A private company's proposal to build the nation's largest drinking water desalination plant at Agua Hedionda Lagoon in Carlsbad cleared its final hurdles Wednesday before the California Coastal Commission.

The decision came at the conclusion of a 10½-hour hearing in Oceanside punctuated by objections from environmentalists and support from elected officials who stressed the crucial need to increase the region's water supply.

“We must diversify our region's water-supply portfolio,” said San Diego Mayor Jerry Sanders. “We cannot conserve our way out of the water crisis.”

San Diego City Attorney Mike Aguirre testified in opposition to the project, arguing that it was the wrong approach to solving the region's water shortage.

“The primary way to gain new water is through reclamation,” he said, referring to the process by which waste water is converted back into drinking water.

A frustrated Mayor Bud Lewis of Carlsbad, urged the commission to make a speedy decision without adding new requirements “so we can get on with building the damn thing.”

The $300 million plant envisioned by Poseidon Resources Inc. of Stamford, Conn., would produce 50 million gallons of drinking water each day, enough to supply 112,000 households.

Nine local water agencies have collectively contracted to buy the plant's entire output of drinking water.

Last November, the commission approved a construction permit for the project and attached 22 conditions.

However, the commission left unresolved the issues of how Poseidon would offset the plant's impact to marine resources as well as the project's contribution to greenhouse gases linked to global warming.

The commission's resolution of those issues Wednesday was important because it sets a precedent for how the agency will handle 20 or so other desalination projects that could be developed along the state's 1,100-mile coastline.

Poseidon's plant would employ a technology called reverse osmosis, a process in which seawater is forced by high pressure through a filtering membrane. Initially, the plant will share the seawater intake system currently used to cool the Encina Power Station, which has been operating at the lagoon since the mid-1950s.

However, Encina's owners intend to phase out the aged power plant and build a new one that would employ air cooling technology that would not require seawater. When the switch occurs, the desalination plant will pump approximately 304 million gallons of seawater daily.

A third of that, 100 million gallons daily, will be demineralized to produce 50 million gallons of drinking water.

The rest of the seawater taken from the lagoon will be used to dilute the super-saline residue before it is discharged into the ocean shoreline.

The desalination plant's intake system will trap and kill an estimated 16 million fish larvae each day and a greater number of plankton, according to a report by commission staffers. In addition, adult fish would be killed each day, including Garibaldi, the state's fish. None of the species affected is rare or endangered.

The project's environmental impact report concluded that, compared to the overall abundance of marine life in the ocean, the loss from the plant's intake system will be insignificant.

Nonetheless, Poseidon is required under the state's Coastal Act to compensate for the loss of ocean productivity by enhancing marine habitats, including wetlands that serve as nurseries for juvenile fish.