Wastewater Treatment Facility Expansion

Owatonna WWTF Upgrade and Expansion Project – Year 3 Summary

Update Provided by Jennifer Svennes - Nero Engineering

June 5, 2025

The Owatonna Wastewater Treatment Facility (WWTF) is three years into construction of its upgrade and expansion project. At the end of construction, the WWTF will be nearly double its current capacity which will serve the City’s expected growth over the next 25 years. The project completion dates have been extended by 210 days, which equates to a revised substantial completion date of October, 27, 2025, and a revised final completion date of January 26, 2026. The contract extension was due to sizable delays in receiving electrical gear, primarily Motor Control Centers (MCCs).

The Owatonna WWTF expansion project includes three new buildings as well as to the existing facility. The first year of construction largely focused on the construction of the three new buildings: the Membrane Bioreactor (MBR) Building, the Fine Screen Building (FSB), and the Solids Thickening Building (STB). The second year of construction focused on the work within the FSB and MBR as well as rehabs of the Primary Clarifiers (PC), Aeration Basins (AB), and site piping. The third year of construction has progressed from the liquid treatment train to the solids treatment upgrades including work in the Return Activated Sludge (RAS)/ Waste Activated Sludge (WAS) Complex (RWC), Solids Thickening Building, and Digester Treatment Complex (DTC).

Overall, the project is nearly 94% completed. Details for each building are as follows:

• Main Lift Station: The work in the Main Lift Station building is completed.
• Fine Screen Building: The fine screen building has been in operation for over a year treating the influent wastewater. The infrastructure for screening the Return Activated Sludge is nearly in place so implementing that operation is expected to occur in the next two months.
• Primary Clarifier: The grit effluent splitter box has been modified to allow peak flow to be diverted to the anoxic splitter box. The electric actuator needed to automate this operation is expected to be installed soon. Two of the three primary clarifier sludge pumps have been replaced and commissioned.
• Aeration Basins: The rehabilitated aeration basins have been operating with the Membrane Bio Reactor since October 2024. One of the anoxic mixers have failed and is being replaced so the anoxic zones are being mixed by diffused air currently. The Dissolved Oxygen control is not yet implemented because the replacement of the blowers hasn’t been completed.
• Process Building: Two of the three turbo blowers in the Process Building have been installed and started up. It was determined that the buried air piping between the process building and aeration basins was severely corroded, and the replacement of this piping with an overhead stainless-steel pipe has been added to the project. Once the new piping is installed which is expected to be in September 2025, the third turbo blower will be commissioned and the Dissolved Oxygen control system with the Aeration Basin’s air control valves and instrumentation will be integrated. The sand filters have been decommissioned.
• Membrane Bio Reactor: The Membrane Bio Reactor was started up in October 2024. The startup of this process included over a month of equipment testing and fine-tuning, but this testing resulted in the system working immediately upon operation. There were a few issues to work through once everything was operating together which is expected with the amount of equipment involved, but the capacity of the system has allowed the team to work through these bugs while still effectively treating the wastewater to an exceptional level. 
• Disinfection Process Structure: The Disinfection Process Structure was sandblasted and recoated during the winter of 2024-2025. The backup disinfection system was tested and started up prior to April 1st, 2025. 
• Return Activated Sludge (RAS)/ Waste Activated Sludge (WAS) Complex (RWC): The work to repurpose the final clarifiers was started after the Membrane Bio Reactor was operational. The pumps in the RWC have been replaced and the piping revisions are in progress. The mixing and aeration systems are being installed in the WAS Storage Tank. Additional flow equalization was added to the project by allowing flow from the Aeration Basins to overflow into the east final clarifier in the event of Membrane Bio Reactor equipment failure. The piping required for this change is scheduled to be installed in the coming months at which time the internal recycle and RAS screening can be commissioned.
• Solids Thickening Building: The processes in the Solids Thickening Building were started up in February and March 2025. Shortly after the thickeners were in operation, one of the drums was discovered to have a damaged screen. It was determined that standard screens did not fit the drum and so the drum needs to be rebuilt. One thickener has adequate capacity for processing Waste Activated S in the meantime.
• Digester Treatment Complex: With the Solids Thickening Building operational, the existing DAF has been decommissioned and the building addition for the biogas equipment is underway. Digester No. 2 has been completely rehabbed and startups for the equipment associated with this tank are in progress. The cover for Digester No. 1 has been constructed and is currently being sandblasted and coated. Once Digester No. 2 is operating as a primary digester, the work to upgrade Digester No. 1 and set the constructed cover will take place. The Motor Control Centers for the Digester Treatment Complex were delivered to the site in March 2025.
• Sitework: A majority of the sitework has been completed including site grading and sidewalk installation. The replacement of the retaining wall on the east side of the Membrane Bio Reactor with a Reinforced Soil Slope wall has been added to the project and is scheduled to occur in July.

