Compost biofilters for highway stormwater runoff treatment
Highway stonnwater runoff negatively affects the water quality of receiving lakes and rivers. Pollutants that build up on roads and other impervious surfaces are harmful to aquatic life and surrounding ecosystems. Woody compost or 'overs' have been found to be effective in removing highway pollutants including suspended solids, heavy metals and petroleum hydrocarbons (PHC). Overs have also been found to be effective as a filtration media when assembled inside a mesh casing to form a compost biofilter. In the summer of 2007 a full scale on site compost biofilter was erected on Hwy. 8 in Kitchener, Ontario. Samples were automatically collected pre and post filtration, and flow data was recorded using weirs and level loggers. A relationship between the upstream water level and the outlet flow rate was developed. The flow through capacity of the compost biofilter was found to be 90-100 L/s. The biofilter was found to be effective in the removal of suspended solids, heavy metals and PHC. The biofilter had a removal efficiency of 42% for TSS, 32% for zinc, 29% for copper and 66% for PHC. The Ontario Provincial Water Quality Objective (PWQO) for TSS was 25 mg/L. In total 9 out of 20 events exceeded the objectives for TSS in Highway 8 runoff (i.e. Inlet Concentration); however, only 4 out of 20 events exceeded the objectives for TSS after filtration through the biofilter (i.e. Outlet Concentration). The PWQO for Zinc is 20 [mu]g/L. In total 15 out of 20 events exceeded the objectives for Zinc in Highway 8 runoff; however, only 5 out of 20 events exceeded the objectives for Zinc after filtration through the biofilter. The PWQO for Copper is 5 [mu]g/L. All 20 events exceeded the objectives for Copper in Highway 8 runoff. Australian Water Quality Guidelines for Fresh Waters (ANZECC, 1992) for Polycyclic aromatic hydrocarbons (PAH) maximum recommended concentration in freshwater is 3 [mu]g/L. In total 18 out of 20 events exceeded the objectives for PAH in Highway 8 runoff; however, only 6 out of 20 storm events exceeded the objectives for PAH after filtration through the biofilter. Using the obtained data from the highway site it was observed that the build-up of pollutants on a highway are governed by antecedent dry days (ADD), and average annual daily traffic (AADT), while the wash-off of the pollutants are influenced by the rainfall depth and rainfall intensity. The compost biofilter was found to be effective in reducing the concentrations of pollutants found in highway runoff. Although a reduction in removal efficiency over time was not observed, the life span for the installed biofilter was estimated to be around 6 years before decommission.