CARLSBAD -- California's coastal watchdog says it wants to know more about how a long-discussed Carlsbad plant that would turn seawater into drinking water would do that if the adjoining Encina power plant stopped using seawater to cool its electricity-making engines.

But officials from the company that has dreamed of building desalting plant, Poseidon Resources, downplayed the Coastal Commission's concerns -- and a preliminary rejection of the plant's permit application -- Monday.

Poseidon Vice President Peter MacLaggan said the company has studied operating the desalination plant if Encina stopped using seawater, and would re-file the application with additional information, perhaps by the end of the month.

Regional water officials have called the proposed $270 million desalination plant "critical" to San Diego County's water supply future.

The San Diego County Water Authority said the plant would provide residents with their first "drought-proof" drinking-water supply, roughly 50 million gallons a day.

If built, the plant would be the largest to turn seawater into drinking water in the Western hemisphere.

Meanwhile, the California Coastal Commission sent Poseidon a letter last week rejecting the company's initial permit application and requesting more information.

Tom Luster, the commission's seawater desalination expert, said the letter did not mean the commission would ultimately reject the Carlsbad project.

However, Luster said the commission believes that the news that the Encina plant could stop using seawater amounted to a "significant change" in the proposed desalination project.

The proposed seawater desalination plant would siphon off a portion of Encina's current cooling seawater to turn into drinking water.

Luster said the commission wants more information about how the plant would work if Encina stopped using seawater; if the plant could pump out less than 50 million gallons a day; if it could be located somewhere away from the coast; and if the plant could use salty groundwater or recycled waste water as a source, rather than seawater.

The commission was created by state voters in 1972 to protect, conserve and restore California's coast. It is expected to pose the Carlsbad project's biggest regulatory hurdle.

MacLaggan said Monday that the company expected the commission to reject the company's initial application.

Both MacLaggan and Luster said that it was common for the commission's staff to reject applications for potential projects several times before deciding it was complete and could be reviewed by coastal commissioners.

MacLaggan, meanwhile, said that Poseidon would have no trouble supplying the commission with information that would ultimately lead to a permit approval.

But MacLaggan also said the company and the project's environmental study had determined that Encina was the "best" spot for the desalination plant, and would create less environmental harm than if located someplace else.

Poseidon's permit application contained the proposed project's voluminous environmental study, technical studies, permits granted by the city of Carlsbad and Regional Water Quality Control Board, and other information.

The company has reached a deal to sell the desalted seawater to the city of Carlsbad and possibly a handful of smaller North County water agencies -- although that deal, with the exception of Carlsbad's participation, remains vague.

Poseidon and regional water officials considered the Encina power plant the perfect spot to build a seawater desalting plant because it had two essential ingredients: access to seawater and power.

The Encina plant sucks seawater from the coast off Carlsbad to swirl around and cool its electricity-producing turbines, then spits it back out into the sea -- a system that has already been given environmental approval.

The desalination plant would siphon off a portion of the seawater intake and use electricity from the plant to force it through salt-extracting filters. Half the filtered water would be salt-free drinking water. The rest would be extra-salty water that would be sent back out to sea.

But Encina plant owners announced in July that they planned to build a new, air-cooled plant, and could shut down the seawater-cooling system within 10 to 20 years.

The commission's letter noted that the proposed seawater desalination plant could exist for 90 years, with an initial 30-year contract and two potential 30-year extensions.

MacLaggan and Poseidon say they have a contractual agreement with Encina's operators that would let the company take over the power plant's existing, already-environmentally approved seawater intake and outfall.

But Luster suggested Poseidon would face tougher environmental questions about marine and fish kill if it assumed control of the system, and that the commission wanted more information about that.

"The (Carlsbad) environmental impact report," Luster said, "was largely built on the desalination plant operating when the power plant would also be in operation. Now we expect the power plant's cooling system to go away."

Connecticut-based Poseidon started studying the idea of building the desalination plant at Encina in 2000, and has a 60-year lease on the Encina site.

- Contact staff writer Gig Conaughton at (760) 739-6696 or gconaughton@nctimes.com.