Treatability of Stormwater Heavy Metals – Or Breaking the Irreducible Concentration Barrier

This presentation reviews the results of related stormwater research projects (funded the EPA, WERF, and the telecommunications industry) that focused on the treatability of stormwater heavy metals. Stormwater heavy metals are strongly associated with particulates, but mostly with the finer sized fractions. The removal of these particles requires rigorous effort. In many cases, significant fractions of stormwater heavy metals are also associated with the filterable fraction of the stormwater. Tests have shown that these metals are mostly in ionic forms, and not as colloids or as organometallic forms. Therefore, the highest levels of heavy metal reductions in stormwater will require a multi-level approach using several complementary unit processes that focus on these challenging stormwater components. This paper summarizes laboratory and fullscale demonstrations of a few options to obtain high level heavy metal reductions in stormwater, including chemical treatment, sedimentation of fines, and ion-exchange/sorption. A large unit, the multi-chambered treatment train (MCTT), is described that has shown to be capable of very high levels of heavy metal (and organic toxicant) control. A smaller unit, more suitable for smaller critical drainage areas, that uses upflow filtration in addition to ion exchange, is also presented. These methods, that incorporate several complimentary unit processes, are shown to result in lower effluent heavy metal concentrations compared to conventional stormwater practices that rely mostly on sedimentation.