Treatment conditions for the removal of contaminants from road runoff

by Hallberg, Magnus

In highly urbanised areas, the existing land use may restrict the building of extensive ponds or wetlands for removal of particulate and dissolved pollutants from road runoff. To optimise treatment facilities for road runoff, treatment conditions must first be assessed. Field studies were conducted in three separate watersheds along the same highway. Based on the EU Directive (1991/271/EEC) maximum of 60 mg/l total suspended solids (TSS) in wastewater, it was found that treatment of the total runoff volume was necessary. The concentrations of dissolved Al, Cd, Co, Cr, Mn and Ni were significantly higher in winter compared with summer, but no significant difference was found for Cu, Pb and Zn. Total concentration of metals showed a good correlation to TSS (r2>0.75). It was possible to estimate the reduction in TSS using an empirical model from the case study at one of the field sites. It was also possible to remove dissolved heavy metals at surface loads 5 to 10 times higher than in previous laboratory studies using a fixed filter bed of blast furnace slag. The mass concentration of metals (mg/kg) in particulate material varied between seasons. Sediment generation (mg sediment/mm precipitation) was found to be constant during summer. The field studies showed that turbidity measurement could be used for process monitoring and controlling treatment of road runoff. The findings of this study could be used to evaluate watersheds where traffic is the dominant source of pollutants, together with existing design methods to optimise treatment systems.