WSP09892

• File Number: 8278.10
• Description: Title I - Yuma Desalting Plant
• Basin: Colorado Mainstem
• Date: 1/1/1986
• Author: USDOI/BOR
• Title: Yuma Desalting Plant Background and Briefing
• Water Supply Pro - Doc Type: Project Overview

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This legislation had two parts: Title One (for salinity control measures downstream of Imperial
Dam), and Title Two (for salinity control measures in the seven Colorado River basin states up-
stream of Imperial Dam).
Until the desalting plant goes on line, drainage water from the farmlands east of Yuma will be
bypassed around Mexico's diversion point at Morelos Dam and carried in a concrete-lined drain to
the Santa Clara Slough at the Gulf of California, The United States does not get treaty credit
from Mexico for this untreated drainage water since Mexico does not use it.
Until the desalting plant goes on line, these bypassed flows are being replaced with water from
upstream U.S. storage reservoirs in order to fully meet the quantity of one and a half million acre-
feet of water allotted each year to Mexico.
When the Yuma Desalting Plant goes on line, it will salvage most of the irrigation return flows
now being bypassed-up to 80 thousand acre-feet per year (or 26 billion gallons per year).
Here's a quick overview of the Yuma Desalting Plant and an explanation of how it works.
The plant is located on a 50-acre tract of land about 5 miles west of Yuma, just south of the
Yuma Valley Levee.
The plant is the world's largest reverse osmosis desalting plant, It has an installed capacity to
produce 73 million gallol11l of desalted water per day from a total of about 100 million gallol11l of
drainage water per day that enters the plant.
If the need later exists, the plant design permits expansion to 96 million gallons per day.
From the farmlands east of Yuma, the irrigation return flow is delivered to the plant through
an existing concrete-lined drain.
The drainage water flowing into the plant ftrst passes through an intake structure containing
a coarse bar screen that removes trash and bulky material from the water.
From the intake structure, the water flows through piping traveling by gravity into the plant.
Before being desalted, the water passes through three pretreatment steps to remove all solids
in the water which would quickly clog the expensive desalting membranes if not removed. Pre-
treating the water extends the life of the membranes to 3 to 5 years.
AB the water flows into the plant, chlorine is added to prevent growth of algae and other organ-
Isms.
Once il11lide the plant, the water ftrst goes through the grit sedimentation basil11l, the ftrst pre-
treatment facility.
Here, the heavy grit, sediment, and sand suspended in the water fall into the bottom of these
basins as the velocity of the water decreases.
From this point, a small pumping plant moves the main water flow up into the second pretreat-
ment facility: the three solids contact reactors, each measuring 180 feet in diameter.
These three giant structures remove more suspended particles in the water which are harmful
to the membrane units used in the ftnal reverse osmosis proosss. The water is also softened by re-
moving most of the calcium.
Lime and ferric sulfate are both used in the solids contact reactors. Both of these chemicals are
stored in silos until needed--lime in the larger silos and felTic sulfate in the smaller ones.
Railroad cars and trucks deliver the chemicals into the plant area.
Just before the chemicals are added to the three solids contact reactors, the lime is put through
a process called slaking, in which it's mixed with water.
The chemicals are then injected into the water in the solids contact reactors and react by bond-
ing to the solids, which makes them heavy enough that they fall to the bottom of the reactors in
the form of sludge.
From the three reactors, the sludge leaves through the bottom and into the sludge thickener,
which removes much of the water from the sludge.
Thickened sludge is transported from the sludge thickener tm-ough a pipeline to lined ponds
for final drying and disposal.
From the three solids contact reactors, as the sludge leaves through the bottom, the product-
or clarifted water-is removed around the top perimeter. It's piped to the dual media filters, the
last step in the pretreatment process.
The dual media filters are made up of graded sand and coal which remove any fine particles re-
maining in the water.