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A federal judge in St. Louis ruled Monday that Ameren Missouri’s coal-fired Rush Island power plant violates the Clean Air Act, a decision that could ultimately force the utility to spend hundreds of millions of dollars on pollution equipment.

Judge Rodney Sippel said the Jefferson County plant “emitted significantly more pollution” after the St. Louis-based utility made major modifications to boost the output of its power-generating units without obtaining proper permits.

Critics of Ameren have said that obtaining those permits would have required the company to install expensive technology to limit the plant’s emissions of pollutants such as sulfur dioxide.

“As a result of the operational increases, the units ran more, burned more coal, and emitted hundreds of tons more of (sulfur dioxide) per year,” Sippel wrote in a 195-page opinion and order.

Such an upgrade to the plant did not constitute “routine maintenance” or defensible changes to accommodate demand growth that would shield the company from liability, according to the ruling.

The projects in question were completed in 2007 and 2010. The Justice Department filed suit on behalf of the Environmental Protection Agency in 2011. In August, a 12-day, nonjury trial was held in the case.

The decision was applauded by environmental advocates.

“Ameren has violated the Clean Air Act and continues to rely on old, outdated coal plants instead of cleaning them up or investing in cheap, clean energy that also creates jobs,” said Andy Knott, a senior representative for Sierra Club’s Beyond Coal campaign in Missouri, in a statement.

Ameren, meanwhile, said the decision contradicts legal precedent and state regulations.

“We are reviewing the ruling but are disappointed with many of the factual and legal conclusions reached by the Court,” Ameren said in a statement.

“In bringing this enforcement action, the Obama Administration argued for legal interpretations and rulings that contradict with the plain language of Missouri’s regulations, positions taken by the Environmental Protection Agency outside this litigation, and in other legal proceedings. Our Rush Island Energy Center burns some of the lowest sulfur coal available on the marketplace and SO2 emissions are considerably lower than when the projects at issue were performed.”

A status conference to discuss remedies has been set for Feb. 15.

Knott is unsure what will be proposed at that time, but he suggested that equipping the facility with a scrubber — a technology that can cost hundreds of millions of dollars — is one way for Ameren to reach compliance.

“They’ll have to reduce their emissions,” Knott said.

“If Ameren had complied with the parts of the Clean Air Act that they violated, they would have had to install a scrubber.”

The two coal-fired generating units at Rush Island were built in 1976 and 1977 and, prior to the upgrades, were originally designed to have an approximately 30-year lifespan, according to court documents.

Monday’s decision said the surrounding area in southern Jefferson County does not currently meet federal air quality standards for sulfur dioxide.

But at the time of the 2007 and 2010 projects, the area was classified as in attainment for levels of the pollutant.

Reporter covering energy and the environment for the St. Louis Post-Dispatch.