Evaluating the simultaneous retention of organic contaminants and Escherichia coli (E. coli) in biochar-amended biofilters

Though organic contaminants and Escherichia coli (E. coli) are very different contaminants, both are ubiquitous in urban runoff, mobile in conventional biofilters, and interact with biochar via hydrophobic interactions. However, limited information is available regarding their simultaneous retention in biochar-amended filtration systems, which was evaluated here via intermittently dosed column tests. Columns amended with commercial biochar (ABC-biochar or WF-biochar) were compared to sand-only controls over treatment of 100 empty bed volumes (EBVs) of creek water, which was augmented with dissolved organic carbon (DOC) and organic contaminants, as well as E. coli during three loading periods. While both biochars demonstrated similar DOC removal, effluents from ABC-biochar columns showed reduced specific ultraviolet absorption (SUVA) and improved organic contaminant retention relative to sand-only and WF-biochar columns. However, biochar-amended filters showed limited improvement in E. coli retention, as sand-only and biochar-amended columns demonstrated up to 1.0 ± 0.5 and 1.4 ± 0.4 log-removal of E. coli, respectively. E. coli log-removal for all columns was reduced to 0.5 ± 0.1 following a freeze-thaw cycle. Drainage rates for ABC-biochar columns were on average approximately 50% higher than the other columns, demonstrating the importance of considering hydraulic conditions when assessing overall filtration performance. Our findings warrant more rigorous validation of the effects of biochar amendment to filtration performance under environmentally relevant conditions and at the field scale.