NORTH COUNTY - A state agency has rejected a petition by environmental groups challenging a plan to build the largest desalination plant in the Western Hemisphere in Carlsbad.

But other hurdles remain before waters of the Pacific Ocean can be converted into drinking water.

The State Water Resources Control Board ruled Tuesday against Surfrider Foundation and Coastkeeper, environmental groups that asserted a regional agency should not have issued a permit last summer to allow the plant to return extremely salty water to the ocean. Because that water would contain higher concentrations of salt than the sea itself, the groups alleged, marine life near the plant would be harmed.

Poseidon Resources Corp., the Connecticut-based firm with offices in San Diego that is proposing to build the $300 million desalination plant at the site of the Encina power station north of Cannon Road, praised the board's decision. The ruling upholds the San Diego Regional Water Control Board's August 2006 decision granting the company a discharge permit.

"It is significant because it means we've cleared another hurdle," said Scott Maloni, a spokesman for Poseidon in San Diego, by telephone. "It is also significant because this is the most senior agency responsible for water quality in the state, and they are saying that the petition of Surfrider and Coastkeeper is without merit."

In a prepared statement, Peter MacLaggan, the company's senior vice president, said the ruling is "evidence that the desalination plant can be operated in an environmentally responsible manner, without negative impacts to the marine environment."

Joe Geever, Southern California regional manager for Surfrider Foundation in Los Angeles, termed the terse statement announcing the ruling disappointing.

"The state board didn't tell us their rationale," Geever said. "It leaves us wondering what they were thinking when they dismissed the petition."

Geever said the groups now will turn their attention to another petition they filed to block the issuance of a permit to allow Poseidon to draw in water from the ocean, the raw material for making drinking water. The California Coastal Commission is expected to decide that issue later this year.

"We're confident we're going to win on that front as well," Maloni said.

Poseidon proposes to pump 100 million gallons from the sea daily and convert it into 50 million gallons of drinking water, and the groups assert that huge numbers of fish will be sucked into plant machinery and killed in the process.

Geever contends Poseidon will find the going rough in the wake of a January court decision, Riverkeeper v. U.S. Environmental Protection Agency, in the Second Circuit Court of Appeal. That ruling ordered the federal government to write rules that significantly curb the number of fish killed when water is drawn from rivers, lakes and oceans for cooling power plants and other purposes.

- Contact staff writer Dave Downey at (760) 740-5442 or ddowney@nctimes.com.