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 NH 4 + -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 TN removal efficiency was greater in biochar-amended sand columns (94.7 - 95.6%) than 100% sand columns (71.2%) due to the additional NH 4 + -N adsorption by biochar. Greater nitrification performance was also observed in biochar-amended columns (87.1 - 96.3%) than 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. This article is protected by copyright. All rights reserved.