Long-term field performance of a conventional and modified bioretention system for removing dissolved nitrogen species in stormwater runoff

Bioretention systems are efficient at removing particulates, metals, and hydrocarbons from stormwater runoff. However, managing dissolved nitrogen (N) species (dissolved organic N, NH4+, NO2−, NO3−) is a challenge for these systems. This paper reports the results of a long-term field study comparing N removal of: 1) a modified bioretention system that included an internal water storage zone containing wood chips to promote denitrification and 2) a conventional bioretention system. The systems were studied, without and with plants, under varying hydraulic loading rates (HLRs) and antecedent dry conditions (ADCs). Both bioretention designs were efficient at removing NH4+ (83% modified, 74% conventional), while removal of NOx (NO2−-N + NO3−-N) was significantly higher in the modified system (81% modified, 29% conventional). Results show that the addition of an internal water storage zone promotes denitrification, resulting in lower effluent TN concentrations (<0.75 mg/L modified, ∼1.60 mg/L conventional). The lowest HLR studied, 4.1 cm/h, provided the longest hydraulic retention time in the internal water storage zone (∼3 h) and had the greatest TN removal efficiency (90% modified, 59% conventional). In contrast to prior short-term studies, ADCs between 0 and 13 days did not significantly affect DOC export or TN removal. A short-term study with Florida friendly vegetation indicated that TN removal performance was enhanced in the conventional bioretention system. This field study provides promising results for improving dissolved N removal by modifying bioretention systems to include an internal water storage zone containing wood chips.