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e..) Q ~ 00 Through various irrigation efficiency improvements to save water, including the neutron probe, infrared thennometer, laser leveling of fields, and flash irrigation, drainage flow has been reduced from 167,000 to a low of I 18,500 acre-feet annually. Lime Softening Pretreatment The major process flows are highlighted in the Yuma Desalting Plant Process Diagram & Mass Balance (previous page), Feedwater is first diverted from the drainage canal, through traveling screens to remove biomass, and then travels in Wlderground pipes by gravity to the grit sedimentation basins for removal of sellleable solids. Reclamation found that lime softening best met the feedwater pretreattnent require- ments, and a design incorporating mixing, flocculation, and clarification in a single vessel was chosen. Nearly all the feedwater pretreattnent requirements (Le" bicar- bonate removal, iron and manganese removal, and suspended solids reduction) are accomplished in the single-unit process. Three I 85-foot-diameter Solids Contact Reactors (SCR) are 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 14 one-acre ponds ten feet deep during each operating year. The next pretreattnent step following the SCRs is the Dual Media Gravity Filters, consisting of anthracite coal over sand. The filters provide the final clarification step. Sulfuric acid is added before and after filtration, and sodium hexarnetaphosphate (as a calcium scale 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 million gallons of desalted water per day. The desalting membrane elements (7 per pressure vessel) are installed in 8-inch-diameter and 24-foot-long pressure vessels and are arranged , in control blocks with a capacity of 1.5 million gallons per day. The L5 million-gal.' lons-per-day control block consists of 48 pressure vessels. The 15 control blocks contain a total of 5,040 elements. The equipment treating the remaining 50 million gallons per day is also the same general configuration. The pressure vessels are 12 inches in diameter and 22 feet long-the largest in the, industry. Control block-capacity for this equipment is 1.1 million gallons per day. The 1.1-million-gallons-per-day control block is made up of 24 pressure vessels, each containing four membrane elements. The 45 control blocks contain a total of 4,320 elements. Feedwater to the desalting units is pressurized to approximately 400 pounds per square inch by up to II high-head vertical turbine pumps that discharge into two headers. One header is for the 8-inch units, and the other is for the 12-inch units, 3