Performance optimization of two-stage constructed wetland-microbial fuel cell system for the treatment of high-concentration wastewater

Constructed wetland-microbial fuel cell (CW-MFC) has attracted much attention because of its dual functions of wastewater treatment and energy recovery. However, its performance in treating high-concentration wastewater is degraded by the decreased dissolved oxygen at the cathode and insufficient electron acceptors. In this study, two CW-MFC systems with cathodic aeration were connected in series to investigate the effects of aeration rate and hydraulic retention time (HRT) on the removal of pollutants and the performance of electricity production in high-concentration wastewater. Results showed that aeration enhanced NH4+-N and TP removal by 45.0-49.8% and 11.5-18.0%, compared with the unaerated condition, respectively. Meanwhile, no significant change regarding COD removal was observed. Aeration enhances the output voltage and power density of the system, especially the first stage CW-MFC, which improved the power production performance by 1 to 2 orders-of-magnitude. Increasing HRT improves the system's pollutant treatment efficiency and power generation performance for high-concentration wastewater. Still, the extension of HRT to 2 days will not contribute much to improving the removal efficiency. Under optimized conditions, the maximum total removal rates of COD, NH4+-N, and TP for the two-stage tandem CW-MFC system were 99.3 +/- 0.2%, 92.4 +/- 1.6%, and 79.5 +/- 3.4%, respectively. Meanwhile, the maximum output voltage and maximum power density of the first-stage CW-MFC were 405 mV and 138.0 mW/m(3), respectively. In contrast, the maximum output voltage and maximum power density of the second stage are 105 mV and 14.7 mW/m(3), respectively.