The state has approved Allegheny Energy Supply Co.'s plan to install scrubbers to reduce sulfur dioxide and soot emissions at its Hatfield's Ferry power plant along the Monongahela River in Greene County.

The $650 million pollution control system will reduce sulfur dioxide emissions by 255,000 tons and soot by 1,900 tons a year after it is installed on the 1,710-megawatt, coal-fired power plant by 2009.

Hatfield's Ferry has long been one of the state's biggest air pollution sources and earlier this year was named the 20th-dirtiest power plant in the United States by the Environmental Integrity Project, a Washington, D.C.-based organization that advocates better enforcement of environmental laws.

"These sharp emission reductions will bring about significant air quality improvements in southwestern Pennsylvania," said Kenneth Bowman, state Department of Environmental Protection regional director.

Allegheny Energy is the fifth-largest emitter of sulfur dioxide and the 10th-largest emitter of nitrogen oxide in the nation.

DEP sued the Greensburg-based company in June in federal court to get it to reduce nitrogen oxide and sulfur dioxide emissions at three of its power plants in Western Pennsylvania and bring them into compliance with state and federal air quality standards.

The state filed the suit after an investigation of the company for unpermitted modifications at Hatfield's Ferry, the Armstrong Power Plant in Adrian, Armstrong County, and the Mitchell Power Plant in Courtney, Washington County.

When power plants make major modifications they must also install state-of-the-art pollution controls, which the DEP said the company did not do. Connecticut, New Jersey, New York and Maryland joined Pennsylvania's lawsuit, which is pending.

David Neurohr, an Allegheny Energy spokesman, said Mitchell has scrubbers already and Armstrong doesn't.

"We will continue to move forward as our financial health allows," he said.