Metals Removal Technologies for Stormwater: Filtration

This study, sponsored by the Water Environment Research Foundation, focused on innovative methods for the capture of metals from stormwater. The two areas of focus were media filters and swales. This paper will focus on the filter studies. Twelve media were chosen for initial evaluation. Equilibrium and kinetic studies were formed on these media to assess their performance in capturing metals from urban runoff. The three best performing media: peat-sand mix, compost, and zeolite were then selected for in-depth study. The results of this investigation emphasize the importance of characterizing the stormwater before selecting a treatment media since the type and quantity of metals, pH, and other runoff characteristics can vary a great deal between sites. Upflow columns proved more effective than downflow columns in the control of detention time and reduction in clogging of the media by solids and associated head loss in the column. Studies on the effect of anaerobiosis on metal retention by filter systems indicated that heavy metals were not mobilized from filter systems under anaerobic conditions. It was found that metal retention by the filters was not different from what was observed in oxygenated environments. Tests also indicate that the heavy metals of concern remain strongly bound to the particulates during long exposures at the extreme pH conditions likely to occur in receiving water sediments. Several of these filter media were also tested in a pilot-scale device using water from a detention pond that drains a medium-density residential area in Hoover, Alabama. In this series of tests, the runoff water was not spiked and as a result, the metals concentrations in the influent were extremely low (near the detection limit of the analysis). On those occasions where the metals concentration was higher, such as shortly after a rain storm, the filters proved effective at removing influent concentrations down to a level of approximately 10 – 15 μg/L. Removals to concentrations lower than that were not possible on a consistent basis.