Water Factory 21 product water must have a total dissolved solids (TDS) content of 500 mg/L or lower. Demineralization of a third of the plant's process water is achieved in the reverse osmosis (RO) process. In addition to removal of TDS, RO is very effective in reducing other minerals, ammonia, and total organic carbon (TOC).

Six million gallons per day enters RO treatment after leaving the multi-media filter beds. Before the water enters the RO membranes, it receives additional pretreatment consisting of antiscalant addition; sulfuric acid addition to lower the pH to 5.5; and cartridge filtration.

Operation of the RO unit requires pressures of 200-325 pounds per square inch (psi) produced by two high-pressure pumps. The RO unit is designed as two parallel 2.5 MGD systems. The basic element consists of six spiral-wound cellulose acetate membranes placed end-to-end inside an eight-inch diameter fiberglass-reinforced vessel 23 feet long. There are six banks of membranes, each containing 42 vessels, arranged in a three-stage "inverted triangle" pattern (24 vessels, 12 vessels, 6 vessels) to provide 85 percent recovery.

Product water which has passed through the membranes has had 90 percent of TDS removed. The concentrated brine (15 percent of the total input) is returned to the County Sanitation Districts for disposal via their ocean outfall.

Membranes are cleaned in situ, using product water with a detergent additive.