LOS ANGELES (Reuters) - Plans for the biggest desalination plant in the Western Hemisphere received final California state approval on Friday, clearing the way for construction to start next year and for the plant to open north of San Diego in 2011.

The California State Lands Commission, which unanimously approved the plant at a meeting in Los Angeles, was the last hurdle before construction can begin.

The $300 million plant will turn seawater from a lagoon off Carlsbad into 50 million gallons of drinking water daily, enough to supply about 110,000 households and about 10 percent of the needs of San Diego County, home to 3 million people.

Desalination is common in the Middle East, but large-scale plants are rare in the United States and Western Hemisphere. There are about 22,000 desalination plants in 120 countries, which together produce about 3 billion gallons per day.

Carlsbad is the first of what is expected to be a wave of approvals for desalination plants in California, where about 20 plants are in various stages of planning.

“This is a historic day for the state of California,” said Peter MacLaggan, senior vice president for Poseidon Resources, the Connecticut-based company behind the Carlsbad plant.

Poseidon is also developing a plant of a similar size in Huntington Beach to the north. Carlsbad will be twice the size of the current largest U.S. plant, on Tampa Bay in Florida.

California officials have set a goal for desalinated water production by 2030 equal to about 10 times the output of the Carlsbad plant.

California Department of Water Resources Director Lester Snow said this week the Carlsbad approval was “essential.”

“Desalination will probably never be a major portion of the water supply, but it’s going to be a critical part of a portfolio of a reliable supply,” Snow said.

The Carlsbad plant will not be ready in time for next year, when there will likely be the biggest drought in state history unless lots of snow falls in the Sierra mountains this winter, he said.

California Lt. Gov. John Garamendi, who is a member of the lands commission, said the plant would make much-needed supply for the San Diego area and replace diminishing supply from the Colorado River and the mountains of Northern California.

The costs of turning salt water into drinking water were about two-and-a-half-times higher 10 years ago. The lower cost and better technology makes the next wave of plants possible, said Scott Maloni, vice president of Poseidon.

The Carlsbad plant will be built by a unit of Spain’s Acciona. The Huntington Beach plant will be built by IDE Technologies, which is jointly owned by Israel Chemicals and Israeli conglomerate Delek Group.