APPENDIX F - YUMA DESALTING PLANT OPERATION

The YDP, located about 6 miles west of Yuma, Arizona, was essentially completed in 1992, and is a central feature in the works authorized in 1974 by Public Law 93-320. The YDP and other works were authorized to enable the United States to comply with its water quality obligations under Minute No. 242 (considered an extension of the Mexican Water Treaty of 1944), without depriving the seven Colorado River basin States of any of their apportioned Colorado River water supply.

Under full-scale operation the reverse osmosis YDP would operate at a capacity of 72.4 Mgal/d to desalt and recover about 73,000 acre-feet per year of irrigation drainage. The operation of the YDP is intended to desalt poor quality drainage water from the WMIDD so that it can be returned to the river for use in meeting the United States 1944 Treaty commitments for the delivery of water to Mexico. Without operation of the YDP, the poor quality irrigation drainage would not be useable for meeting deliveries to Mexico and would have to be bypassed to the Cienega in Mexico, lost from use, and would require additional releases from upstream reservoir storage.

When the YDP is operating, WMIDD irrigation drainage (feed water) will be diverted from the Main Outlet Drain Extension through the YDP intake structure, chlorinated, and then passed through the grit sedimentation basins where solids and biomass will be removed. The water will then be pumped to three solids contact reactors (SCRs). A single-unit partial lime softening pretreatment process will remove calcium, bicarbonates, iron, and manganese and further reduce suspended solids. Under normal operation, the pretreatment process will use about 100 tons of lime and produce about 360 tons of calcium carbonate sludge by-product per day. The by-product will be piped to sludge disposal ponds located in the 5-mile zone near the Mexican Border.

The 5-mile zone was acquired by Reclamation for the construction of a well field to intercept groundwater entering Mexico from the United States and deliver that water in partial satisfaction of the 1944 Treaty with Mexico. This was included under Title I of the Salinity Control Act.

From the SCRs, the water will flow by gravity to dual-media filters (anthracite over sand), which will provide the final clarification. From the filters, the water will flow to a clear well. Then the water will be pumped to the reverse osmosis desalting equipment, which is the heart of the YDP. This reverse osmosis equipment consists of "pressure vessels" that are 8 inches and 12 inches in diameter and 20 feet long. Each pressure vessel will contain spiral-wound, semipermeable membranes. These membranes, through a process known as reverse osmosis, will remove dissolved materials (mostly mineral salts) from the pretreated water.

When operating at full capacity, the YDP is sized to accommodate about 108,000 acre-feet of irrigation drainage annually with a salinity of about 2,900 to 3,200 ppm. About 97,000 acre-feet of saline irrigation drainage will be fed into the YDP, producing about 68,500 acre-feet of desalted water (product water) with a salinity of about 250 to 300 ppm and about 28,000 acre-feet of reject stream with a salinity of about 9,400 ppm. Under the planned full scale operation, the 68,500 acre-feet of desalted water would be blended with about 4,500 acre-feet of untreated irrigation drainage. This 73,000 acre-feet of blended water would then be returned to the Colorado River, mixed with other Colorado River water, and then delivered to Mexico at the NIB, upstream from Morelos Dam. The 28,000 acre-feet of reject stream from the YDP will be discharged into the Bypass Drain where it will be combined with excess untreated WMIDD irrigation drainage (if any) and conveyed to the Cienega in Sonora, Mexico.

With three SCRs the YDP has the flexibility of operating at one-third, two-thirds or full capacity. The YDP is not currently being operated while being maintained in standby status. However, it is plausible that during the next 5 years, the YDP could be operated at one third capacity. The main outlet drain is expected to discharge between 110,000 and 132,000 acre-feet per year with a quality of about 3,000 ppm. Operating the YDP at one-third capacity would reduce the annual drain flow to between 76,000 to 98,000 acre-feet annually, and increase the salinity of the discharge water to the range of 3,560 to 3,700 ppm. The plant would yield about 26,200 acre feet of useable blended water.