METALS REMOVAL TECHNOLOGIES FOR STORMWATER

This study, on innovative methods for the capture of metals from stormwater, focused on two major areas of investigation: media filters and swales. The test water for both the filters and the swales was a stormwater which was collected from a parking lot during wet weather events. The characterization of the stormwater showed that the most prevalent metals detected in the runoff were iron, zinc, copper and small amounts of particulate bound lead. Ranges of metals were within national ranges reported by other studies.In the filter studies twelve media were chosen for initial evaluation. Equilibrium and kinetic studies were completed 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. Additionally determining the range of metal concentrations of the runoff to be treated is crucial to selecting the best media, since the removal efficiencies of the media relative to each other changed with varying metal concentrations. Upflow columns proved more effective than downflow columns in the control of detention time, 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.In the swale study, the hydraulic characteristics of grass swales appear to be more important than grass species for removing heavy metals from stormwater during single storm events. Many of the concentration reductions were quite large, but some “negative removals”, possibly associated with scour of previously deposited materials, were also noted. Because of the potentials for both sediment deposition and scouring, swales can improve or deteriorate water quality during individual storm events. Long term performance considering infiltration has shown significant heavy metal retention in swale systems. Data from the phytoremediation study suggests that there is a relatively similar behavior among the different grass species. However, in areas where it thrives, centipede is possibly the best choice for its resilience to drought, its nutritional frugality, and its greater ability to accumulate key contaminant metals such as Cu, Zn and Pb.