After considering a number of alternatives, the City ultimately selected a Submerged Attached Growth Reactor (SAGR) system as its preferred solution. Great West and the City worked with Nexom, the developer of the SAGR system, to design a lagoon system that would be followed by a SAGR polishing reactor with focus on ease of operation while still allowing consistent removal and low effluent levels for BOD, TSS, and Ammonia year-round.

The SAGR allows the system to overcome factors in typical lagoon systems that would otherwise prevent nitrification which include:

• When facultative lagoons freeze, atmospheric oxygen is no longer available to support the nitrifying bacteria.
• Nitrifying bacteria thrive when they have a surface to grow on, which is not abundant in a lagoon system.
• Heterotrophic bacteria consume BOD dominate nitrifying bacteria, it’s difficult for nitrification to take place before BOD levels are lowered significantly.
• In cold temperatures, the heterotroph metabolism slows, which requires more lagoon retention time for BOD removal and reduces the available retention time for nitrification to take place.
• As water temperatures drop, nitrifying bacteria have a lower metabolism and reproduction rate. New biomass does not develop quickly enough to overcome the reduced consumption of Ammonia resulting from the slower metabolism.

In order to overcome these factors, the SAGR provides an environment that promotes the growth of nitrifying bacteria. The SAGR is a fully aerated polishing reactor constructed of sacrificial wood walls lined with a synthetic liner and filled with a clean rock. Lagoon effluent flows horizontally through the bed, which is sized to provide full BOD polishing and nitrification regardless of temperature. The SAGR offers a step-feed process to prebuild and store nitrifying bacteria during months in which the water is warm, so they are in place when winter temperatures arrive. The rock in the bed provides surface area to support nitrifying bacteria growth and buffer the water temperature to avoid shocking the bacteria with cold influent.

With construction on the SAGR system completed in June of 2020, it was time to test the new treatment system. In samples taken in July through September 2021 and May through September of 2022, BOD, TSS, and Ammonia concentrations in the effluent were < 1 mg/L, < 2 mg/L, and non-detect, respectively.  Results for BOD and TSS were less than 2 mg/L and Ammonia was less than 1 mg/L based on sampling completed in October- March 2021 and 2022. The influent samples were measured as high as 287 mg/L for BOD, 328 mg/L for TSS, and 28 mg/L for Ammonia, demonstrating a significant reduction in waste stream amounts for these parameters, leading to improved water quality in receiving water. Additionally, using this innovative technology, the City was able to save money in capital expenses and annual operation and maintenance costs by implementing a solution that cost less to construct and maintain than other alternatives considered.

Inadequate wastewater infrastructure in communities such as Council can become a barrier to residential and commercial growth and the discharge of poorly treated water to local ground and surface waters can adversely impact the environment. This project is a long-term solution that will promote community growth and prevent contamination of the local bodies of water now and well into the future.

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