Apparatus and method for purifying a solvent such as water, using a reverse osmosis process. The apparatus includes a reverse osmosis (R.O.) unit, a storage tank for storing permeate produced by the reverse osmosis unit, and a flushing arrangement for cleansing the concentrate side of a reverse osmosis membrane at the conclusion of a purifying cycle. A flow regulating arrangement 34, 40, 42 maintains one of two pressures at the concentrate output. A system controller controls the communication of feed water to the R.O. unit and the concentrate pressure. A flushing accumulator receives permeate during a purifying cycle. When the cycle is terminated, the permeate stored in the accumulator is transferred to the input side of the R.O. unit and flushes the membrane. The permeate is driven from the accumulator by a feed water generated force exerted on the permeate by an isolated pressurization chamber forming part of the accumulator. According to a feature of this embodiment, the R.O. unit, the flushing accumulator and the system controller are integrated in a unitary package which is relatively maintenance free.