In-ground Nitrogen Reducing Biofilters for Onsite Wastewater Treatment

Authors:

  • Damann Anderson, Josefin Hirst - Hazen and Sawyer
  • George Heufelder - Barnstable County Department of Health and Environment

In 2009, the State of Florida initiated the Florida Onsite Sewage Nitrogen Reduction Strategies (FOSNRS) Project to develop more passive methods for nitrogen reduction from onsite wastewater systems (OWS). Numerous two stage nitrogen reducing biofilter (NRB) concepts were developed and tested, both in pilot and full scale. Tank-based and in-ground systems were evaluated, and several designs resulted in nitrogen removals of over 90% using lignocellulosic materials such as wood chips, mulch, or sawdust as carbon sources for denitrification. The Massachusetts Alterna-tive Septic System Test Center (MASSTC) located in Barnstable County, Massachusetts, and run by the Barnstable County Department of Health and Environment, further tested and developed the successful in-ground NRB design concepts. The goal of these systems is to provide options for reducing nitrogen inputs to watersheds where OWS have been identified as a significant source of nitrogen.

This paper presents results from in-ground NRB testing associated with the FOSNRS project and additional work carried out at the MASSTC. The in-ground NRBs utilize a two-stage passive bio-filtration concept treating septic tank effluent (STE). The first stage provides ammonification and nitrification via a porous media biofilter. The second stage provides denitrification via an anoxic biofilter with reactive media (such as lignocellulose).

FOSNRS Project: Based on the results from treatment media testing, a prototype in-ground NRB was designed and constructed at an OWS test facility at the University of Florida Gulf Coast Re-search and Education Center (GCREC) for more thorough testing. The site for this prototype unit consisted of a somewhat poorly drained fine sandy soil, and wet season water table elevations required a mounded system be designed. The mound system design consisted of 18 inches of fine sandy soil media for the stage 1 biofilter placed over a 50/50 mixture of sand and southern yellow pine sawdust lignocellulosic media in a V-shaped liner (stage 2a). An effluent collection pipe ran along the center of the liner, and discharged the stage 2a effluent to a small upflow stage 2b bio-filter containing elemental sulfur reactive media for further denitrification as necessary. The final effluent from the system was discharged to an infiltration trench consisting of plastic chambers. Figure 1 shows a schematic of the prototype in-ground NRB that was constructed and monitored as part of the FOSNRS project. The in-ground pilot NRB was operated over 500 days and aver-aged 90% total nitrogen reduction in effluent off the liner. This was reduced further by the stage 2b sulfur biofilter to 3.5 mg N/L, for a 95% overall nitrogen reduction by the system prior to discharge to the infiltration system trench.

Based on the successful performance of the pilot system, a full-scale vertically stacked in-ground NRB was designed and tested at a home in Longwood, Florida. The system was monitored over an 18 month period and averaged >84% total nitrogen reduction in the stage 2 liner effluent. This system also included an elemental sulfur biofilter tank after the stage 2 liner, and the entire system achieved 96% total nitrogen reduction after the additional denitrification process. Effluent from this system was used for turf irrigation at the home, where additional treatment will occur.

In addition to the treatment performance, groundwater quality was monitored at the full scale in-ground NRB site before and after installation. Prior to the NRB installation, a groundwater mon-itoring network was established, which included over 60 groundwater monitoring wells down gradient of the home’s existing conventional OWS drainfield. Shallow groundwater total nitrogen concentrations as high as 40 mg/L, primarily as NO3-N, were measured at several locations just below and down gradient of the existing drainfield before installation of the NRB. Following installation of the NRB, total nitrogen concentrations in the shallow groundwater were generally < 5 mg/L, approaching background concentrations at the site.

MASSTC Testing: The Massachusetts Alternative Septic System Test Center aimed to determine the simplest, most cost-effective modification of a soil treatment system to enhance nitrogen re-moval in Cape Cod’s geological setting. The configurations included designs that incorporated saturated and unsaturated Stage 2 (sand/lignocellulose) mixtures as summarized in Table 2. The unsaturated media testing utilized finer textured media mixed with sawdust to test the performance of a moisture holding denitrification layer in place of a liner.

The MASSTC preliminary results from prototype and full scale in-ground NRB suggest that at least 50% and up to 90% nitrogen removal occurred in the first year of the test. In Massachusetts colder climate, nitrogen removal performance degraded slightly in colder winter months for some of these trials.

These validating experiments suggest that a lignocellulose-sand mixture can be integrated into an OWS soil system in such a way as to promote denitrification. Further work to refine these systems is ongoing in Massachusetts, Long Island, and Florida.

For more information, please contact the author at danderson@hazenandsawyer.com.

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