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Recommendations of the On -Site Wastewater Treatment Systems Nitrogen Reduction Technology Expert Review Panel February 2014 Summary of Recommendations The On -Site Wastewater Treatment Systems Nitrogen Reduction Technology Expert Review Panel (OWTS Expert Panel) was tasked with identifying and recommending on-site wastewater treatment technologies or modifications to existing wastewater treatment systems that would reduce total nitrogen (TN) loads to the Chesapeake Bay watershed. The OWTS Expert Panel was instructed not to address the issue of nitrogen attenuation in the native soils between the edge of the treatment system (drainfield) and the edge of the stream, since the Chesapeake Bay Program Office (CBPO) and a future Expert Panel will review and address this issue. The OWTS Expert Panel also reviewed the existing scientific research and provided recommendations for TN reduction credits that can be assigned for specific OWTS technologies and system modifications. To the extent possible, the associated TN reduction credits were linked to the planning, design, installation, and operational elements of OWTS. Recommendations were also made regarding verification of best management practice (BMP) performance. This report is intended to be an internal technical document for the CBPO to use to adapt the existing Chesapeake Bay Model and BMP crediting program. A number of other valuable resources are available to assist regulators, designers, and owners in making decisions about the type of systems to install based on the benefits, drawback, costs, and other characteristics of specific systems. Appendix A lists these resources. As a starting point, the OWTS Expert Panel used existing CBPO guidance and reviewed recent literature to develop baseline TN load estimates for use in modeling and BMP performance comparisons. This exercise required the OWTS Expert Panel to determine how much TN was discharged per capita as a baseline necessary to model system performance. The OWTS Expert Panel concluded that 5 kg TN/person/year or a 60 mg/L concentration of TN at a flow rate of 60 gallons per capita per day (gpcd) could be reasonably estimated as the TN loading associated with the septic tank effluent (STE) applied to the drainfield from a conventional septic tank system. The OWTS Expert Panel agreed that the current CBPO assumption that a 20 percent TN reduction occurs within a conventional gravity flow drainfield was reasonable. Based on these assumptions, the OWTS Expert Panel also concluded that the TN load discharged at the edge-of- drainfield can be estimated to be 4 kg TN/person/year, as currently defined in the Chesapeake Bay Model. The OWTS Expert Panel divided appropriate BMPs into two main categories: ex situ BMPs that occur prior to the drainfield, and in situ BMPs that are implemented as enhancements to the soil treatment unit, including the drainfield. Reduction credits for ex situ systems were compared to the baseline of 5 kg TN/person/year associated with STE. Reduction credits for in situ and combined BMPs were compared with the baseline edge-of-drainfield performance of 4 kg TN/person/year that was used to model the performance of a conventional septic tank coupled with a gravity -flow drainfield. Tables ES -1, ES -2, and ES -3 summarize the OWTS Expert Panel recommendations for ex situ BMPs, in situ BMPs, and combined BMPs, respectively. The OWTS Expert Panel recommends continuing to offer the existing 5 percent TN reduction credit for pumpout of septic tanks and 100 percent TN reduction (transfer from the on-site sector to the National Pollutant Discharge Elimination System [NPDES] sector) for septic systems that are decommissioned and connected to NPDES (discharging) facilities. O TETRA TECN.INC. 9