Nitrogen Removal in Recirculating Sand Filter Systems with Upflow Anaerobic Components

Septic systems can present a risk to human health by releasing highly soluble nitrate–nitrogen into the groundwater. A research and demonstration study undertaken in Black River Falls, Wisconsin, evaluated several promising biofilter technologies for on-site nitrogen removal. Duplicate recirculating sand filter-upflow anaerobic systems with a design hydraulic loading rate of 954?L/day (250?gal/day) were used to treat septic tank effluent from a correctional institution and produced a treated wastewater with a total nitrogen concentration of 15.2?mg/L for System 1 and 18.2?mg/L for System 2, or 72.0 and 63.0% nitrogen removal, respectively. The differences between the two systems appear to have been the result of process configuration changes made over the duration of the study. This paper evaluates the nitrogen removal performance of the recirculating sand filter-upflow anaerobic systems and the effect of operational and environmental factors, including the recirculation ratio, BOD5/NO3?, and temperature. Nitrogen removal was limited by the recirculation ratio with the maximum total nitrogen removal of 70.1% when the recirculation ratio = 3. Improved performance was also noted for temperatures ≥ 20°C and BOD5/NO3? ≥ 8. Low temperatures adversely affected nitrification and low BOD5/NO3? adversely affected denitrification. The relationships among nitrogen removal, recirculation ratio, BOD5/NO3?, and temperature are also discussed.