Suffolk County Executive Steve Bellone's sweeping plan to provide sewers or upgrade septic systems for 209,000 homes will cost about $7 billion, according to an initial estimate released Thursday.

Consultants from IBM and staff from the county planning department made the estimate Thursday during a presentation to Suffolk elected officials, water providers and environmentalists.

Dorian Dale, Suffolk's director of sustainability, estimated it would cost $30,000 to $35,000 per house to install an upgraded on-site septic system, and about $50,000 to connect to a sewer. More than 70 percent of Suffolk homes are not connected to sewers.

Bellone administration officials say high levels of nitrogen pollution, coming primarily from urine in septic tanks and cesspools and fertilizer runoff, can damage wetlands that help mute storm surges. Nitrogen also can cause algal blooms that can kill fish and shellfish in bays. High levels can be dangerous in drinking water, according to the World Health Organization.

Jan Bowen, an IBM water consultant from Britain, said it is urgent that Suffolk address nitrogen pollution.

"Doing nothing is not an option," Bowen said.

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The IBM consultants did not address how to fund the wastewater upgrades in Suffolk.

Bowen said in Ireland, water infrastructure upgrades were covered by property tax increases and charges on water service.

Dale suggested that increases in water rates could help fund the program.

Bellone spokesman Justin Meyers said the "administration is looking at a myriad of different avenues to fund wastewater infrastructure and water quality improvement projects."

Meyers said general discussions about funding sources have occurred, but that other than state and federal grants, no other directions have been officially identified.

James Gaughran, chairman of the Suffolk County Water Authority, which provides drinking water to 1.2 million residents and businesses, declined to take a position on raising rates to fund the antipollution program. He said the discussion was premature because state legislation would be needed before SCWA could institute a charge for sewers.

Gaughran, who is running as a Democrat for county comptroller, added that "our rates are the lowest of any comparable agency in the state or the country. We're very proud of that and would like to keep it that way."