Field evaluation of a biochar-amended stormwater filtration system for retention of nutrients, metals, and Escherichia coli†

The objective of this study was to assess the impacts of biochar and iron-enhanced sand (IES) on the comprehensive contaminant retention performance of a field-scale stormwater filtration system. The system distributed runoff from a parking lot into three filters containing sand, sand amended with biochar (custom-produced via pyrolysis of red pine wood chips at 550 °C), or IES. Over the first two field seasons of operation flow into the testbed and out of each filter were continuously monitored, and influent and effluent samples were collected during 21 precipitation events and analyzed for various contaminants and water quality parameters. To account for variations in flow distribution between the filters, long-term filter performance was assessed based on comparison of apparent cumulative input and output contaminant loads over the study duration (i.e., apparent cumulative contaminant retention). The IES filter showed the most effective phosphorous retention performance (>90% net retention of total phosphorus, TP), reflecting results from previous studies. The biochar-amended filter showed improved retention of zinc and total inorganic nitrogen (TIN) relative to the sand filter, which may be attributed to: (i) enhanced electrostatic interactions between zinc and oxygen-containing functional groups on the biochar surface, and (ii) improved attenuation of ammonia-N due to reduced nitrification and/or enhanced adsorption of ammonium. The biochar-amended filter did not show improved retention of total organic carbon or Escherichia coli, in contrast to some previous studies, potentially due to differences in biochar material properties (e.g., reduced hydrophobic interactions due to the custom biochar's relatively polar surface chemistry) or operational conditions (e.g., differences in flow rate or biofilm development between the filters). These findings demonstrate the complexities surrounding the application of biochar as a stormwater filter material for broad contaminant removal, and warrant the development of best practice recommendations for biochar selection and performance testing.