The City of Otsego, MN, is located outside of Minneapolis with a population of 17,000. The WWTP is approximately 4 MGD and historically produced Class B biosolids via aerobic digestion for liquid land application. As a result of population growth, the plant required an expansion. Additional residential housing led to a reduction of available liquid land application sites near the facility and eliminated the possibility of expanding the plant’s footprint. The city needed a new residuals approach to meet its changing needs.
The city evaluated several alternatives including lime stabilization and digester expansion, as well as off-site solids processing. They ultimately elected to install dewatering equipment along with Schwing Bioset’s Bioset lime stabilization process. By selecting the Bioset process, the facility eliminated the need for digesters by processing WAS directly to Class A. The facility, optionally, can operate the Bioset in a Class B mode, which allows the plant to reduce sludge processing costs when sending sludge to the landfill during the winter months. The Bioset’s small footprint meant that they were able to achieve the same level of treatment without expanding the site.
The final detailed design of this pre-negotiated equipment was completed, and the plant was brought online in 2021. The plant is currently producing Class A sludge for a comparable operational cost to Class B aerobic digestion, but with more flexibility in operation modes and disposal options. With the Bioset process, the facility was able to improve its residuals treatment, maintain operational costs, and reduce the overall footprint of the plant.