GREAT FALLS (AP) -- A proposed state permit for an $800 million, coal-fired power plant that would be built near here has been altered, and now includes the addition of equipment to capture microscopic particles released from the plant's boiler.

Responding to pressure from environmental activists, Southern Montana Electric Generation and Transmission Cooperative wants permit terms that include a $5 million equipment addition at the Highwood Generating Station eight miles east of Great Falls. The equipment would be for control of particulate 2.5 microns or smaller, particulate known as PM2.5.

The equipment would be "costly but cost effective," said Jeff Chaffee of Bison Engineering. The firm advises Southern Montana Electric, a coalition of rural utilities trying to build the 250-megawatt Highwood project. The city of Great Falls is an investor, as well.

The amended, proposed permit calls for installation of a system intended to control liquid acid gases. The gases are components of PM2.5 and can turn into particulate if not captured. The U.S. Environmental Protection Agency says PM2.5 can lodge deep in human lungs. The EPA requires that states monitor PM2.5 levels, but has not produced specific emission-control guidelines for industrial facilities.

In May, the Montana Board of Environmental Review considered a state air-quality permit for Highwood and decided the portion dealing with PM2.5 needed more work. That decision followed an appeal by the Montana Environmental Information Center and Citizens for Clean Energy.

"We'll be going through this with a fine-tooth comb and making sure that the public is as protected as possible," MEIC's Anne Hedges said after the Montana Department of Environmental Quality released the amended permit on Monday.

State officials have concluded that PM2.5 emissions at Highwood can be controlled satisfactorily under terms of the amended permit, said Dave Klemp of DEQ. Southern Montana Electric analyzed the best control technology available, Klemp said.

Public comment on the latest version of the permit will be accepted until Nov. 5, after which DEQ will decide whether to issue a final air-quality permit.

A two-tiered permit structure applies to the Highwood project. One tier applies to the boiler and the other to different aspects of the project. Southern Montana Electric has until Nov. 30 to begin construction or face expiration of the permit segment not involving the boiler. A deadline considerably later would apply to the boiler segment.

"They can build everything else but the boiler," Klemp said.