The Environmental Protection Agency will leave it up to states to decide how nuclear power plants and other facilities that use large amounts of water for cooling should be required to minimize harm to fish and other aquatic creatures, avoiding a specific mandate for cooling towers that environmental groups have been pressing for.

In response, environmental advocates vowed Wednesday to continue their push for cooling towers at the Millstone Power Station in Waterford and the nation's other nuclear power plants.

"We're incredibly disappointed," said Reed Super, attorney for Riverkeeper, one of the 19 environmental groups that sued the EPA in 1993 for failing to enforce the Clean Water Act by not requiring cooling towers. He said he will continue the fight in federal court.

The EPA's ruling, issued Monday, means Millstone will not be forced by federal regulators to retrofit the plant with the massive concrete structures that plant owner Dominion contends would cost $2.6 billion and reduce energy output. Instead, the EPA said facilities that withdraw more than 2 million gallons a day from marine and fresh waterways can comply with Clean Water Act standards, which require that the best available technology be used to reduce fish mortality, by employing one of seven options. The list of available options includes cooling towers, but also installing equipment to ensure intake water speeds are slow enough so that healthy fish can escape, or installing special types of screens to trap marine life before it enters the plant. The option chosen would be based on the unique design of each plant and would have to meet the approval of state regulators.

The new EPA rules also require plants to conduct a study to determine the site-specific equipment that should be used. Public input would be required.

Dominion said a detailed analysis of the 559-page ruling is underway, but that its initial review indicates that the ruling responds to its need for flexibility while also providing the "business certainty" companies need by identifying the types of technologies that can be used.

"This reasonable approach will minimize costs to our customers and recognizes our responsibility to protect the reliability of the electric grid," the company said in a statement.

Ken Holt, spokesman for Millstone, said he could not comment beyond the company's statement. The two operating units at the power station draw in more than 2 billion gallons of water per day from Long Island Sound to help remove waste heat generated in the production of electricity, discharging warmed water back into the estuary. The intake pipes are fitted with 3/8-inch mesh screens that catch many fish and other marine life before they enter the plant. The plant also curtails water use by 40 percent during winter flounder spawning season, and recently installed equipment to reduce the amount of water used.

The EPA, in the announcement about the ruling, said an estimated 2.1 billion fish, crabs and shrimp are killed annually when they are pinned against water intake structures or drawn into cooling water systems. The ruling applies to about 521 factories and 544 power plants nationwide, the EPA said.

Roger Reynolds, attorney for the Connecticut Fund for the Environment, said the EPA's ruling overlooks evidence that cooling towers are the only technology that meets the "best available technology" standard of the Clean Water Act requirement to reduce environmental harm. His organization was one of those that joined Riverkeeper in the lawsuit, contending that cooling towers would reduce Millstone's water use by 98 percent.

"It's clearly the superior technology and should be applied," he said. "This ruling is not going to be good for Long Island Sound and won't cut into the 2 billion gallons a day" that Millstone uses.

While some cooling towers are huge structures rising 500 feet, smaller, mechanical cooling towers could be used at Millstone that would be as effective and be less of an eyesore to the surrounding neighborhood, Reynolds said. Mechanical towers could range from about 6½ to about 40 feet tall, he added.

The state Department of Energy and Environmental Protection has been awaiting the EPA ruling to determine how it will proceed on Millstone's pending application for renewal of its water discharge permit. Its current permit expires in 2015. DEEP officials handling the permit application could not be reached to comment Wednesday.

As a condition of its last permit renewal in 2010, Millstone was required to study its cooling technology, reduce water use and screen more fish from intake pipes. The new requirements were the result of a settlement between DEEP, Soundkeeper and the Connecticut Fund for the Environment. Millstone's study, submitted to DEEP in 2012, concluded that the equipment currently in use already meets the "best available technology" standard, and that there is no upgrade that would be a substantial improvement. Connecticut Fund for the Environment disagreed with the conclusion.

Douglas Dixon, program manager for fish protection research at the Electric Power Research Institute, said there were no surprises in the EPA ruling, and he does not expect that state regulators will interpret it to force plants to make significant changes.

"There's a lot of flexibility in this rule," he said. "It does bring some clarification."

The most costly effect of the new rule may be the complicated site-specific study the EPA is requiring to ensure best available technology is being applied, Dixon said.

The Nuclear Energy Institute, an industry organization, said its initial reading of the rule indicates that its concern that the unique conditions of each plant be taken into account appears to be addressed.

"Different water bodies, fish populations, geography and plant engineering require site-specific analysis to determine the most effective measures to protect fish at a particular power plant," Richard Myers, the organization's vice president for policy development, said in a statement. "A one-size-fits-all requirement imposed nationwide, such as replacing once-through cooling systems with cooling towers, would result in adverse environmental consequences."

j.benson@theday.com