In addition, using warm cooling water from the power plant as source water in lieu of collecting colder sea-water directly from the ocean decreases the power demand for reverse osmosis salt separation by 5% to 10%. It also reduces the impact on marine environment by not collecting additional water from the ocean.
The Carlsbad plant also boasts ultra-filtration (UF) membrane pretreatment technology to remove suspended particulates, pathogens, and debris from the sea-water prior to reverse osmosis membrane separation. When built, the plant will be the largest facility in the world employing such UF pretreatment technology.
The pretreatment system doesn't use coagulants (such as ferric and aluminum salts) to condition the seawater prior to filtration. As a result, the spent filter backwash is environmentally safe because it contains only particulates that naturally occur in the seawater. In comparison, commonly used granular media pretreatment systems use manmade coagulants and flocculants.
The plant's pressure exchanger-based energy recovery system allows recovery and reuse of 40% to 45% of the total initial energy applied for salt separation. Harnessing, transferring, and reusing the energy applied for salt separation at a very high efficiency dramatically reduces the overall amount of electric power needed for desalination.
As a result of the use of this technology, the total energy needed for the Carlsbad desalination plant to produce fresh drinking water from seawater for one household per year (about 2000 kW per year) is less than the energy used to run the household's refrigerator.
The power plant co-location configuration—along with the benefits that stem from the economy of scale and the use of state-of-the-art pretreatment, salt separation, and energy recovery technologies—reduces production cost for the Carlsbad project by 20% to 50% over other stand-alone desalination plants with separate intake and discharge facilities.Huntington Beach
The 50 mgd, $250 million Huntington Beach seawater desalination plant uses most of the technology and design features incorporated in the Carlsbad project. This project also is co-located with a power plant, although the intake and discharge configuration are slightly different.
The plant will collect warm cooling water from the power plant outfall line, process this seawater through pretreatment filters and reverse osmosis system, and discharge the high-salinity byproduct (concentrate) and the spent backwash water from the pretreatment system back to the power plant discharge outfall, several hundred feet downstream of the point of cooling water intake.
Water from this plant will cost $2.70 to $2.90 per 1000 gallons, about half of which is associated with operations and maintenance. Although this cost is higher than the cost of imported water today, technology improvements and economies of scale will eventually lower this cost.
“Higher energy rates will drive up imported water costs,” says Kevin Hunt, general manager of the Municipal Water District of Orange County. “Desalinated and imported water prices are expected to intersect in about a decade.”
Orange County—where the Huntington Beach plant is located—has more local water resources than Carlsbad and currently imports 45% to 50% of its water supplies from the Colorado River and the Bay-Delta. The 12 cities and water utilities in this county also have another large groundwater recharge project that, when completed later this year, will supplement 70 mgd of the county's water resources. As a result, the desalinated water produced by the Huntington Beach seawater desalination plant would constitute only 6% of the current county-wide water supplies.
— Voutchkov is senior vice president, technical services with Poseidon Resources Corp., Stamford, Ct.
Related Articles and Tables
- Read California is Not Alone:Other states also turn to seawater and brackish sources for drinking water
- Table of desalination projects in California