Heavy metals and pathogens are two of the most common pollutants found in urban stormwater, and pathogens are one of the most difficult pollutants to remove by stormwater biofilters, leading to impairment of surface water. Limited removal capacity of filter media used in biofilters and potential growth of pathogens retained in biofilters often lead to net negative removal - that is effluent concentration of pathogen is higher than influent concentration. The goal of the study was to test the capacity expanded shale, clay, and slate (ESCS) aggregates to remove metals (Pb, Cu, and Zn) and pathogen indicator (E. coli) and test the hypothesis that adsorbed metals can enhance bacterial removal with the aging of the filter media. To test the hypothesis and understand mechanisms of pollutant removal by ESCS aggregates both batch and column tests were conducted in the laboratory using ESCS aggregate, and their surface characteristics were examined using spectroscopic tools such as FTIR and SEM. The results show that amending biofilters with ESCS aggregates could remove both heavy metals and pathogens. Sand amended with ESCS removed Pb, Cu, and Zn from contaminated stormwater, and no breakthrough was observed after injection of 500 pore volume of stormwater. Interestingly, the adsorbed metals not only reduced the growth of E. coli in the biofilter columns during extended drying periods but also reduced the remobilization of the previously adsorbed E. coli during the intermittent flow of stormwater. The results suggest that metals adsorbed with the aging of ECSC aggregates act as a natural disinfectant coating to increase adsorption and inactivation of the adsorbed E. coli, thereby regenerating the filter media capacity to remove more E. coli in the following rainfall events. Thus, the use of ECSC aggregates to remove metals and pathogen indicators can improve treatment efficiency and the overall long-term cost of biofilters.

Annesh Borthakur

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