Desalination 

Southern California resident have long been reliant upon water imports from northern California and the Colorado River. With an increasing population and limited groundwater sources, certain municipalities are now looking to desalination—in this case, the desalting of abundant local supplies of ocean water—to help quench their thirst.

Water conservation and reclamation/reuse are still vitally important, especially in crowded areas of the arid Southwest. But with recent improvements in technology, desalination is emerging as a viable alternative where water is needed most.

The Economics of Water
For decades, engineers have dreamed of converting the world’s abundant salt water to drinking and industrial process water. Although people have been desalting water since antiquity, the challenge has been making the desalination process economically feasible.

Now population growth, industrial development, and droughts have depleted readily accessible sources of water in much of the world, raising production costs significantly. As the economics of water have changed, desalination technology has improved, too. We are no longer dependent on energy-intensive evaporative processes.

Modern municipalities are now considering desalination as part of their water supply portfolio. In Southern California, for example, the rising cost of energy needed to deliver 90 percent of its water from external sources—combined with membrane technology improvements—now make large-scale desalination an economically feasible alternative.

In June 2007, Engineering News-Record magazine named Tetra Tech the nation’s number one firm in Water Treatment/Desalination. Tetra Tech’s engineering services for water reclamation and water supply, including desalination, are world-class.

Primary Water Sources, New Technologies, and Energy Issues
Twenty-one thousand desalination plants worldwide produce 3.5 billion gallons of water every day using three sources: seawater, brackish water, and reclaimed wastewater. Tetra Tech designs and builds desalination systems that use all three sources to help increase water supply.

Reverse osmosis (RO) is a membrane technology that is used to turn saltwater into freshwater. Energy is required to generate pressure that pushes water through the membrane, leaving concentrate (dissolved minerals and other contaminants) on one side and fresh water on the other. The saltier the water, the more pressure is required and the more energy is needed to produce freshwater.

Brackish water RO is the most cost effective of the desalination technologies, followed by seawater, and finally wastewater. Its salinity is the lowest of the three sources but requires as much energy as brackish water to produce freshwater because it requires additional treatments to remove other dissolved solids and micropollutants.

Recent Projects
Florida has the most desalination plants of any state. The Floridan Aquifer underlies almost the entire state, which provides a prolific brackish source of supply for several municipalities.

Tetra Tech has been designing, permitting, and managing construction of desalination plants in Florida since the 1990s.

 For the City of North Miami Beach, Florida, Tetra Tech designed, permitted, and managed construction of the Norwood-Oeffler Water Treatment Plant Expansion, to be completed in 2008.Tetra Tech expanded its capacity from 15 mgd of lime softening to a combined 32 mgd.

Tetra Tech designed its first California desalination plant—the Corona Temescal Desalter—in 2001. Completed this year, the Orange County, California, Groundwater Replenishment Project is a 70 mgd facility that uses microfiltration, RO, and ultraviolet light to treat and purify municipal wastewater before injecting it into the aquifer to prevent seawater intrusion.

Tetra Tech also helped the City of Long Beach, California, design and build the Seawater Research Facility to test the ability of nanofiltration to treat seawater at lower cost. Funded by Long Beach, Los Angeles, and the U.S. Bureau of Reclamation, this pilot plant is the largest seawater desalination research facility in the nation.

Environmental Considerations
Desalination plants have three major unit operations: feedwater supply, treatment (pre-treatment and desalination process), and concentrate discharge. Environmental impacts must be considered at all stages of operations, but the largest impacts may come from the final stage, when the concentrates (salts) stripped out during desalination must be disposed.

On the California coast, where the Pacific Ocean is deep close to shore, desalination plants can send concentrate to ocean outfalls with little impact because the concentrate can mix safely with seawater.

In Florida, it’s more difficult. Ocean outfall has recently become less viable due to increased permitting requirements for surface water disposal of concentrate. The Atlantic Ocean and especially the Gulf of Mexico are shallower closer to shore and require longer transmission for the concentrate to mix and dilute adequately.

Small amounts of low-salinity concentrate can be sent to wastewater treatment plants for final treatment and disposal. The concentrate can also be injected into groundwater through deep injection wells.

Disposal by deep injection has fewer environmental and regulatory challenges in Florida, but it’s also the most expensive. Deep injection wells are most cost-effective for large capacity desalination plants. Lastly, a small percentage of low salinity concentrate can be re-used as a source for irrigation water.

Water Where It Is Needed Most
Tetra Tech has been active in the desalination field for almost two decades. At present, our specialists are advancing the application of membrane technologies worldwide. Better plant designs and improvements in manufacturing membrane elements are increasing capacity, lowering operating costs, and reducing environmental impacts.

The next challenge is getting water to the people. This involves the infrastructure of transmission and delivery systems that must address problems of topography, seismic considerations, and energy costs.

With its well-respected international experts on desalination, and a host of experience on both U.S. coasts, Tetra Tech’s long-term market position in desalination is undeniably strong. The company’s infrastructure expertise to deliver water makes that future appear even stronger. For communities faced with increased water demands and few available local sources of water, desalination will continue to be an integral part of the regional water supply solution.

For more information on Tetra Tech’s desalination capabilities and experience, contact This e-mail address is being protected from spambots. You need JavaScript enabled to view it .