A self-contained, transportable reverse osmosis water purification unit has a limited throughput of no more than about 20 gpm (88 l/m) of permeate in a single pass operation with reverse osmosis ("RO") modules which can be cleaned in place. When raw water is particularly fouled, the unit is operated in a double pass operation in which the permeate from the first pass is used as feed for a second RO module. The unit, referred to as a "purification container" "PC" or an Air Deliverable Reverse Osmosis Water Purification Unit "ADROWPU" because the PC is air-deliverable, comprises an enclosed weathertight housing having a base on which the major components are removably mounted. For transportability, the PC has the overall dimensions of an ISO container, and is fitted with means to load and unload it on standard carriers. A high pressure pump is required to feed water freed from suspended solids to the RO modules. The pump also delivers concentrate to a bladder tank where the concentrate is stored under pressure (about 100 psig, about 6.9 bar) to flush a coarse filter automatically when it is plugged, which is often. Fine filters for solids removal are not plugged too often and can be replaced when the unit is not producing permeate. The ADROWPU is unique because of the economy of components used to produce a relatively large amount of permeate from any available raw water source. The placement of the components symmetrically about the center of mass of the PC is essential to allow the PC to be loaded and unloaded uneventfully, even if it is delivered by a helicopter.