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<br />World's Largest
<br />RO Plant--
<br />uma Desalting Plant.
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<br />Looking for high quality water?
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<br />Satisfying U.S. and Mexican Water Quality, Quantity Needs
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<br />Contact YDP
<br />staff for marl' info
<br />about buvinp wat~r.
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<br />Why the Yuma Desalting Plant was built
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<br />In 1961 an irrigation district in Arizona, the Wellton-Mohawk Irrigation and Drainage District
<br />..~ (WMIDD), began discharging irrigation drainage flows, with an average salinity of 6,000 parts
<br />~:,..:per million, into the Gila River. The Gila River joins the Colorado River near the Arizona city of
<br />~'-i.;;' Yuma and below Imperial Dam. The Colorado River then flows to Morelos Dam on the
<br />. I ",i,; U .S./Mexico border, where Mexico diverts it to irrigate farmlands in the Mexicah Valley, south
<br />, of California's Imperial Valley.
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<br />Prior to 1961, WMIDD was able to irrigate its crops without pumping drainage water into the
<br />I Gila River. However, high groundwater conditions forced the district to put in drainage wells to
<br />lower the groundwater table and maintain crop yields.
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<br />Salinities caused by WMIDD's discharge of drainage flows into the Gila River had not been a
<br />major problem until the U.S. began filling Glen Canyon Dam, in the 1960s. Up to that time, the
<br />U.S. had been delivering between four and six million acre-feet per year to Mexico; this large
<br />. amount of water had been diluting the salty drain flows being delivered to Mexico. However.
<br />upon completion of the Glen Canyon Dam, the U.S. reduced deliveries to Mexico to the amount
<br />I required by the U.S.-Mexico Water Treaty of 1944, 1.5 million acre-feet per year.
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<br />" ,F4.t""" Because there was less water in the river to dilute the drainage flows, problems resulting from
<br />"'~' ~,,(~~ high salinity became more evident. WMIDD's saline drainage return flows in the Gila River
<br />..' "- rr:"~ raised the salinity of the Colorado River, and damaged crops being grown in the Mexicali Valley.
<br />~/ ~ ~ -~'~ Mexico complained about the salt. 'The U.s. negotiated an agreement, Minute No. 242, that
<br />, ~.~ _ ~.~ _ I required controlling salinity levels in the water being delivered to Mexico. That agreement
<br />.'" -...r,;,.. ~ resulted in the Colorado River Basin Salinity Control Act (CRBSCA), which gave rise to the
<br />" "'" ' .),~ construction of a 73-million- gallon-per-day desalting plant in Yuma.
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<br />During the construction of the desalting plant, Reclamation controlled the salinity oftlows in
<br />water It delivered to Mexico by bypassing WMIDD return flows around the river system and
<br />sending the water to a wetland in Mexico known as the Cienega de Santa Clara. Reclamation
<br />replaces the bypassed return flows with conserved water from California. In recent years nature
<br />helped Reclamation maintain the quality of water delivered to Mexico-- several periods of high
<br />Colorado River flows occurred during YDP's construction and freshened the water being
<br />delivered to Mexico.
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<br />Reclamation completed construction of the YDP in 1992 and began operating the plant at
<br />one-third capacity, but in 1993 a SOO-year flood event occurred on the Gila River, dumping
<br />almost 5 million acre-feet of water into the Colorado River. This allowed Reclamation to take the
<br />plant offline until salinity again rises in the Colorado River, thus saving taxpayer dollars.
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<br />How YDP desalts water
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<br />The YDP takes in drainage water, pretreats it, and uses reverse osmosis membranes to remove
<br />the salt and other constituents. Pretreatment consists of several steps: chlorinating to kill algae
<br />~;"lI. and bacteria, removing suspended sand and other large particles at grit sedimentation basins,
<br />. removing smaller particles in solids contact reactors (using lime and ferric sulfate), then sending
<br />the water through dual media gravity filters composed of a layer of sand and a layer of anthracite
<br />. coal. After the water has been filtered, the pH is lowered and it is chlorinated again, then it
<br />travels to a holding tank prior to being pressurized and pumped into reverse osmosis membranes.
<br />~.'-7'~~./ "14~ Before the water is processed, the chlorine resic:tual is converted to chloramine by adding
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<br />05/02/97 II: 10:36
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