Left: The Carlsbad, Calif., seawater desalination demonstration plant is a 30,000 gpd plant, it has a potable water sampling station that allows visitors to taste the desalinated water. Above: The reverse osmosis system consists of two four-element pressure vessels in series. This configuration allows collection of permeate from one or both ends of each vessel and testing of different numbers of membrane elements. Photos: Nikolay Voutchkov
An ongoing marine aquarium test places aquatic organisms indigenous to power plant discharge areas in water with a salinity equivalent to what would be experienced with the full scale desalination plant in operation. After nine months, the species are healthy and tolerant of the environment. Photo: Nikolay Voutchkov

Seawater desalination is riding a rising tide of interest as many municipalities and utilities in drought-stricken southern California are turning to the ocean as a source of more reliable and drought-proof water. Less than a dozen small seawater desalination plants are operating in California. In the recent past, the cost of desalinated water was significantly higher than that of traditional sources—ground-water and surface water.

Technology breakthroughs in the past 10 years, however, have improved the affordability of seawater desalination and brought it back into the limelight to respond to water-shortages and dwindling traditional sources. Currently, there are five large seawater desalination projects in various stages of development in southern California.

Two of the projects, the Huntington Beach and the Carlsbad desalination plants, are being developed in a public-private partnership between Poseidon Resources, Stamford, Conn., and local municipalities and utilities. The plants will be located at existing coastal electrical power generation stations. The Huntington Beach and Carlsbad desalination plants are projected to have an ultimate water capacity of 50 mgd and may be developed in one or more phases. The environmental impact reports for the two projects were released for public review in April 2005. Currently, they are in a process of environmental feasibility review and permitting and are planned to begin construction by spring 2006.

On Sept. 28, 2004, Carlsbad reached an agreement with Poseidon Resources for purchasing 25 mgd of the desalination plant's fresh product water at a cost of $861 per acre-foot ($2.64 per 1000 gallons). The rest of the plant production will be procured by other local municipalities and utilities.

The Metropolitan Water District of southern California has been supportive of the development of new water resources and plans to subsidize the cost of water produced at several desalination facilities with a $250 per acre-foot ($0.77 per 1000 gallons) credit. With the credit, the cost of desalinated water will become comparable to the cost of water imported from northern California and the Colorado River. The desalinated water, however, will be of lower salinity and will have better overall quality than traditional water resources. More importantly, seawater desalination will create a new, and more reliable, local drought-proof water resource.

Up to six projects are expected to be operating by 2010 that cumulatively will provide more than 150 mgd of fresh water for southern California. Although this is a significant amount, it would satisfy only a small portion of California's commitment to reduce its use of Colorado River water and accommodate ever-growing water demands. Other water sources that could be used to meet future water demand are increased water reuse, conservation, and development of new groundwater resources.

Carlsbad's Demonstration Plant

To demonstrate the feasibility of coastal desalination and the benefits of co-location of large desalination plants with existing power plants in southern California, Poseidon Resources, assisted by local specialty contractor Enaqua and Hydranautics, have constructed a 30,000-gallons-per-day (gpd) desalination demonstration plant at the Encina power plant in Carlsbad. The demonstration facility has a raw water intake feed pump station; two pre-treatment filtration systems configured to operate in parallel; filtered water transfer pumps; a membrane system feed seawater storage tank; a 5-micron cotton cartridge filter; a 45-gpd high-pressure reverse osmosis (RO) feed pump; a single-stage RO system; a permeate lime conditioning system; and an ultraviolet disinfection system.

The demonstration plant also has a potable water sampling station that allows visitors to taste the desalinated water. In addition, this facility is equipped with a number of ports for water quality sample collection. The demonstration plant is fully automated and is designed for remote monitoring and control via the Internet. The pilot plant's state-of-the-art design, configuration, and multifunctional use has been recognized by the San Diego section of the American Planning Association, which, in 2003, granted the project a first place award for innovative technology.

The source of feed seawater for the demonstration facility—and for the planned full-scale plant—is the warm cooling water from the Encina power plant. This once-through power generation station withdraws cooling water from the Pacific Ocean via the Agua Hedionda Lagoon. After passing through the power plant intake structure, trash racks, and traveling screens, the cooling water is pumped through the power plant's condensers. The power plant has five power generators and, depending on the number of units in operation, pumps 200 mgd to 820 mgd of cooling water through the condensers.

The warm cooling water from all condensers is directed to a common discharge tunnel and lagoon leading to the ocean. The full-scale desalination facility is planned to tap into this discharge tunnel for desalination plant feed water and for discharging high-salinity concentrate downstream of the intake.