An environmentalist who is suing Northport power plant operator National Grid and a state agency said yesterday that the plant is a "giant fish-killing machine" and its continued operation with an antiquated cooling system is "an act of theft" on a public resource.

Terry Backer, who operates the Connecticut environmental group Soundkeeper, used the 48-year-old plant as a backdrop and took sharp aim at the New York State Department of Environmental Conservation for a 5-year delay in issuing a new water-pollution discharge permit for the plant.

"We'd be arrested," if ordinary citizens took a fraction of the billions of fish killed each year in the plant's once-through cooling system, the onetime lobsterman said. The cooling method draws upward of a billion gallons of water a day to cool the plant, spewing a "lifeless soup" when it's returned to the waterway each day, Backer said.

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Backer said the DEC is taking a "lackadaisical, dilatory and laissez-faire" approach to regulating the plant.

National Grid's discharge permit to operate the plant expired in 2011, but under state rules it can operate while its application is reviewed. DEC officials have already said state plants should adopt improved technology called closed-cycle cooling, which uses minimal amounts of water.

Reed Super, an attorney for Soundkeeper, said the cost to upgrade to closed-cycle cooling would amount to "pennies" per month for PSEG Long Island ratepayers, who cover all costs to operate and maintain the plant.

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National Grid spokeswoman Wendy Ladd said the utility has proposed a "suite" of solutions for addressing impacts that the old cooling system "may have" on aquatic life. She did not elaborate.

DEC spokesman Tom Mailey said the agency "does not comment on pending litigation."

Adrienne Esposito, executive director of Citizens Campaign for the Environment, said plants such as Northport are "literally sucking the life right out" of the Long Island Sound, which supports about $30 billion a year in economic activity.