Advanced treatment of first flush roof runoff via a dry-wet polymorphic constructed wetland system: Performance and mechanistic insights

Controlling runoff pollution is crucial to improving ecological environments in the context of urbanization and climate change. However, a significant research gap remains in the treatment and reuse of roof runoff, particularly during the first flush. To address this, a novel dry-wet polymorphic constructed wetland (DWP-CW) system was developed to purify first flush runoff efficiently and reliably. The performance and stability of the DWP-CW system were evaluated under varying conditions, including rainfall intensities, pollution load levels, and antecedent dry days (ADDs). The purification mechanisms and pollutant metabolism were further analyzed using bioinformatics. Results demonstrated that the DWP-CW system achieved excellent pollutant removal efficiencies, with average removal rates exceeding 99% for chemical oxygen demand (COD) and ammonia-nitrogen (NH₃-N), and stable removal rates above 80% for total nitrogen (TN) and total phosphorus (TP). The dominant aerobic microbial community played a key role in the system's purification process, while the superior water-retention capacity of the upper sandy soil layer provided a favorable environment for microbial survival during ADDs. This study offers robust theoretical support for practical application of DWP-CW systems in runoff pollution control.