Biochar and woodchip amended bioreactor extending reactive volume for enhanced denitrification in stormwater runoff

Bioretention has been increasingly used in stormwater runoff treatment. However, its performance was often restricted by poor treatment or even the net leaching of nitrate. To solve this problem, a biochar-woodchip modified bioretention system was developed in this study. The modified system greatly enhanced the nitrogen removal than the conventional sand bioretention system. The nitrate removal efficiency in the modified bioretention system ranges between 46.8 ± 21.1% and 73.6 ± 6.5%, much higher than the conventional system (only −38.6 ± 18.4% and 24.4 ± 6.0%). The nitrogen removal process, pore water variation, and functional gene analysis further demonstrate that the biochar and woodchip amendment extended the reactive volume in the bioreactor, facilitating nitrogen removal in both the upper layer and internal water storage zone (IWSZ). Biochar amendment in the upper layer increased volumetric water content, ammonium adsorption, nitrification, and denitrification. Although a slight amount of ammonium generation was found, woodchip addition greatly enhanced the denitrification rate in the IWSZ. In contrast, the upper layer of the conventional system cannot completely adsorb and transform ammonium. The denitrification performance in both the upper layer and IWSZ of the conventional system was poor as well. And the lack of nosZ in the IWSZ also made it potentially become a nitrous oxide emission source. Overall, the biochar woodchip modified system greatly extended the reactive volume and enhanced nitrogen removal, making it become a promising alternative to conventional sand bioretention system for stormwater runoff nitrogen treatment. [Display omitted] •Bi-layer biochar woodchip modified bioretention system was established.•TN removal in modified reactor was about 5 times higher than conventional system.•Biochar addition extended the reactive volume of bioretention system.•Woodchip greatly increased the denitrification rate and abundance of the nosZ gene.•Both upper layer and internal water storage zone contribute to enhanced denitrification.