Removal of Nutrients from Stormwater Using Biochar and BOF Slag Filter Media in Bioswales

Urban stormwater runoff often carries excessive nutrients such as phosphate and nitrate, which leads to the eutrophication of surface water bodies that endangers the environment, ecology, and human health. The concept of bioswales has garnered significant attention in recent times, whose function is to avoid flooding while simultaneously removing various pollutants from the runoff. However, conventional filter media such as sand, used in bioswales, is inadequate in removing these harmful nutrients. Consequently, there is a growing need to develop innovative filter media capable of efficiently removing various pollutants including nitrate and phosphate from stormwater runoff. In this study, basic oxygen furnace (BOF) slag (a byproduct of steel manufacturing) and biochar (resulting from the gasification of waste pinewood) were investigated as filter media for the removal of nitrate and phosphate from stormwater runoff. Basic physicochemical and geotechnical characterization was performed to inform bioswale design, while batch and column tests were performed to assess the efficacy of these materials in removing the nutrients. Results of batch tests showed adequate efficiency of both materials to remove nitrate, with adsorption capacities of 78.74 and 129.87 mg/kg of slag and biochar, respectively. Results also showed an exceptional ability of BOF slag in removing phosphate with an adsorption capacity of approximately 16,667 mg/kg of slag, while in comparison biochar had significantly lower adsorption capacity of 48.54 mg/kg of biochar. Subsequently, column tests were performed to assess the efficiency of these materials under dynamic flow conditions. The results from the column tests agreed with the findings from batch tests, except for the case of phosphate adsorption using biochar, in which case biochar showed very low removal efficiency. Overall, both biochar and BOF slag (both waste materials) show promise to be effective in nutrient removal as filter media in bioswales, while the actual performance in field scale scenario with runoff containing wide-ranging contaminants is yet to be assessed.