Climate change isn’t what’s driving some U.S. coal-fired power plants to shut down. It’s the expense of stricter pollution controls on their wastewater.

Dozens of plants nationwide plan to stop burning coal this decade to comply with tougher federal wastewater guidelines, according to regulatory filings, as the industry continues moving away from the planet-warming fossil fuel to make electricity.

The new wastewater rule requires power plants to clean coal ash and toxic heavy metals, including mercury, from plant wastewater before it is dumped into streams and rivers. The rule is expected to affect 75 coal-fired power plants nationwide, according to the Environmental Protection Agency.

Those plants had an October deadline to tell their state regulators how they planned to comply, with options that included upgrading their pollution-control equipment or retiring their coal-fired generating units by 2028.

The national impact of the wastewater rule is still coming into focus, but at least 26 plants in 14 states said they will stop burning coal, according to the Sierra Club, which has been tracking state regulatory filings. Twenty-one of the plants intend to shut down, and five indicated they may switch to natural gas, the environmental group said.

The rule will reduce the discharge of pollutants into the nation’s waterways by about 386 million pounds annually, according to EPA estimates. It’s expected to cost plant operators, collectively, nearly $200 million per year to implement.

Those that intend to close include two of Pennsylvania’s largest coal-fired power plants, Keystone and Conemaugh outside Pittsburgh, which said they will stop using coal and retire all of their generating units by Dec. 31, 2028, regulatory notices show.

The plants opened more than 50 years ago and together employ about 320 full-time workers and 170 contractors. They generate enough power for perhaps 1.5 million homes, according to industry averages for coal plants of their size.

In addition to Pennsylvania, states with power plants that plan to stop using coal by 2028 are Arkansas, Georgia, Indiana, Louisiana, Maryland, Michigan, North Carolina, Ohio, Oklahoma, South Carolina, Tennessee, Texas and West Virginia, according to Sierra Club data.

Power producers that say they will shutter coal-fired units as a result of the new rule include Atlanta-based Southern Co. and Houston-based NRG. Southern, which operates electric utilities in Georgia, Alabama and Mississippi, said it will shutter 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 sector has spent years transitioning to cheaper, cleaner-burning natural gas and renewables like wind and solar. Nationwide, about 30% of generating capacity at coal plants has been retired since 2010, according to the Energy Information Administration. (Coal use at power plants is expected to surge more than 20% this year because of sharply higher natural gas prices — the first such increase since 2014 — but the energy agency said it expects that trend to be temporary.)