San Diego, California, imports virtually all of its water supply from other parts of the state. New sources of imported water are not readily available, and the availability of existing supplies is diminishing. The city is thus actively investigating advanced water treatment technologies for reclaiming municipal wastewater that is presently being discharged to the Pacific Ocean. Preliminary experiments were conducted at the bench-scale (0.02 x 10⁶ gal/day) Aqua I facility in Mission Valley from 1981 to 1986. the pilot-scale (0.3 x 10⁶ gal/day secondary, 0.05 x 10⁶ gal/day advanced) treatment Aqua II Total Resource Recovery facility operated at Mission Valley from 1984 through 1992. the full-scale demonstration Aqua III facility (1 x 10⁶ gal/day secondary, 0.5 x 10⁶ gal/day advanced) was constructed in Pasqual Valley and began full-time operation in October 1994.

The Aqua II pilot facility uses channels containing water hyacinths for secondary treatment, followed by a 50,000 gal/day advanced treatment system designed to upgrade the secondary effluent water to a quality equivalent to raw water for potable reuse. The tertiary and advanced process trains were selected in 1985 by a technical advisory committee in conjunction with the city. Tertiary to produce a low-turbidity water suitable for reverse osmosis feedwater was provided by a package water treatment plant, with ferric chloride coagulation, flocculation, sedimentation, and multimedia filtration. The system included ultraviolet light dissinfection, cartridge filtration, chemical pretreatment, reverse osmosis using thin-film composite membranes, aeration tower decarbonation, and carbon adsorption. The final process train produces water that meets U.S. drinking water standards.