Hydraulic flow direction alters nutrients removal performance and microbial mechanisms in electrolysis-assisted constructed wetlands.

In this study, an electrolysis-assisted down-flow constructed wetland (E-DFCW) was successfully established, and achieved simultaneously efficient removal of PO-P (93.6% ± 3.2%), NO-N (97.1% ± 2.0%) and TN (80.6% ± 5.4%). When compared with electrolysis-assisted up-flow constructed wetland (E-UFCW), E-DFCW allowed significantly lower concentrations of PO-P, NO-N, total Fe and SO-S in effluents. In addition, microbial community and functional genes prediction results indicated that hydraulic flow direction significantly altered microbial nitrogen, sulfur and carbon metabolisms in electrolysis-assisted constructed wetlands (E-CWs). Specifically, multi-path denitrification facilitated NO-N reduction in cathodic chamber of E-DFCW, whereas autohydrogenotrophic denitrification might dominate NO-N reduction in cathodic chamber of E-UFCW. More abundant and diverse denitrifiers in cathodic chamber of E-DFCW contributed to enhanced denitrification performance. Overall, this work provides microbial insights into multi-path nitrogen metabolisms in electrolysis-assisted denitrification systems in response to hydraulic flow direction.