||A 50,000 gpd Desal ion exchange pilot plant for the demineralization of lime treated, dual media filtered, tertiary sewage effluent has been built and tested at Santee, California. The plant removed up to 86 percent of the total dissolved solids (TDS) as CaCO3. Influent TDS ranged from 700 to 1000 mg/l during the two month operating period. Anion resin (45 cu ft of IRA-68) capacities of 11.4 and 9.7 Kgr (as CaCO3)/cu ft were obtained with a tertiary effluent flow of 1 gal./cu ft/min and an operating pressure of 40 psig. The system was designed to demineralize a portion of the influent stream and blend it with influent water to make a product TDS equal to or better than the Colorado River drinking water supplied to Santee. Because of the short operating time, emphasis was placed on the demineralization process, especially the anion IRA-68 resin performance. The estimated total cost, based on Santee, for 64,370 gpd of 500 mg/l (as CaCO3) blended effluent is $1.00 per 1000 gallons. This estimate calls for activated carbon adsorption influent, two complete operating cycles per day, and a properly functioning blending system.