Biochar based constructed wetland for secondary effluent treatment: Waste resource utilization

[Display omitted] •The sustainable reuse of sludge and wetland macrophyte litter was realized.•SBC and CBC promoted nutrient removal and reduced N2O emissions fluxes in CWs.•Biochar-derived DOM can supplement carbon source for denitrification in CWs.•SBC significantly enhanced the key enzyme activities and ETSA in CWs.•Biochar enhanced the enrichment of nitrogen and phosphorus removing microorganisms. Constructed wetlands (CWs) are widely used to treat the effluent of wastewater treatment plants (WWTPs) due to their energy-saving and environmentally friendly advantages. Wetland macrophyte litter and sludge are wastes produced by CWs and WWTPs, respectively. However, there are few studies on the reuse of sludge and macrophyte litter as CW filling materials. In this study, sewage sludge and cattail litter were selected as raw materials to prepare biochar, which was used as CW filling materials to enhance the advanced treatment of secondary effluent. The results showed that the sludge biochar CWs (SBC-CWs) and the cattail biochar CWs (CBC-CWs) possessed better total nitrogen removal efficiency (91% and 81%, respectively) compared with the control (67%). Furthermore, SBC-CWs significantly improved total phosphorus removal efficiency by 20% than that of the control (p