Written by Chuck Wanstrom
Home to both St. Olaf and Carleton colleges, the City of Northfield, MN, is located approximately an hour south of Minneapolis and has a population of 20,000. The wastewater plant is approximately 3 MGD and has historically produced Class A biosolids via an open alkaline and thermal stabilization process. Disaster struck in May 2018 when a fire destroyed all of the Class A biosolids processing equipment, as well as the surrounding dewatering and odor control equipment in the building.
Rather than simply replace the old equipment, the city of Northfield evaluated current available technologies and elected to convert from belt presses to screw presses for its dewatering needs and has purchased two machines from Schwing Bioset to accomplish this. Additionally, the city will continue with Class A biosolids production, but they are converting to Schwing Bioset's Bioset process. The Bioset process is a closed process that contains odors and dust that does not require supplemental heat and has also been approved by the USEPA through the PFRP process to operate at temperatures below those specified in the 503 regulations.
Final detail design of this pre-purchased equipment is underway and the plant is scheduled to be operational later in 2019. To help the plant bridge the gap in biosolids processing while the new facility is in being constructed, the city is also renting a mobile screw press and Bioset trailer to process their current biosolids production.