Nitrogen removal mechanisms in biochar-amended sand filters treating onsite wastewater

The performance of biochar-amended sand filters treating septic tank effluent (STE) was investigated in bench-scale columns. Softwood biochar showed higher NH4+-N adsorption capacity (1.3 mg N g(-1)), and its water holding capacity (0.57 g ml(-1)) was significantly higher than sand (0.26 g ml(-1)). Two biochar amendment ratios (10% and 30%) were selected for STE treatment in short-term (20 days) and long-term (8 months) studies. During the short-term experiment, the overall total nitrogen removal efficiency was greater in biochar-amended sand columns (94.7%-95.6%) than in 100% sand columns (71.2%) due to the additional NH4+-N adsorption by biochar. Greater nitrification performance was also observed in biochar-amended columns (87.1%-96.3%) than in 100% sand columns (61.4%) during long-term operation when alkalinity was insufficient. The nitrification performance in biochar-amended columns resumed more quickly (<7 days) after sufficient alkalinity was amended. The density of total biomass and nitrifying bacteria in biochar-amended columns (30%) were significantly higher at all experimental stages, suggesting biochar served as a growth media for enhanced biomass growth. The alkalinity changes and STE composition fluctuation had little impact on the nitrification performance of the 30% biochar-amended sand columns. In addition, biochar surface functional groups and zeta potential changed little after long-term STE filtration. Collectively, the results demonstrated proper biochar amendment ratio (30%) could enhance the nitrification performance of sand filters treating STE by increasing the system hydraulic retention time, providing additional alkalinity for nitrification, and serving as a growth media for enhanced biomass growth.

Automated summary

The performance of biochar-amended sand filters in treating septic tank effluent (STE) was studied. Softwood biochar demonstrated higher adsorption capacity for NH4+-N and greater water holding capacity compared to sand. Addition of biochar improved total nitrogen removal efficiency and nitrification performance, especially when alkalinity was insufficient. Biochar amendment also increased biomass growth and had little impact on nitrification performance during fluctuation in STE composition.