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The installed capacity of the desalting plant will produce up to 72.4 million gallons of product water per day. The plant will produce an average of about 67,000 acre-feet of product water per year at about 300 milligrams per liter. This flow will be blended with raw drainage waterto develop a total of73,000 acre-feet per year of blended water to be delivered to the Colorado River. Through various irrigation efficiency improvements to save water, including the neutron probe, infrared thennometer, laser leveling of fields, and flash irrigation. the drainage flow was reduced from 167,000 to 108,000 acre-feet annually. The desalting plant is designed and constructed so capacity can be increased to 96 million gallons per day of product water, jf necessary. Lime Softening Pretreatment The major process flows are highlighted in the Yuma Desalting Plant Process Diagram & Mass Balance (next page). Feedwater is first diverted from the drainage canal and conveyed to grit sedimentation basins for removal of settleable solids and biomass. The Bureau of Reclamation found that lime softening best met the feedwater pretreatment requirements, and a design incorporating mixing, flocculation, and clarification in a single vessel was chosen. Nearly all the feedwater pretreatment needs (i.e., bicarbonate removal, iron and manganese removal, and suspended solids reduction) are accomplished in the single-unit process. Each of the three l85-foot-diameter Solids Contact Reactors (SCR) is expected to use approximately 100 tons of lime per day at a dosage of 200 milligrams per liter, The calcium carbonate sludge produced by the lime softening process will be piped to specially prepared evaporation ponds for disposal. Under nonnal operation, 360 tons of sludge will be generated per day, which will fill about 6 acres per year. The next pretreatment step following the SCRs is the Dual Media Filters, consisting of anthracite coal over sand, which provides the final clarification step. Sulfuric acid is added before filtration, and sodium hexametaphosphate, as a calcium carbomate sulfate precipitate inhibitor, is added after filtration. Reverse Osmosis Desalting The heart of the desalting plant is the reverse osmosis desalting equipment. One group of membrane-equipped pressure vessels can produce 22.4 mil- lion gallons of desalted water per day. The desalting membrane element (7 per pressure vessel) are installed in 8-inch-diameter and 20-foot-long pres- sure vessels and are arranged in control blocks with a capacity of 1.5 million gallons per day. The 1.5 million-gallons-per-day control block consists of 48 pressure vessels, The 15 control blocks then contain a total of 5,040 elements. 2