contaminated water involving passing water containing contaminants as a stream through a filter to remove the contaminants and form a purified water stream, directing the purified water stream to an effluent discharge outlet and accumulating a portion of the purified water in a reservoir under pressure, preferably through action of a trapped air pocket in the reservoir, and regenerating the filter by using the purified water in the reservoir to back-flush the filter.