Performance and microbial characteristics of a novel continuous flow nitrogen removing biofilter for onsite wastewater treatment

Conventional soil-based biofiltration system is an economically feasible option to remove nitrogen from onsite wastewater. However, it requires large footprint to accommodate the design loading rate with limited treatment capacity. In this work, we developed a novel continuous flow nitrogen removing biofilter (CF-NRB) with small footprint for onsite wastewater treatment. Compared with other soil-based onsite wastewater treatment systems (OWTSs), the CF-NRB achieved efficient nitrogen removal (81.6%) at high hydraulic (0.12 m³ m^-2 d^-1) and recycle ratio (R=3). Nitrous oxide (N2O) production (17.0 mg N2O-N m^-2 d^-1) was observed in the CF-NRB, and denitrification columns were the major source of N2O production (>80%). In addition, the abundances of total biomass and nitrifying microorganisms decreased significantly as the nitrification columns depth increased, while homogeneous microbial distribution was observed in the denitrification columns. Collectively, this study demonstrated that the CF-NRB is a promising technology that could efficiently remove nitrogen from onsite wastewater at high hydraulic loading with a small footprint.