The efficacy of bioretention systems amended with iron-modified biochar for the source-separated and component-specific treatment of rainwater runoff: A microbiome perspective.

Bioretention systems offer advantages in controlling non-point source pollution from runoff rainwater. However, the systems frequently encounter challenges, including insufficient stability of nitrogen and phosphorus removal. Limited research has been performed on bioretention systems which integrate actual data from non-point source pollution cases for the quantitative and qualitative refinement of initial and non-initial rainwater. Moreover, the potential linkages between amended media and microbial communities in bioretention systems with the addition of novel functional filler have not been explored. In this study, a system for treating both initial and non-initial rainwater was established through measurements including iron-modified biochar (FeBC) packing and the optimization of the layer structures. In system treating initial rainwater, the systems loaded with FeBC maintained stable NH-N and NO-N removal rates of over 95% and 80%, respectively under 12 rainfall simulation events. After a 10-day antecedent drying duration (ADD), the removal rates for NH-N and PO-P remained above 78% and 85%. In systems designed to process non-initial rainwater, increasing the height of the transition layer effectively enhanced the NH-N removal stability. Meanwhile, increasing the height of the drainage layer could promote PO-P removal rates to over 75%. The addition of FeBC facilitated the growth of certain denitrifiers improved overall NO-N removal during successive rainfall events. The microbial communities may adapt to variations in the external environment by enhancing the synthesis of ribosome and the metabolism of pyrimidine and purine, further improving the stability of NH-N removal. This study provides a theoretical basis for the precise enhancement of nitrogen and phosphorus removal and the design of bioretention systems for differentiated treatment of rainwater, guiding their design and applications in different regions.