Population and commercial growth put stress on Folkston, Ga.'s wastewater treatment plant.

The city had expanded the facility a decade ago by adding a storage pond, land application system and three-cell constructed wetland system. During the drier periods in the summer, the land system and discharge to the Spanish Creek can handle the flows from the wetlands, but during wetter periods, the two systems cannot treat the increased flow. Also, the plant had to contend with expansion of a nearby prison that added to the wastewater plan's load.

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 facilitates high-rate ammonia removal in the first stage, where the bulk ammonia concentration was highest, and low-rate polishing in the third stage. Staging allows for a smaller footprint and less media volume to help minimize equipment costs. Each stage contains one 10" dia wedge-wire retention screen, sized for peak hydraulic flows with minimal head loss. The screen is kept free of debris by aeration headers located directly underneath that also keep carriers from continually knocking into the screen. Airflow to the MBBR is controlled by a control system from Siemens.

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

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

Ammonia removal for the MBBR occurs within the biomass carrier and is measured in a g NH3-N/m2/d. The specific removal rate across the entire MBBR at Folkston averaged 0.05 g/m2/d up to a maximum of 0.95 g/m2/d.

For more information on Siemens Water Technologies and to view this case study video, go to www.siemens.com/water, type "Georgia town" in the search box, then click on "Georgia Town Meets Regulations and Increases Capacity with MBBR System."