FACTBOX: Key facts about water desalination

(Reuters) - The U.S. West is investing in desalination to cope with growing water shortages. Here’s how the technology works and where it is used:

-- The primary method of desalting water is reverse osmosis, which pushes water under high pressure through fine membranes that separate the salt. This uses high amounts of energy, the major cost.

-- The top three desalination countries are Saudi Arabia with 17 percent of global output, United Arab Emirates with 13.4 percent, and the United States with 13 percent.

-- Most of the world’s high-capacity desalination projects are in the Middle East, where energy is cheaper and environmental regulations are more lax than in the United States.

-- Most of the U.S. desalting plants process inland brackish water, accounting for 54 percent of desalination. But seawater, which accounts for just 7.4 percent and is more expensive to process, will be the main source of future growth as costs decrease and technology improves.

-- In California, total capacity of desalination plants is 83,000 acre feet per year, equal to about 11 percent of the water used in Los Angeles. An additional 475,000 acre feet per year in new capacity are in various stages of planning.

-- A new desalination plant in San Diego County, using seawater, will be the biggest in the Western Hemisphere when it opens by 2012, with output of 56,000 acre feet per year.

-- Environmental concerns include: the brine byproduct that is much saltier than ocean water and must be processed before being put back in the ocean; and the impact of the suction intake pipes on sea life.

-- Desalination will not be a major supply of California water until after 2025 due to the cost differential with conventional freshwater supplies.

-- Desalinating seawater at major plants nearing construction in Southern California will cost between $950 to $1,100 per acre foot. It costs Southern California cities about $650 per acre foot for imported drinking water, which is to go up to $800 or more per acre foot in three years.

Sources: California Water Plan Update 2009; California State Desalination Task Force, California Department of Water Resources, International Desalination Association, Santa Ana Watershed Project Authority.

An acre foot is the amount of water it takes to cover an acre 12 inches deep. It is enough water to serve two four-person U.S. households.

Reporting by Bernie Woodall, Editing by Mary Milliken