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Abstract
Stormwater engineering has grown to include not only the management of water quantity but also water quality. Bioretention systems are designed to address both concerns by retaining stormwater to mitigate flooding and, at the same time, improve water quality through biofiltration. Although there are well established and widely accepted removal efficiencies for many common pollutants through bioretention systems, the transport and removal of Microplastics as an emerging contaminants of concern is not well understood. In order to begin assessing the capability of bioretention systems to remove microplastics, the component that makes up the majority of the soil filter media must be analyzed. The sand component of bioretention media was tested through a series of laboratory experiments designed to evaluate the effects of variable stormwater and constituent parameters on microplastic removal efficiency. Significant changes in removal efficiency were observed based on variations in particle size, while removal rates were consistent across polymer types. Removal efficiency dropped from an average of 97.3% to 96.5% to 66.68% as polystyrene particle size dropped from 33.1 μm to 10.7 μm to 3.93 μm. Microplastics of similar size but different type showed comparable results as 22-27 μm polyethylene particles produced a 95.47% removal rate. The 33.1 μm polystyrene microspheres were tested in the presence of two common stormwater co-contaminants: heavy metals represented by ZnCl and oil and grease represented by motor oil. The presence of co-contaminants in levels observed in stormwater did not significantly change removal rates. Average removal rates of 98.52% were observed in the presence of ZnCl and average removal rates of 97.90% were observed in the presence of motor oil. The 3.93 μm polystyrene microspheres were tested in the presence of oil and grease and produced an average removal rate decrease from 66.68% to 59.36%. The sand component of bioretention media proved to be an effective filter media for the removal of microplastics of various types, and in the presence of co-contaminants; however removal rates significantly drop as particle size decreases below 10 μm and particles of that size appear to be more affected by the presence of co-contaminants.