Biochar amendment of stormwater bioretention systems for nitrogen and Escherichia coli removal: Effect of hydraulic loading rates and antecedent dry periods

[Display omitted] •High surface area and cation exchange biochar retained more NH4+, E. coli and DOC.•Biochar enhanced nitrification during aerobic periods between storm events.•Biochar retained moisture, favoring denitrification and total nitrogen removal.•Stable N and E. coli removal observed under varying hydraulic loading rates. Bioretention systems improve stormwater infiltration and water quality; however, limited total nitrogen (TN) and fecal indicator bacteria (FIB) removal is observed in sand-based bioretention media. In this study, the fate of nitrogen and E. coli in bioretention systems was investigated through batch and column studies using sand media, with and without biochar addition. Variables investigated included biochar characteristics, hydraulic loading rate (HLR) and antecedent dry period (ADP). Total ammonia nitrogen (TAN), dissolved organic carbon (DOC), and E. coli removals were significantly higher in biochar-amended columns due to biochar’s high cation exchange capacity and specific surface area. TAN adsorption resulted in increased nitrification during the ADP when aerobic conditions developed. Moisture content data revealed that saturated conditions prevailed toward the bottom of biochar-amended columns for several days, favoring denitrification and TN removal. Biochar amended columns also showed more stable TAN, DOC and E. coli effluent concentrations under varying HLR and ADP.