A federal judge has ordered the U.S. Environmental Protection Agency to move forward with a stalled rule governing coal-ash disposal but offered few details on how the EPA should proceed.

In a Sept. 30 order, Judge Reggie Walton of the U.S. District Court for the District of Columbia said he would issue an opinion with more specifics by Oct. 30. EPA proposed the coal-ash regulation in 2010.

Environmental groups filed suit in 2012 against EPA, seeking to have the agency designate coal ash as a hazardous material under terms of the Resource Conservation and Recovery Act.

Doing so would enable EPA to regulate coal-ash disposal and phase out wet impoundments, such as the one at a Kingston, Tenn., powerplant, the site of a 2008 coal-ash spill that was the largest in U.S. history.

But industry groups say coal ash is not harmful when recycled and encapsulated in concrete or other building materials. They argue that designating coal ash as "hazardous" would stigmatize its re-use.