CARLSBAD - After issuing an "incomplete" verdict for the fourth time in 10 months on the application for a long-discussed plan to turn seawater off the coast of Carlsbad into drinking water, the agency that watches over the state's coastline suggested a sit-down meeting to discuss the plan.

Representatives from the California Coastal Commission, which has to give its approval for the $300 million seawater desalination plant to be built, issued a letter to Poseidon Resources, Inc. officials, saying the commission still needed more information before it could even bring the issue to its board for consideration.

Commission spokesman Tom Luster said the agency still had substantial financial and environmental questions about the proposed plant. The commission has long been considered the proposed plant's biggest hurdle, because it has questioned whether private companies should be allowed to control potential water supplies.

Poseidon officials, who began studying the idea of building the plant at the Encina Power Station in Carlsbad in 2000 and applied for a commission permit 10 months ago, did not return calls for comment Tuesday.

However, Vice President Peter MacLaggan said last month that the company believed it had answered every possible question about the project and that the commission should approve the application and schedule a hearing with the commission's board.

When Poseidon submitted its most recent application in the first week of June, MacLaggan said, "this submittal … thoroughly addresses any issues we think remain open."

Tuesday afternoon, Tom Luster, the coastal commission's desalination expert, said he and the commission staff believe that Poseidon still had not answered questions about the project that the commission had "asked over and over again" during the 10-month process.

Because of the questions, Luster said, the most recent rejection letter sent to Poseidon also suggested that the company, the commission's regional board and the State Lands Commission, which also needs to issue a permit, should meet "to identify ways to move forward with Poseidon's proposal."

Luster said the agency wants to know more about how much the water that the plant would produce would cost; whether the plant should be smaller; and whether the plant would increase greenhouse gas emissions.

Poseidon, working with the city of Carlsbad and several water agencies, wants to build the largest seawater desalination plant in the United States at the Encina power plant. The proposed plant would produce up to 50 million gallons of desalted drinking water a day, giving county residents their first "drought-proof" water supply. The plant would use part of the water that the Encina plant already uses to cool its electricity-producing turbine engines. It would then use electricity from the plant to force the seawater through high-tech membranes, creating a stream of drinking water and a stream of twice-as-salty brine to be returned to the sea.

Environmental critics - and the coastal commission staff - have questioned whether the plant would cause environmental harm to the ocean's ecosystem.

Poseidon officials said in June that they still hope to get their application approved in time for the commission's board to consider the project and issue a permit in November.

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