Abstract The U.S. Army Mobility Equipment Research and Development Command is developing transportable reverse osmosis (RO) equipment to produce potable water from fresh, brackish and sea water sources and to remove chemical and radiological contaminants from these waters. Of the various water purification processes available the Army has determined that reverse osmosis has the greatest potential for achieving this multi-purpose function. A 600 gallon per hour (gph) trailer mounted unit, the smallest unit in a family of equipment to be developed, has undergone field testing producing potable water at rated capacity from the turbid Potomac River, a brackish well supply, and the Atlantic Ocean. In addition, the unit has successfully removed three toxic chemical agents from a natural water source. Principle components of the unit are a multi-media filter using the addition of cationic polyelectrolyte to aid preclarification of the water and 10 pressure vessels containing 20 spiral wound reverse osmosis elements. Special dry cellulose acetate membranes have been developed that have been found successful on fresh and brackish water operations in which the feed water is prechlorinated. The CA elements are wet-dry reversible so that the unit may be completely drained for transport and freezing environments. Replacement or spare elements may be stored in a dry condition without impairing membrane performance. In addition, a physically interchangeable dry polyamide (PA-300) RO element has been successfully used to produce drinking water in a single pass from sea water.