Denitrifying Bioretention Systems for Control of Non-Point Nitrogen Sources

Side-by-side pilot-scale experiments were conducted with bioretention systems utilizing both elemental sulfur and wood chips as substrates for denitrification at varying hydraulic loading rates (HLR), influent concentrations and wetting and drying periods. Total N removal efficiencies greater than 88% were observed in both units with synthetic stormwater. In field tests with dairy farm runoff and high HLR, moderate removal efficiencies were observed for COD (46%), suspended solids (69%), total P (66%), and total N (65%). During the second season, operational changes on the farm resulted in lower organic, solids and nutrient loadings resulting in improved bioretention system effluent quality, especially with regard to suspended solids (81% removal) and total N (82% removal). The systems were not hydraulically overloaded even at 20 times the normal HLR. Somewhat better overall performance and cold temperature tolerance was observed in the wood-based unit than the sulfur based unit.