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

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 < 0.05), and reduced nitrous oxide emissions by 66% compared with the control (p < 0.05). The dissolved organic matter (DOM) released in the biochar was beneficial to replenish carbon sources and accommodate adaptive microorganisms. Electrochemical characterization and molecular methods revealed that the sludge biochar had stronger electron transfer capacity (ETC) than the cattail biochar, which could better promote the activities of key enzymes of the pollutant removal procedure. High-throughput sequencing showed that microorganisms such as Thaurea, Rhodocyclaceae, Hydrogenophaga and Fusibacter related to nitrogen removal were well enriched in the SBC-CWs and CBC-CWs. This research shows the potential for the simultaneous energy-saving advanced treatment of WWTP tailwater and waste resource recovery.