Enhancing Nitrate Removal From Urban Stormwater In An Inverted Bioretention System

Bioretention systems are widely used in sponge city to reduce the pollution caused by urban stormwater. However, limited nitrogen removal is observed in conventional bioretention systems due to the low nitrate elimination efficiency. In this study, an inverted bioretention system was developed by setting a gravel layer above a soil layer, with an anoxic zone formed to enhance biological denitrification. Variables investigated included bioretention configurations, hydraulic loading rate and inundation time. The results indicated that, in inverted bioretention systems the water detention time was increased 110.2-139.5%, and the hydraulic retention time was increased by 3.0-6.6 h. Besides, the total runoff reduction rate and the runoff peak reduction rate of the inverted bioretention system was higher than the conventional bioretention system by 11.8-29.4% and 30.4-62.7% respectively. Using inverted bioretention system, 72.9-100% of the NO3--N was removed as compared with -126.6-46.1% by the conventional bioretention system. Significant correlation between the effluent concentration of NO3--N and the inundation time was observed. When the inundation time was prolonged to 24 h, good performance on nitrate removal was exhibited in both conventional and inverted bioretentions.