AN INNOVATIVE RECIRCULATING SAND FILTER (RSF) SYSTEM SOLVES A SMALL WISCONSIN COMMUNITY'S LARGE WASTEWATER TREATMENT PROBLEMS: A CASE STUDY

The Village of Stetsonville, Wisconsin, had a problem with its wastewater treatment system. The former two-cell aerated lagoon treatment system had not been in compliance with its Wisconsin Pollutant Discharge Effluent Standards (WPDES) permit for several years. High effluent ammonia nitrogen levels during cold weather and excessive algae growth during warm weather periods contributed to high biological oxygen demand (BOD) and suspended solids (SS) levels, continually placing the plant out of compliance. A practical and effective wastewater treatment solution was needed to bring the plant into compliance.An innovative approach that used the existing lagoon system for pretreatment followed by secondary treatment through a recirculating sand filter (RSF) was designed and implemented. In this design, the primary lagoon was split into two stages by a new dike. The first stage incorporates pre-aeration followed by settling to provide pretreatment before transfer to the RSF system. The pretreatment zone level is allowed to vary for storage of higher flow rates during wet weather. The pretreatment zone also is covered by a grid of floating insulating panels to prevent heat loss during cold weather and to shield the wastewater from sunlight, preventing algae growth during warm weather. Flow from the pretreatment zone to the sand filter recirculation tank is paced by a flow control system. The second stage of the primary lagoon is used for emergency storage of excess wet weather wastewater flow, which is returned to the pretreatment zone via a new lift station when emergency conditions have ceased.A 300-by-100-foot recirculating sand filter with eight zones was constructed to provide secondary stabilization of pretreated wastewater. A programmable logic control (PLC) system is used to control the effluent dosing to each zone in the sand filter. The control system is designed to provide maximum flexibility of operation. This allows for adjusting the recirculation ratio, which impacts the degree of nitrification achieved under different conditions.The results of this project have proven to be successful. The Village is now in compliance with its WPDES permit limits for BOD, SS, and ammonia nitrogen during warm and cold weather conditions. This paper will describe the different stages of this project, including funding development, design, construction, and start-up. Additionally, this paper will present a summary of the plant's first year of operation, including the data collected that indicates the system's ability to meet specific treatment requirements under varying seasonal conditions.