The Sierra Club and Montana Environmental Information Center filed the federal lawsuit on Wednesday against the six co-owners of the massive Colstrip coal-fired power plant, seeking to force the installation of more pollution controls at the decades-old facility.

If the suit is successful, it will require Colstrip to apply for a new emissions permit and potentially install new pollution equipment.

The penalties for the period in which it was in violation of pollution limits would be in excess of $30,000 a day under federal law, potentially totaling many millions of dollars for violations over 20 years, according to the lawsuit.

The plant's six owners are PPL Montana, Avista, Puget Sound Energy, Portland General Electric, Northwestern and PacifiCorp, which were all named as defendants in the lawsuit.

PPL Montana operates the plant on behalf of the six owners.

Under the Clean Air Act, old plants such as Colstrip were not required to comply with tougher pollution control standards because it was argued they would eventually be retired.

At issue is whether changes made to the plant amount to major modifications that no longer classify Colstrip as an older plant.

According to the lawsuit, those changes include several turbine replacements, plant overhauls and other work, dating back as far as 1993.

The environmental groups argue that Colstrip has been upgraded and expanded without the same pollution controls as newer plants.

But according to The Billings Gazette, PPL Montana external affairs director David Hoffman said the changes generally were needed to maintain the 2000 megawatt plant or meet government mandates and as such did not qualify as modifications.

"Part of this might be a lack of understanding on their part about what it takes to maintain a coal plant of that size," Hoffman said.

"We closely examine that every time we have an overhaul.”