Each violation refers to an instance when DTE’s visible emissions exceeded “opacity” limits, according to the Sierra Club, which filed a federal lawsuit in US District Court Tuesday.

The lawsuit comes four months after the environmental group announced its intent to sue, as part of a 60-day heads up required by the Clean Air Act.

The suit – which also lists DTE Energy subsidiaries DTE Electric and Detroit Edison as defendants – claims the emissions at DTE’s Belle River, River Rouge, St. Clair and Trenton Channel coal plants contain dangerous pollutants, including mercury, sulfur dioxide, carbon dioxide and nitrogen oxide.

The Sierra Club said in the filing that each of DTE’s power plants had electronic continuous opacity monitoring systems that measure and record the opacity at each boiler, then reports it quarterly to the Michigan Department of Environmental Quality.

“Opacity is a general indicator of the amount of particulate matter emitted from the source in question. The higher the opacity percentage, the more particulate matter emitted by the source,” the report stated.

According to the federal Environmental Protection Authority, particulate matter is "a complex mixture of extremely small particles and liquid droplets made up of a number of components, including acids (such as nitrates and sulfates), organic chemicals, metals, and soil or dust particles.”

Sierra requested the data from MDEQ, which revealed violations of the Clean Air Act by exceeding opacity limits allowed in pollution permits on numerous occasions in 2007 to 2011.

Trenton Channel unit was allegedly the highest polluter.

Sierra has called for an order requiring the companies to meet Clean Air Act standards and to remedy any harm to public health or the environment caused by the violations, but ideally they would prefer DTE use alternate forms of energy, rather than coal.

According to the Detroit Free Press, DTE responded to the November announcement, defending its record.

"The Sierra Club's announcement is clearly a ploy to grab some headlines and is part of their crusade to end the use of coal. All of our plants, including those cited in the news release, operate in compliance with state and federal emissions regulations, which are designed to protect the environment and public health," DTE Energy spokesman Len Singer said in an email.

The company also defended its record on installing pollution-control technology.

"DTE Energy has invested $2 billion in the last 10 years to install emissions controls to ensure compliance with current and future regulations. We anticipate spending another $1 billion to $2 billion to meet new regulations," the email said.