With a population of more than 67,000 residents, Missoula, Mont., is home to the University of Montana and a number of impressive natural features. Known as the “Hub of Five Valleys,” the city is surrounded by five mountain ranges and divided by the largest river in the state by volume, the Clark Fork River.
The Missoula Wastewater Treatment Plant operates in close proximity to the university, city residents and the river. Officials at the wastewater treatment plant (WWTP) have learned to adjust to the ever-changing social and environmental landscape of the surrounding community.
“Treatment plants are always built in the middle of nowhere,” said Starr Sullivan, superintendent of the city’s wastewater division. “Then one day you wake up and find yourself right smack dab in the middle of a city.”
Sullivan and his team had to employ new methods and technologies to protect the surrounding environment, as well as the interests of the residents of what he describes as “a pretty progressive green community.”
Past vs. Present
When the Missoula WWTP opened in 1964, it was a fraction of the size that it is now. What was once a primary, 1-million-gal-per-day (mgd) treatment plant, is now designed for 12 mgd. According to Sullivan, the plant operates at an average capacity of 8 mgd.
In the mid-1970s, in response to the Clean Water Act, the plant transitioned into an activated sludge secondary treatment process. In 2004, it was upgraded to an advanced secondary treatment plant with biological nutrient removal.
The plant features a headworks process primary clarification system with two trains of bioreactors and a secondary clarification system. Additionally, the effluent undergoes an ultraviolet (UV) disinfection process before the water is discharged into the Clark Fork River.
Environmental & Social Concerns
The Montana Department of Environmental Quality listed the Clark Fork River as impaired on its 303(d) list in the state’s Water Quality Integrated Report for its elevated algae levels.
“You have to have three things to grow algae: sunlight, water and nutrients,” Sullivan explained. “That’s why we went to a biological nutrient removal process. We wanted to reduce the amount of phosphorous discharged into the river to control algae.”
Ultimately, the multimillion-dollar project was a success. The WWTP was given limits of 1 part per million (ppm) of phosphorous and 10 ppm of nitrogen. According to Sullivan, the average phosphorous and nitrogen levels are well below that limit, at 0.3 ppm and 7.5 ppm, respectively.
The facility also has adopted methods to cater to the growing needs of the community. The 2004 transition from chlorine to UV disinfection was done in part due to the public’s interest in removing chemicals from the water.
Last year, the Missoula WWTP adopted another process as a response to complaints from area residents: a photoionization odor control system, which uses UV light and carbon filters to tame the odors emitted by the plant.
Another unique green aspect of the Missoula WWTP is its nearly four-decade-long public-private partnership with the facility located right next door. Since 1977, the biosolids produced from the WWTP have been sent to Eko Compost, where they are incorporated into the company’s compost soil products.
“It’s worked very well over the years,” Sullivan said.
As for the future of the plant, there are no new upgrades on the horizon. This is due in part to a recent grant that allowed for the completion of an energy audit, as well as the resulting projects, which included the addition of a new air compressor with a larger receiver tank for air storage and an increase in the size of the suction line to three sludge pumps.
“The energy savings were pretty tremendous,“ Sullivan said. “We saved about $10,000 a year for just the two projects that we were able to complete, with a relatively short payback time.”
However, the division does plan to expand a recent land application experiment. The 1.6-acre hybrid poplar tree plantation is receiving irrigation from the treated effluent, thereby reducing the amount that is discharged into the Clark Fork River. Next year, the plot will be expanded to 130 acres.
“In the future, we’ll definitely be doing more and more land application,” Sullivan concluded.