MarkWest Liberty Midstream & Resources, which operates natural gas gathering systems and processing plants in Butler County, will upgrade its facilities after the U.S. Department of Environmental Protection alleged the company was violating federal clean air regulations.

Specifically, MarkWest will install appropriately sized emergency vents on its condensate tanks and revise its maintenance schedule at all 14 of its compressor stations in Butler and Washington counties, according to a statement released by the EPA on Tuesday.

MarkWest’s holdings in Butler County include the Bluestone Gas Processing Plant facility in Jackson Township and the Voll Compressor Station in Connoquenessing Township.

A 2011 EPA study of 11 different MarkWest facilities concluded that the Atlasburg, Washington County, company was violating the Clean Air Act. As part of the settlement to upgrade its facilities, MarkWest did not admit or deny guilt regarding the allegations.