A design change will require Clearwater Paper to alter the air-quality permit for the company’s $160 million upgrade to its Lewiston pulp and paper mill.

The Idaho Department of Environmental Quality opened a public comment period Monday on the permit to construct a continuous pulp digester and associated polysulfide generator. The project, which is underway and expected to be complete in September, is designed to reduce production costs at the mill and increase efficiency.

The permit, required for any construction or changes of operation that could affect air emissions, was last updated in September 2015 as the company planned its first significant upgrade at the Lewiston site in decades. At that time, company engineers believed the polysulfide generator would operate with a wet scrubber to control emissions of volatile organic compounds or VOCs. The generator converts sodium sulfide into polysulfide and increases the amount of pulp produced from wood chips.

“It originally was (designed) with a wet scrubber, but putting in a condenser system instead allows us to reuse water in that system versus sending it to the sewer, so it’s a better design,” said Matt Van Vleet, spokesman for Clearwater Paper at Spokane.

According to Clearwater Paper documents posted on the DEQ website, company officials learned from a vendor that wet scrubbers have not been shown to work on polysulfide converters and its own engineering showed that a wet scrubber would swamp its wastewater treatment system.

Dan Pitman, an air-quality permit engineer for DEQ at Boise, said new calculations by the company show that after the upgrade is complete, the decline in VOC emissions will be greater than anticipated. The permit requires Clearwater Paper to verify emissions once the new system becomes operational.

Van Vleet termed the reductions as “slight.”

The proposed permit can be viewed on the department’s website at https://bit.ly/2hNtdgb. It is open to comment through Jan. 18 at 4 p.m.