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,\ After the water leaves the filters, it is stored in the clearwell, a large underground concrete storage tank. The large pumping plant forces the water from the clearwell to the desalting units. The pH of the water is lowered by adding acid to prevent scaling in the membrane units and to increase the life of the membranes, The heart of the desalting plant is the reverse osmosis desalting equipment. Reverse osmosis is the separation of one component of a solution from another component (in this case, the salt from the H2 0); this is accomplished by the pressure exerted on semipermeable plastic membranes. A total of about 9,000 membrane elements, inserted into fiberglass pressure vessels will desalt the water. While the pressure tubes are all 20 feet long, some membranes have a diameter of 12 inches, while the diameter of others is 8 inches. A desalting element is made up of a number of sheets that are rolled into a spiral-wound mem- brane. Looking at an unrolled membrane element you see that the membrane is attached to a center product water tube. The membrane has several leaves of dacron sail cloth with a thin coating of cellulose acetate attached. Between each leaf is a net spacer (made of plastic material) that al- lows the water to flow across the leaves. The separation of the salt from the product water is both a chemical process and a physical dif- fusion process. The water is forced through the walls of the cellulose scetate membranes by apply- ing pressure at about 400 pounds per square inch, allowing only the freshly desalted water to pass through. This process removes about 97% of the salts from the water. The product water is forced by pressure toward the center tube. Most of the salts remain outside the membrane in the reject water which passes out of the pres- sure vessels at a salinity of about 10 thousand parts per million. This reject water-<>r waste water from the plant-is bypassed via the existing concrete-lined drain to the Santa Clara Slough at the Gulf of California. After the final desalting process, the product water is collected and combined with untreated drainage water to produce the desired salinity level. The untreated water has a salinity of around 3,000 parts per million, and the plant product water has been lowered to an average salinity of about 300 parts per million. The blending of these two has been found to be the most efficient and economical way of producing water with the desired salinity level. The Yuma Desalting Plant is the major feature of the Colorado River Basin Salinity Control Project. The desalting plant is capable of saving extra water in the dry southwest by salvaging up to 80 thousand acre-feet per year of southwestern water to beneficially use. All major structures at the plant are completed, including the intake system, pumping plant, electrical switchyard and transmission line, pretreatment facilities, desalting building, test train, and the 53-mile concrete-lined bypass drain, Until the plant goes on line it's in a shakedown mode in preparation to operate the plant. Located in the desalting plant is its Test Train, a smaller desalting plant capable of desalting one million gallons of water per day. The Test Train tests desalting processes and researches new desalting technologies after they are invented or developed. The Yuma Desalting Plant is scheduled to be available to use by late 1992.