When the City of Fond du Lac, Wisconsin, faced stringent phosphorus discharge limits, the wastewater treatment team transformed a tough regulatory mandate into a scalable model for operational resilience. By optimizing existing infrastructure rather than investing in a costly tertiary treatment upgrade, the facility achieved remarkable efficiency.
$82.000
Annual chemical savings
29%
Energy intensity reduction
Challenge
The regional wastewater treatment facility serves around 75,000 people across the city and 18 neighboring communities. To protect local ecosystems like Lake Winnebago, Wisconsin enforces some of the strictest phosphorus limits in the nation. The utility faced a challenging total phosphorus discharge limit of 0.19 milligrams per liter.
The original system was built to handle ammonia, not deep biological phosphorus removal. Initial efforts to improve phosphorus removal fell short because nitrate concentrations remained too high for the necessary organisms to thrive. The facility needed a new approach to meet regulations without relying on expensive, chemical-heavy processes.
Solution
Having previously partnered with Xylem on energy-saving initiatives, the utility co-developed a smarter, biological treatment retrofit. They implemented CASPERON, Xylem’s advanced simultaneous nitrification-denitrification system. This technology allows ammonia and phosphorus removal to happen in the same tank by creating specific oxygen-rich and oxygen-poor zones.
The team used a phased approach, upgrading one aeration basin at a time. Dense diffuser grids and dynamic aeration controls created spatial oxygen gradients, which suppressed nitrate interference and stabilized the biological process. Now, sensor data feeds directly into automated control systems, adjusting oxygen levels in real time as daily loads fluctuate.
Results
The phased retrofit delivered impressive operational and financial improvements. Between 2018 and 2024, energy consumption dropped by nearly 29 percent. Reliable biological phosphorus removal also led to a sharp decline in chemical use, resulting in over $82,000 in annual chemical savings. Ultimately, the new system paid for itself within two years.
Beyond the financial metrics, plant operators experienced a major shift in their daily work. The team moved from reacting to immediate problems to proactively optimizing the system through sensor maintenance and data monitoring.