The Chesapeake Bay On-site Wastewater Nitrogen Removal BMP Expert Review Panel was reconvened to specifically evaluate two proposed BMPs:
- Drip irrigation (at a higher TN reduction credit than currently given under the “Shallow-Placed, Pressure-Dosed Dispersal” BMP) and
- Peat biofiltration systems discharging to a pad or trench
Upon review of available data, the Panel recommends creating a new, creditable BMP for Drip Irrigation, which has been shown to result in a 50 percent net TN reduction through Zone 1 in the drainfield. Qualifying characteristics for the new BMP, which are described in detail in the report, require the use of pressure compensating emitters, maximum not to exceed loading rates for three different soil types and exclusion of the credit for drip systems installed in Type I (sand textured) soils.
The Panel recommends that peat systems discharging to a pad or trench not be included by the CBP as a new BMP. In particular, existing data were from studies not designed to explicitly address nitrogen removal and the TN results were highly variable and thus inconclusive. The data do appear to support crediting peat filters a 20 percent net TN reduction as an ex situ BMP, consistent with similar technologies which fall under creditable BMPs: Intermittent Media Filters and NSF Standard 40 Systems.
The Panel further recommends that the CBP track efforts underway in EPA Regions 1 and 2 to develop nitrogen sensors specific to monitoring on-site wastewater systems and to consider using such sensors (or other appropriate methods) to verify the performance of BMPs that have been approved and are being implemented in the watershed. Recommendations for outstanding research questions are also provided (the reader is further referred to the 2014 predecessor report on OWTS Nitrogen Removal BMPs and the 2016 OWTS Nitrogen Attenuation report for additional recommendations for the CBP).