Hydrologic and pollutant removal performance of stormwater biofiltration systems at the field scale

Biofiltration systems are a recommended and increasingly popular technology for stormwater management; however there is a general lack of performance data for these systems, particularly at the field scale. The objective of this study was to investigate the hydrologic and pollutant removal performance of three field-scale biofiltration systems in two different climates. Biofilters were shown to effectively attenuate peak runoff flow rates by at least 80%. Performance assessment of a lined biofilter demonstrated that retention of inflow volumes by the filter media, for subsequent loss via evapotranspiration, reduced runoff volumes by 33% on average. Retention of water was found to be most influenced by inflow volumes, although only small to medium storms could be assessed. Vegetation was shown to be important for maintaining hydraulic capacity, because root growth and senescence countered compaction and clogging. Suspended solids and heavy metals were effectively removed, irrespective of the design configuration, with load reductions generally in excess of 90%. In contrast, nutrient retention was variable, and ranged from consistent leaching to effective and reliable removal, depending on the design. To ensure effective removal of phosphorus, a filter medium with a low phosphorus content should be selected. Nitrogen is more difficult to remove because it is highly soluble and strongly influenced by the variable wetting and drying regime that is inherent in biofilter operation. The results of this research suggest that reconfiguration of biofilter design to manage the deleterious effects of drying on biological activity is necessary to ensure long term nitrogen removal.