Investigating N2and N2O Production in Nitrogen Removing Biofilters for Onsite Wastewater Treatment

Conventional onsite wastewater treatment systems (OWTSs) remove BOD and pathogens from wastewater but are not designed to remove nitrogen (N) which percolates through receiving soils to groundwater. Nitrogen removing bio-filters (NRBs) are OWTSs designed to achieve significant N removal by coupling biological nitrification and subsequent denitrification by naturally occurring soil microbes. NRBs consist of a nitrifying sand layer above a denitrifying mix of sand and lignocellulose (i.e. carbon source). Two conditions are required for N removal in NRBs: (1) an oxic nitrification layer and (2) sufficient labile C available under anoxic conditions in a denitrification zone. Production of N_xO intermediates has been reported under low dissolved oxygen (DO) both in bench-scale experiments and in wastewater treatment plants. However, there are few studies characterizing the product-side of N transformations in NRBs.

This study investigated N transformations in NRBs by monitoring (1) dissolved N₂in slurries of nitrified percolate and aged denitrifying soil matrix in sealed bench-scale incubations and (2) emissions of N₂O in static chambers above in-ground NRBs. Lab incubations showed an average 0.82 mols N- N₂produced /mol N-NO₃^-consumed. Some portion of the remaining fraction may have been due to one or a combination of (1) analytical error; (2) N bio-assimilation by growing microbes and (3) production of N_xO. Measured DO was > 0.9 mg L^-1in all incubations and frequently between 2- 3 mg L^-1.

Initial monitoring of in-ground NRBs showed N₂O emissions increased from air equilibrium values, yet these emissions accounted for less than 1% of influent N. Observed values were consistent with reports from full-scale wastewater treatment plants. If N₂O production constituted a substantial portion of the N deficit in the bench-scale incubations, the result may have arisen from prevailing sub-oxic conditions and would not characterize denitrification in in-ground NRBs generally. Efforts to comprehensively understand N transformations in NRBs are ongoing.