Optimized Parameters and Mechanisms for Simultaneous Nitrogen and Phosphorus Removal in Stormwater Biofilters: A Pilot Study

Nitrogen (N) removal efficiency through denitrification is usually not ideal and stable in biofilter systems due to the deficiency of organic carbon supply and difficulty of formation of anaerobic status, which could be enhanced by regulating the inflow carbon/nitrogen ratio (C/N), outflow water level, retention time, and addition of plant detritus through the pilot-scale stormwater biofilter study. Removal efficiency of ammonium (NH4+-N), soluble reactive phosphorus (SRP), and total phosphorus (34-90%) was significantly higher than that of nitrate (NO3--N), dissolved total nitrogen (DTN), and total nitrogen (TN) (9-25%) in the biofilter systems. The addition of plant detritus (especially herbaceous plant) in biofilter systems can enhance the nitrogen (N) and phosphorus (P) removal efficiency by >24%. Through further regulation of inflow C/N, retention time, and outflow water level to promote denitrification, removal efficiency of NO3--N, DTN, and TN was significantly enhanced and reached up to 83%, 68%, and 73% on average, respectively. However, P leaching increased due to SRP release from iron-bound P caused by anoxia. In the overall consideration of N and P removal, optimized inflow C/N, retention time, and outflow water level are estimated as 10-30, 1-2 h, and 20-40 cm, respectively, which makes a crucial contribution to simultaneous N and P removal.