Efficient nitrogen removal from onsite wastewater by a novel continuous flow biofilter

Soil-based passive biofiltration system is an economically feasible technology for nitrogen removal from onsite wastewater. However, the conventional design requires a large system footprint with limited treatment capacity. In this study, a novel continuous flow biofilter (CFB) with adjustable recirculation and continuous flow pattern was developed for onsite wastewater treatment with a small footprint. Efficient total nitrogen removal (80.1–97.5%) was observed at various hydraulic loadings (0.03–0.12 m3 m−2 d−1), nitrogen loadings (1.1–8.6 g N m−2 d−1) and recycle ratios (2–3) when treating septic tank effluent (STE), with low effluent TN (0.7–13.6 mg N L−1). Nitrous oxide was observed in the denitrification effluent indicating incomplete denitrification at elevated dissolved oxygen levels (3.3–5.8 mg L−1). Nitrogen removal rate (2.9–7.0 g N m−2 d−1) and ammonium removal rate (2.4–7.2 g N m−2 d−1) were positively correlated with nitrogen loadings increase (1.1–8.6 g N m−2 d−1) but were not significantly impacted by the hydraulic loading rate change (0.08–0.12 m3 m−2 d−1). The total biomass abundance and nitrifying microorganisms decreased significantly as the nitrification columns depth increased, while homogeneous microbial distribution was observed in the denitrification columns. The abundance of ammonium oxidizing archaea (AOA) increased significantly at increased hydraulic and nitrogen loading rate, while the ammonium oxidizing bacteria (AOB) abundance remained steady. The abundance of functional genes involved in denitrification process (nirS, nirK and nosZ) responded differently when hydraulic and nitrogen loading rate changes. Collectively, this study suggested the CFB could efficiently remove nitrogen from onsite wastewater with fluctuating influent compositions and various hydraulic loadings. [Display omitted] •Efficient N removal (80–98%) was achieved by the continuous flow biofilter.•N loading and the pre-aeration condition highly impact the N removal efficiency.•Functional microbial abundance changed differently with operational changes.•Increase of internal recycle ratio (R) did not impact the N removal efficiency.•Nitrous oxide emission was mainly from the denitrification columns.