The Sierra Club and a member, Marilyn Wall of Cincinnati, alleged the J.M. Stuart Generating Station has repeatedly violated the Clean Air Act and put public health at risk with emissions of soot, nitrogen oxide and sulfur dioxide.

The Sierra Club wants the U.S. District Court to order the utilities to stop the violations, install the best available pollution control technology and pay fines of up to $27,500 for each day of violations.

The Sierra Club said in July that it would file the lawsuit if the U.S. Environmental Protection Agency did not act to enforce the law. A message seeking comment was left with EPA spokeswoman Cynthia Bergman in Washington, D.C.

The Stuart plant is jointly owned by Dayton Power & Light Co., Cincinnati Gas & Electric Co. and Columbus Southern Power Co. DP&L operates it.

The owners have spent more than $400 million in clean-air technology on the plant in the past 15 years and believe that it complies with all federal, state and local requirements, said Amy Wright, DP&L's director of environmental management.

Wright said the improvements include $175 million in equipment that began operating in May to reduce nitrogen oxide emissions that can combine with sunlight to form ozone, a breathing irritant for the elderly, children and those with respiratory ailments.

The four-unit plant, along the Ohio River near Aberdeen about 50 miles southeast of Cincinnati, has been operating since the 1970s. It has a capacity of about 2,400 megawatts, making it one of Ohio's largest electricity generating plants. It can burn more than 5 million tons of coal annually, the lawsuit said.