Improving the nitrogen removal efficiency of recirculating sand filters

Recirculating Sand Filters (RSFs) are commonly used to treat septic tank effluent from individual homes and small communities. It is generally accepted that the RSFs can provide partially nitrified effluent, with nitrogen removal efficiencies up to 40 percent. However, for applications such as schools and restaurants, where the influent total nitrogen concentrations can be as high as 70 to 80 mg/L, RSFs can fail to achieve sufficient nitrogen removal to comply with the permit requirements. The RSF system that will be described in this paper is located in Chaparral, New Mexico. The RSF unit currently serves one elementary school and the middle school of the community. After the installation of the RSF unit, the Biochemical Oxygen Demand (BODs) and Total Suspended Solids (TSS) removal efficiencies were about 98 and 90 percent, respectively. However, the nitrogen removal efficiency was sporadic, with an average of about 20 percent. In order to enhance nitrogen removal and to comply with the 27 mg/L effluent nitrogen requirements of the discharge permit, we applied simple process modifications to the 25,000 gallon per day RSF treatment facility. The process modification included installation of a pump and a recycle line to return the nitrified RSF effluent to the septic tank. The described process modification is a low-cost and effective method of enhancing nitrogen removal, especially on existing systems without changing major design components of a treatment facility. With the improvement, the nitrogen removal efficiency observed at the plant increased to 54 percent.