City of Columbia Falls to Re-evaluate Wastewater Treatment Plant Capacity
Council approves $50,000 study to determine the plant’s available capacity and whether its rated capacity should be revised
By Lauren Frick
Columbia Falls city councilors on Monday unanimously approved a roughly $50,000 agreement with HDR Engineering to study the capacity of the city’s wastewater treatment plant, following concerns earlier this year about how it could limit future growth.
The agreement comes about a year after the city completed a roughly $4 million bioreactor expansion meant to improve the plant’s nitrogen removal, a goal city officials say is being met.
“This is what we’ve been waiting for,” City Manager Eric Hanks told councilors at their Monday regular meeting. “We’ve been waiting for a year of operational data to update our operational capacity of our wastewater treatment plant.”
HDR’s analysis will give the city and the Montana Department of Environmental Quality (DEQ) current engineering data to establish the plant’s available capacity and, if the analysis supports it, a revised rated capacity.
“We’ll advocate for a capacity increase,” Hanks said. “It’s very clear we’re not asking for increased discharge into the river. This is just an update of our design and capacity, and because our performance has been exceptional.”
In February 2023, during the plant’s improvement design and discharge permit renewal, HDR evaluated treatment capacity using plant operating data through Dec. 31, 2022. Based on that analysis, the city’s rated capacity was reduced from 0.85 million gallons per day (MGD) to 0.63 MGD, a reduction of more than 2,500 people in estimated population capacity, according to city documents.
The drop was driven largely by an “unusually high peaking factor,” the ratio of the maximum sewage flow rate to the average daily flow rate. The peaking factor was inflated by a historic 2022 rainfall event that caused the Flathead River to flood.
Since that analysis and the completion of the bioreactor expansion project, the city has accumulated operating data showing lower peaking flows and “materially improved treatment performance,” including better nitrogen removal and levels that remain “well below” current nitrogen permit limits.
“Our peaking factors have clearly decreased based on how they do calculations, which should increase our ability to handle more flow in the wastewater treatment plant, which should equate to more population growth and not premature requirements to expand our wastewater treatment plant,” Hanks said.
If the analysis supports revising the plant’s design capacity, the city intends to request a mid-cycle DEQ permit update addressing design flow capacity. If DEQ doesn’t approve a mid-cycle revision to the design capacity, the updated engineering analysis will support the city’s next discharge permit renewal process in 2030.
The plant’s capacity, and its reduction in the 2023 design model, became especially prominent as the city updated its growth policy under the Montana Land Use Planning Act (MLUPA), a process meant to plan for 20 years of estimated growth and development.
Earlier this year, city staff, the planning commission and the city council realized that the plant’s remaining capacity, enough for population growth of roughly 1,894 people, falls short of the more than 2,000 people accounted for in approved and current or near-term development applications.
The HDR agreement is meant to find additional room in that capacity. The firm will also help the city prepare a DEQ-approved Development Plan, another tool Montana provides rapidly growing communities to coordinate public water and wastewater capacity with development.
The plan doesn’t eliminate the need for a future plant expansion but instead sets a pathway for “identifying available capacity, growth assumptions, capacity thresholds, required capital improvements, and the points at which design and construction of additional improvements must begin.”
The framework gives the city, DEQ, developers, and the public “greater predictability while maintaining regulatory safeguards,” the city said.
“This plan … allows us to allocate capacity under a very strict plan with DEQ, and we’re monitoring growth with development and expansion of the wastewater treatment plant under certain milestones,” Hanks said. “None of this would allow us to exceed our permit, and we wouldn’t have even qualified if we had exceeded our permit.”
In developing the plan, HDR will consider historical wastewater use, growth projections, capacity analyses under applicable DEQ design standards and planned capital improvements, and will coordinate with DEQ throughout.