Climate change isn’t the reason that some coal-fired power plants in the United States are closing down. It is the cost of tighter pollution controls on their wastewater.

According to state regulatory filings, dozens of plants across the country plan to stop burning coal this decade in order to comply with more stringent federal wastewater guidelines, as the industry continues to shift away from the planet-warming fossil fuel.

The new wastewater rule requires power plants to remove coal ash and toxic heavy metals like mercury, arsenic, and selenium from plant wastewater before dumping it into streams and rivers. According to the Environmental Protection Agency, the rule will affect 75 coal-fired power plants across the country.

These plants had until October to notify state regulators of their plans to comply, with options including upgrading pollution-control equipment or retiring coal-fired generating units by 2028.

The national impact of the wastewater rule is still being determined, but according to the Sierra Club, which has been tracking state regulatory filings, at least 26 plants in 14 states have stated their intention to stop burning coal. According to the environmental group, twenty-one of the plants intend to close, and five have indicated that they may switch to natural gas. According to EPA estimates, the rule will reduce pollutant discharge into the nation’s waterways by 386 million pounds per year. Implementation is expected to cost plant operators nearly $200 million per year.

Two of Pennsylvania’s largest coal-fired power plants, Keystone and Conemaugh outside Pittsburgh, have announced plans to phase out coal and retire all of their generating units by December 31, 2028, according to regulatory notices.

The plants, which have been in operation for more than 50 years, employ approximately 320 full-time employees and 170 contractors. According to industry averages for coal plants of their size, they produce enough electricity to power approximately 1.5 million homes.

According to Sierra Club data, states with power plants that plan to phase out coal use by 2028 include Arkansas, Georgia, Indiana, Louisiana, Maryland, Michigan, North Carolina, Ohio, Oklahoma, South Carolina, Tennessee, Texas, and West Virginia.

Atlanta-based Southern Co. and Houston-based NRG are among the power companies that have stated that they will close coal-fired units as a result of the new rule. Southern, which owns and operates electric utilities in Georgia, Alabama, and Mississippi, has announced the closure of two-thirds of its coal fleet, including units at the nation’s two largest coal-fired power plants, Scherer and Bowen, both in Georgia. NRG said it plans to stop burning coal at its domestic plants outside Texas, and install new pollution controls at its two Texas plants.

The electric power industry has spent years transitioning away from coal and toward cheaper, cleaner-burning natural gas and renewables such as wind and solar. According to the Energy Information Administration, approximately 30% of coal plant generating capacity has been retired since 2010.

Pennsylvania, the nation’s third-largest coal-producing state after Wyoming and West Virginia, has seen a long-term shift away from coal. Coal’s share of state electrical power generation fell from nearly half in 2010 to 10% last year, as operators took advantage of a statewide boom in natural gas drilling in the Marcellus Shale, the nation’s largest gas field. Seventeen Pennsylvania coal plants have been retired since 2009.

According to federal data, Pennsylvania and neighboring Ohio have accounted for 20% of all coal-fueled power plant shutdowns in the United States in recent years.

According to experts, the majority of the megawatts lost during last winter’s massive Texas blackout were generated by gas, coal, and nuclear plants.

In Pennsylvania, the planned retirements of Keystone and Conemaugh come as building trade unions, industry groups, and coal communities oppose the state’s planned membership in the Regional Greenhouse Gas Initiative, a multi-state consortium that imposes a price on carbon dioxide emissions from coal, gas, and oil-fired power plants. Pennsylvania would be the first major fossil fuel state to adopt such a carbon pricing policy.

The planned shutdowns could leave Homer City Generating Station as the state’s last large, traditional coal-fired power plant operating by the end of the decade. Homer City, which is located east of Pittsburgh and is Pennsylvania’s largest coal plant, has informed state regulators that it intends to continue operations while adhering to the new wastewater limits.