Laboratory Study of Stormwater Biofiltration in Low Temperatures: Total and Dissolved Metal Removals and Fates

Stormwater biofilters, which are recommended for application in both Water-Sensitive Urban Design and Low Impact Development, can remove up to 80% or 90% of total metals found in stormwater. However, their winter operation is a common concern. That was addressed in this study by investigating the metal removal effectiveness of replicate laboratory biofilter mesocosms at 2°C, 8°C and 20°C. As recommended for cold climate bioretention, coarse filter media were implemented and in the top 100 mm layer topsoil was added to increase the sorption capacity. Cd, Cu, Pb and Zn concentrations measured in the biofilter effluent were far below those in the influent and this significantly improved the treated stormwater quality. Contrary to a common notion that coarse media in the main filter body impair dissolved metal sorption, satisfactory removals of dissolved metals were found in this study with most metal burdens retained in the top layer of the filter in which the sorption capacity was enhanced by topsoil. Some metal uptake by the plants was also detected. Temperature did not affect Cd, Pb and Zn removals in general, but Cu removals increased with decreasing temperatures. This was explained by increased biological activities in the filters at warmer temperatures, which may have led to an increased release of Cu with dissolved organic matter originating from root turnover and decomposition of organic litter and debris. Furthermore, plant uptake and biofilm adsorption may also be influenced by temperature. However, even in the worst case (i.e. at 20°C), Cu was removed effectively from the stormwater. Further research needs were identified including the effects of road salts on stormwater biofiltration during the winter period.