Fosnrs 3: The Performance of a Full-Scale 2 Stage Passive Biofilter System.

As part of the FOSNRS project to identify and test different systems to address nitrogen reduction from OWS, 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 OWS have been identified as a significant source of nitrogen.  This paper provides an overview of the field testing results from a full-scale two-stage passive biofilter system at a single family residence in Florida.

This full-scale PNRS studied utilized the two-stage passive biofiltration concept at a home located in Hillsborough County, FL, just southeast of Tampa.  The nitrogen reducing OWS for the 3 bedroom single family residence was installed in September 2012.   Primary treated wastewater, or septic tank effluent (STE) from the home’s existing septic tank is discharged to a two-stage treatment system consisting of a first stage unsaturated porous media recirculating biofilter for ammonification and nitrification, followed in series by a second stage saturated anoxic upflow porous media biofilter for denitrification.  The Stage 1 unsaturated biofilter utilized an expanded clay porous media.  Effluent from the Stage 1 biofilter was pumped to the Stage 2 biofilter and also recirculated back to the Stage 1 biofilter at a ratio of approximately 3:1 recirculation flow R to forward flow Q.   The Stage 2 anoxic biofilter utilized lignocellulosic and elemental sulfur electron donor reactive media for heterotrophic and autotrophic denitrification, respectively.  Crushed oyster shell was added for alkalinity control in the stage 2 biofilter.  The denitrified treated effluent was discharged into the home’s existing drainfield. 

The system was monitored over an 18 month period, receiving STE with an average total nitrogen (TN) concentration of 50 mg N/L. The Stage 1 biofilter with recirculation of nitrified effluent has consistently produced a nitrified effluent with ammonia N less than 3 mg N/L and a total nitrogen concentration averaging under 20 mg N/L.  The Stage 1 recirculation scheme has resulted in an average 61 percent reduction in TN through the first stage alone. 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 however, resulting in an average TN concentration in the overall system effluent of 3.5 mg N/L, a reduction in total nitrogen of over 93%.  TN in the effluent from the two stage system consisted of approximately 40 percent organic nitrogen and 60 percent ammonia N.  Thus, increasing the nitrification performance of the stage 1 biofilter could further enhance nitrogen removal from these systems, and investigations into this are underway.  While these are preliminary results, they suggest the potential to significantly reduce N input to sensitive watersheds from OWS.  Six additional full-scale PNRS are currently under early stages of evaluation, and results from these systems will provide key additional data regarding PNRS performance.