The city of Caledonia, Minnesota, sits in the hilly countryside of southeastern Minnesota. The Caledonia Wastewater Treatment Facility (WWTF) serves the city’s 2,800 residents as well as local industry. The treatment facility currently receives an average of 200,000 gallons per day with a peak flow capacity of two million gallons per day.
Built in the 1970s, the Caledonia WWTF was aging and struggling to treat modern wastewater. Cold weather and stormwater, industrial wastewater from a local food hauler, and strict effluent limits were stressing the original sequencing batch reactor (SBR) system beyond its capacity. As a result, the plant consistently exceeded its nitrate, coliform, and other permit limits.
The impact of these exceedances was particularly dire because the WWTF discharges to groundwater, the local source for farm irrigation and city drinking water. Concerned about the risk of health impacts of high nitrates in water, such as Blue Baby Syndrome, the Minnesota Pollution Control Agency directed Caledonia to build a treatment process that could reliably meet permit.
But the city had design constraints. The new plant had to fit the current plant’s footprint; the existing plant was landlocked and could not be expanded. Like many rural towns, the plant operators held various public works responsibilities, such as road work and animal control. The new plant could not require more of their time or more employees.
“We needed a system that only required one person to run it at a time,” explained Jake.
Solution
Caledonia’s consulting engineer, Donahue and Associates, evaluated the city’s options and ultimately picked The Orbal System, due to its small footprint, proven denitrification performance, and process engineering support from Xylem.
The Orbal System for biological treatment uses an activated sludge process designed to address today’s nutrient and stormflow demands.
The simultaneous nitrification/denitrification (SND) process features dedicated zones that operate in series during the treatment process. The system is designed to be easily modified to meet a large variation of effluent limits.
Donohue designed a first-of-its-kind multi-barrier Orbal® system that simultaneously addressed stormwater flows, industrial wastewater loading, and upcoming state nutrient limits.
Key design decisions included:
- Repurposed existing tanks as equalization tank to fit the landlocked footprint
- Excluded primary clarifiers to retain carbon in the system
- Installed advanced sensors to monitor dissolved oxygen, nitrogen, and oxidation-reduction potential—easing operator burden, save energy, and stabilizing the treatment process.
Results
Within months of completion, the Caledonia WWTF consistently achieved total nitrogen levels of 1.2 mg/L, far below the 10 mg/L permit limit that the original plant struggled to meet.
While the plant’s nitrate removal was performing as designed, it was discovered that the plant was also removing phosphorus at concentrations below 1 mg/L,
which was not accounted for in the design. Upon further investigation, it was determined that the plant had cultivated the bacteria Azonexus.
Previously identified only in laboratories, Azonexus has the capability to simultaneously remove nitrogen and phosphorus, performing the work of multiple bacterial communities and putting Caledonia ahead of upcoming phosphorus limits without additional chemical or biological inputs.
Key results:
- Total nitrogen: 1.2 mg/L (permit limit: 10 mg/L)
- Phosphorus: 0.096 mg/L reported in March and April of 2026.
- Aeration demand: reduced through the Orbal® system's low dissolved oxygen approach
- Green Project Reserve grant: over $800,000 awarded for the plant's sustainable performance and environmental benefits
“It’s been operating great. It frees up quite a bit of time for my operators because there’s a lot that they don’t have to do anymore,” said Dickson. “The system handles everything and that is not how we were operating before. We were having alerts constantly.”
Looking ahead
As wastewater treatment facilities across Minnesota are preparing to meet the Minnesota Nutrient Reduction Plan, many small towns are studying Caledonia’s success with the Orbal System for biological treatment.