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Pretreatment Process

Before being desalted, the water passes through several pretreatment steps to remove all solids, which would quickly clog the expensive desalting membranes. Pretreating the water extends the life of the membranes to 3 to 5 years. Without pretreatment, membranes would last approximately 1 hour.

Saline drainage water from farmlands east of Yuma flows in a concrete-lined drainage canal to the desalting plant and enters the plant at an intake system where screens remove algae and large debris, such as tree branches, from the water. As the drainage water flows by gravity through underground pipes into the plant, the water is treated with chlorine. This chlorination kills micro-organisms and stops the growth of algae, which would plug or damage the plant's filters and membranes.

Pretreatment Process Diagram Desalting Process AreaDesalting Process AreaDesalting Process AreaHigh-Pressure PumpsClearwell and SHMP SilosDual Media Gravity Filters and Sulfuric Acid ProcessSludge Thickener and Sludge Disposal Site Solids Contact ReactorsLime and Ferric Sulfate Process Grit sedimentation process

 

1. Grit Sedimentation Basins

Grit sedimentation basin
Grit sedimentation basin

Once inside the plant, the water goes through grit sedimentation basins, the first pretreatment facility. There the rapidly flowing water slows down and spreads out, allowing gravity to pull the sediment to the bottom. At the bottom of the basins, automated rakes scrape the sediment buildup to the side. From the basins, the sediment is piped to a sludge handling area, where most of the water is removed from the sediment and the sediment is discarded. The partially-cleaned water near the top of the basins flows over the basin walls and into the intake pumping plant, which pumps the water to the next pretreatment process, the solids contact reactors.

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2. Lime and Ferric Sulfate Process

Lime silos
Lime silos

Railroad cars and trucks transport most of the chemicals into the plant. Pebble-sized quicklime and ferric sulfate are stored in silos until used. When needed, the quicklime is blown from the bottom of the silos to the square lime slaker building. There, the quicklime is mixed (or slaked) with water to form a lime slurry (water thick with lime, about the consistency of cream). The lime slurry is then pumped from the slaker building to the solids contact reactors and mixed with the water in the reactors. The ferric sulfate is mixed with water and also injected into the solids contact reactors.

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3. Solids Contact Reactors

Aerial view of three solids contact reactors
Aerial view of three solids contact reactors

The three large solids contact reactors each measure about 56 meters (185 feet) in diameter and about 8 meters (26.2 feet) deep. Each one can hold about 18,000 cubic meters or 18 million liters (4.71 million gallons) of water. Inside these three giant structures, lime and ferric sulfate remove more suspended particles in the water and soften it by taking out most of the calcium. Ferric sulfate helps the lime form sludge and settle the suspended particles to the bottom of the solids contact reactors.

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4. Sludge Thickener and Sludge Disposal Site

Evaporation ponds
Evaporation ponds

The sludge in the solids contact reactors is scraped to the center and transferred to the sludge thickener, where the sludge can be concentrated further by settling. Pumps then force concentrated sludge through an underground pipeline to a disposal site consisting of evaporation ponds. Each acre-size evaporation pond will be filled with sludge. When the sludge dries, the ponds are covered over with soil to blend into the desert landscape. The buried sludge eventually becomes a limestone deposit. The sludge can also be used in scrubbers on air pollution control systems or can be recycled and used for soil treatment on farmlands.

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5. Dual Media Gravity Filters and Sulfuric Acid Process
The water clarified by the solids contact reactors flows through pipes called launders, which are located near the top of the reactors, then flows to the dual media gravity filters. Before the water reaches the dual media gravity filters, sulfuric acid is added to reduce the pH of the water from 10 to 7.5. (pH is an indicator of how acidic or alkaline water is.) Adding sulfuric acid prevents calcium carbonate plating, which would plug and cake the filters. Very tiny suspended solids remaining in the water are removed as the water flows by gravity through the anthracite coal and sand filters. The dual media gravity filters consist of 28 filter cells. Each cell filters approximately 13,500 cubic meters or 13.5 million liters (3.6 million gallons) of water per day. The tiny solids that are filtered out of the water are left behind in the sand and anthracite coal, and these tiny solids are cleaned out periodically by backwashing the filters.

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6. Clearwell and SHMP Silos
The filtered water flows from the dual media gravity filters into a large underground storage tank called a clearwell, which can hold about 13,500 cubic meters or 13.5 million liters (3.6 million gallons) of water. Concrete that covers the top of the clearwell keeps dust from blowing into the cleaned water. As the water flows into the clearwell, additional chlorine is added to stop the growth of micro-organisms and algae; SHMP (sodium hexametaphosphate) is added to the water from the SHMP silos to help prevent scale buildup in the reverse osmosis desalting membranes; and again, the pH of the water is lowered to about 5.5 to prevent scaling of the desalting membranes. Scaling would occur if the pH is too high, and other damage would occur if the pH is too low.

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7. High-Pressure Pumps

High-pressure pumps
High-pressure pumps

Fourteen high-pressure pumps at the clearwell force water from the underground tank into the processing area and into the reverse osmosis membranes. The water enters the membranes at an average pressure of about 2,500 kilopascals (362 pounds per square inch). After going through the desalting process, a stream of pressurized reject water is sent through energy recovery units on three of the high pressure pumps. The stream of fast moving water turns the turbines of these energy recovery units and powers the pumps. This helps reduce overall energy consumption and costs. Ammonia is added into the pump discharge pipe to convert the remaining chlorine to a less aggressive biocide called chloramine, which prevents mold and bacteria from growing in the spiral-wound membranes.

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8. Desalting Process Area
Reverse osmosis is the process of separating salt from water through the exertion of pressure on semipermeable membranes. Salty water is forced through cellulose acetate membrane walls which do not allow salt to pass through.

The desalting equipment can produce about 837 gallons of desalted water per second.


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Date last updated: 11/14/14 11:20 AM