City Upgrades Grit Removal & Washing to Solve Odor Problems

Odor problems at the Missoula Wastewater Treatment Plant were so bad at one point recently, that the city received more than 200 odor complaints from the public in one calendar year and a violation from the county health department. 

“The nature of wastewater treatment is that there are odors,” said Gene Connell, Treatment Supervisor for the Wastewater Division in Missoula. “But we got some pretty severe complaints from the public.” 

After a study commissioned by the city, a number of solutions were identified to solve the odor problem at the treatment plant. At the top of the list was a recommendation for a new headworks facility to replace the existing 25-year old facility. 

The headworks equipment at this facility was uncovered, technologically outdated, very high maintenance and worst of all not doing its number one job: efficiently removing grit and other materials. Not only was the poor performance of these systems contributing to the odor problems in the city, just as importantly it was exposing downstream equipment to unnecessary damage.

After consultation with a representative from manufacturer Smith & Loveless, the city launched a new headworks facility in 2012 that included two PISTA® Grit Removal Systems and two PISTA® TURBO™ Grit Washers. 

The new headworks facility achieves 95% grit removal (down to 140 mesh), allowing it to better protect downstream equipment and reduce odors at the plant. The superior washing action of the new grit washers further reduces odors at the plant, which are now at a negligible level according to Connell. 

“As operators, you can look and see the performance,” Connell said. “It’s clearly better.”

Gone are the days of frequent public complaints and visits from the health department. With state-of-the-art grit removal and washing systems, the new headworks facility is set for decades of solid performance. 

“The PISTA® Grit Removal System is the industry standard,” Connell said. “It’s certainly worked out for us.”


The treatment plant opened in 1962, and at the time it was located well outside the limits of the city / nose of the public. Five decades later, the plant found itself surrounded by the quickly growing city.
This population growth contributed to wear on the city’s wastewater equipment and an increase in treatment plant odors. Complicating the situation was the fact that these odors were also much more offensive to the public now that they were so much closer to them. 

Public complaints started coming in left and right—in one recent calendar year the city received more than 200 of them—and the odors were even starting to get the attention of local health officials, who at one point cited the city for public nuisance.

The city committed itself to solving its odor problems and a new headworks facility was at the top of the list. The old headworks sytem, built in 1984, allowed sewage to flow openly and uncovered—allowing odors to escape into the atmosphere—and had poor removal efficiencies that often required some manual removal of both organic and inorganic material. It was also very high maintenance and would break down at times. 

“It was really tired,” Connell said. “It just wasn’t built to today’s standards and there wasn’t any practical opportunity for expansion or upgrade.”

As part of its new headworks facility, the city began exploring equipment that offered covered channels, high grit removal efficiencies and odor-reducing grit washing technologies.


Based on the desired criteria, the PISTA® Grit Removal System and PISTA® TURBO™ Grit Washer were obvious choices for the city. 

The PISTA® Grit Removal System, featuring the exclusive V-FORCE BAFFLE™, provides the high grit removal efficiency—95% down to 140 mesh— necessary to effectively protect downstream equipment and reduce odors.

The PISTA® TURBO™ Grit Washer utilizes patented TRI-CLEANSE TECHNOLOGY™ to clean and dry the removed grit like no other system. The system produces grit that is less than 5% putrescible organic material and 10% water content, making it much more agreeable as far as odors are concerned. 

Combined with atmospheric control equipment, these headworks systems have significantly reduced odors at the treatment plant. According to Connell, public complaints about odor are down to a negligible level.

Furthermore, both of the systems require very little maintenance to operate effectively, unlike the previous headworks equipment. This allows city operators to focus their efforts on other important wastewater activities. 

“It’s very low maintenance,” Connell said. “We really haven’t had to do very much. They just run.”

The systems were designed for an average daily flow of 7 MGD and a peak flow of 13 MGD, so there are no concerns about a population that continues to grow in the city. Connell has been very impressed by the robust construction of the systems—including heavy gauged stainless steel and oversized bearings—and expects the systems to provide decades of dependable service.

“It’s all built for the long haul,” Connell said. “It’s going to be there a long time.”

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