Challenge

Folkston Georgia has seen residential and commercial growth over the years, which has created a need to upgrade the wastewater treatment plant.

In 1990, the city expanded the wastewater treatment facility by adding a storage pond, land application system (LAS) and a three-cell constructed wetland system.  During drier periods in the summer, the LAS and discharge to Spanish Creek are able to handle the flows from the wetlands; however, during the wetter periods, these two systems cannot treat the increased flow.

Additionally, the expansion of the D. Ray James Prison also created problems for the city’s wastewater treatment plant.

Solution

The solution was a moving bed biological reactor (MBBR) system to upgrade the lagoon. The MBBR is a once-through system that incorporates a biofilm process in a completely stirred reactor, with no sludge recirculation required.

Biofilm develops on the inside of plastic carriers, which move freely in suspension in the reactor tank, oxidizing ammonia nitrogen in the wastewater.  Oxygen is delivered to the carriers through coarse bubble aeration which also keeps the carriers mixed and in suspension. Media is retained in the tank via stainless steel cylindrical retention screens.

Each MBBR train has three individual MBBR reactors or stages. Staging was used to facilitate high-rate ammonia removal in the first stage, where the bulk ammonia concentration was high, and low-rate polishing in the third stage. This staging allowed for a smaller footprint and less media volume, which helped minimize equipment costs. Each stage contains one 10-inch diameter wedge wire retention screen, sized for peak hydraulic flows with minimal headloss. The screen is kept free of debris by aeration headers located directly underneath, and carriers continually knocking into the screen. Airflow to the MBBR is controlled by the Siemens control system.

After the wastewater leaves the MBBR system, it flows to the newly installed effluent pump station where the water is pumped to the existing constructed wetlands, then to Ultraviolet (UV) disinfection, and finally discharged.

Results

The MBBR system began meeting the effluent ammonia limit of 5 mg/l less than two weeks after start-up. Influent and effluent samples were analyzed regularly for the first four months, the average ammonia discharge from the MBBR was  <1 mg/l.

Ammonia removal for the MBBR occurs within the biomass carrier and ismeasured in g NH3-N/m2/d. The specific removal rate across the entire MBBR at Folkston averaged 0.50 g/m2/d up to amaximum of 0.95 g/m2/d.

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