Backyard BNR: Passive nitrogen reduction system research for onsite wastewater treatment

As part of the Florida Onsite Sewage Nitrogen Reduction Strategies (FOSNRS) project, passive nitrogen reduction systems (PNRS) were developed and pilot tested and are now being evaluated at homes in Florida. The goal of these systems is to reduce nitrogen inputs to watersheds where onsite wastewater systems (OWS) have been identified as a significant source of nitrogen. Results from a full-scale PNRS installed at a 3 bedroom single family residence in Hillsborough County, Florida are presented here. This PNRS utilizes a two-stage passive biofiltration concept treating septic tank effluent (STE). The first stage provides ammonification and nitrification via a recirculating porous media biofilter. The second stage provides denitrification via an anoxic biofilter with reactive media. The system was monitored over an 1 8 month period, receiving STE with an average total nitrogen concentration of 54.7 mg N/L. The Stage 1 biofilter with recirculation of nitrified effluent has consistently produced a nitrified effluent with ammonia N less than 4 mg N/L and an average total nitrogen concentration of 31 mg N/L. The second stage biofilter has consistently produced a final effluent with NO2-NO3 N concentrations below the method detection limit of 0.02 mg N/L. Residual ammonia nitrogen in the effluent from the Stage 1 biofilter passes through the Stage 2 biofilter resulting in an average TN concentration in the overall system effluent of 2 .5 mg N/L, a reduction in total nitrogen of over 95%. These results suggest the potential to significantly reduce N input to sensitive watersheds from OWS.