The WWTF Upgrade and Expansion project has been under construction for just over two years. The Fine Screen Building is fully operational and a majority of the rehabilitation work for the existing liquid train processes has been completed. The star of the liquid train processes, the Membrane Bioreactor (MBR), has been completed, marking a major milestone in the project. The MBR is the final step of the biological treatment process, and it physically separates the liquid from the solids allowing only a high level of water quality to be discharged into the Straight River. All wastewater flow coming into the facility is now treated through the MBR process before discharging to the Straight River. 

The solids that are removed from the liquid train will be treated in a new thickening process and an upgraded digestion process. The work on these processes is well underway. The digestion process will produce energy in the form of biogas that will ultimately be treated and injected into the natural gas grid. Other work anticipated to begin or continue over the next few months is the remodeling of the Administration Building, site work, and the construction of a new Septage Receiving Station. 

Other recent work at the WWTF has included the construction of the anaerobic digester covers which are being built and painted next to the existing digesters and then will be lifted into place on the tanks. Progress has been made in the Solids Thickening Building and other areas throughout the plant including process piping, electrical and mechanical systems installation. The remodeling effort in the Administration Building is well underway and will result in updated office spaces, larger conference and meeting rooms, a new kitchen, upgraded showers and locker rooms and a plant control center.

Year 1 Summary - July 2023

The Owatonna Wastewater Treatment Facility (WWTF) is one year into construction of its upgrade and expansion project. At the end of the approximate three-year construction, the WWTF will be nearly double its current capacity which will serve the City’s expected growth over the next 25 years.

The Owatonna WWTF expansion project includes three new buildings as well as upgrades to the existing facility. The first year of construction largely focused on the construction of the three new buildings: the Membrane Bioreactor (MBR) Building, the Fine Screen Building (FSB), and the Solids Thickening Building (STB).

MBR milestones included:

• August 2022: Excavation for the MBR building, which has the largest footprint of the three new structures. 
• September 2022: The project’s first concrete pour, the MBR base slab.
• October 2022: Concrete pours continue for the base slabs of the 7 membrane tanks.
• November 2022 – March 2023: Concrete walls were poured throughout the winter despite the cold and snowy conditions.
• April 2023: Deck pour for upper level.
• May 2023: Tanks tested for water tightness. 
• June 2023: Precast walls and roofs installed. The MBR building walls are texturized and colored to match the existing tanks onsite.

FSB milestones included:

• September 2022: Excavation and footings for the tallest new building—FSB which will house three fine drum screens to remove solids larger than 2 millimeters.
• October 2022: Buried piping around building installation and connection of piping from the Main Lift Station.
• November 2022: Interior masonry wall installation. 
• December 2022: Precast walls, floors and roof were set in place. FSB is the tallest new structure and the brick façade blends in nicely to the existing buildings at the WWTF site entrance.
• January 2023: Concrete slab pours within building.
• February 2023: Three fine drum screens flown in through the roof and installed on the upper level of FSB.
• March 2023: Roof installation and plumbing, electrical, and mechanical work within the building.
• April 2023: Interior painting.
• May 2023: Interior stairways installation.

STB milestones included:

• December 2022: Excavation for the STB which is the deepest structure. A temporary earth retention system was installed to support the existing adjacent structures during construction.
• March 2023: Plumbing and conduits installed and base slab formed.
• April 2023: Base slab concrete pour.
• May-June 2023: Concrete wall pours.

Other first-year milestones included scheduling, coordination, and equipment deliveries. With two of the three new structures enclosed, the new equipment is continually being set in place. The next year of construction will start to have a larger impact on the existing WWTF operations which will certainly pose some challenges. However, the implementation of new equipment and processes is an exciting step towards operating the improved facility.

To prepare for the plant’s expansion, the City began implementing incremental rate increases ten years ago so it can continue to meet the financial needs of the plant’s operations while maintaining affordability for residents and businesses. In addition to covering more than $3 million in project planning costs already, the City has also set aside $10 million in reserve funds for this expansion project. An additional $8 million in grant funds will be used to help cover project costs.

On May 22, the Minnesota legislature approved $22 million in funding to support Owatonna’s Wastewater Treatment Plant expansion project. Another $4.5 million was approved for Medford to modify its wastewater collection system so it can connect to Owatonna’s plant. By combining the resources of the Cities of Owatonna and Medford with state funding, this regional project maximizes the use of tax dollars through economies of scale, a lower cost per gallon for treatment, better performance and reliability, greater staff efficiencies and reduced permitting, sampling and testing. It is also the most environmentally sound option for the region’s waterways. This regional facility will meet current and future capacity needs for both cities.

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