The separation of the salt from the product water is both a chemical process and a physical diffusion process. Three-fourths of the saline water is forced through the walls of the cellulose acetate membranes by pressure. About three percent of the salts pass through the membranes along with the water, lowering the water salinity from about 3,000 parts per million to approximately 300 parts per million.
Reject water, or brine, contains the concentrated salts left behind as the "product" water flows through the membranes. The reject water, with salinity levels at about 10,000 parts per million, drives the energy recovery turbines at the clearwell pumping plant, then flows to the Santa Clara Marsh (Cienega de Santa Clara) at the Gulf of California.
Cienega de Santa Clara
Untreated saline drainage water from farmlands east of Yuma can be bypassed around Mexico's diversion point at Morelos Dam and carried in a 95-mile-long concrete-lined drainage canal to the Santa Clara Marsh. However, the United States does not receive Treaty credit for this water because salinity levels exceed the limits of Minute No. 242. So, to deliver the entire quantity of water owed to Mexico under the 1944 Treaty, the United States replaces this bypassed untreated drainage water with water from upstream storage reservoirs; and this replacement water counts as part of the approximately 1.85 billion cubic meters (1.5 million acre-feet) of water allotted annually to Mexico under the terms of the 1944 Treaty.
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Date last updated: 7/27/12 9:59 AM