Filtered metals control in stormwater using engineered media

To address water quality concerns and numeric effluent limits, the designers of bioinfiltration/bioretention systems will need to integrate water and soil chemistry into the selection of filtration media mixtures. For the "dissolved" metals, designers will need to consider the ratio of valence states of the metals as they consider the proportion of ion exchange resins versus organic-based media in the final media mixture. As the correlations between pollutant capacity and soil/media chemistry showed, metals' capacity is directly related to organic matter content and the effective cation exchange capacity of the soil. Available stormwater treatment organic media provides a wide range of treatment sites, but possibly smaller numbers of each site type, compared to ion exchange resins such as zeolites. An activated organic media, such as granular activated carbon (GAC), will have an increased number of surface active sites potentially available for treatment, but this media may not sustain plant growth and may not be desired as a component of bioretention media. Other trade-offs also have to be considered in a complete analysis of a potential media component, such as the trade-off between organic content for plant growth versus nutrient leaching. Finally, there is a lower limit to treatment, after which no further pollutant removal occurs, especially given the contact time requirements based on draindown times required for many bioretention devices. Slightly improved removals for many metals may be achieved with much longer contact times, but the substantial increase in surface area devoted to bioinfiltration may not be cost-effective (given the draindown time requirements). This paper uses "dissolved" copper as an example of how to use both soil and water chemistry to design an optimal bioretention media from a subset of potential mixture components.