Large-scale multi-stage constructed wetlands for secondary effluents treatment in northern China: Carbon dynamics

摘要

AbstractMulti-stage constructed wetlands (CWs) have been proved to be a cost-effective alternative in the treatment of various wastewaters for improving the treatment performance as compared with the conventional single-stage CWs. However, few long-term full-scale multi-stage CWs have been performed and evaluated for polishing effluents from domestic wastewater treatment plants (WWTP). This study investigated the seasonal and spatial dynamics of carbon and the effects of the key factors (input loading and temperature) in the large-scale seven-stage Wu River CW polishing domestic WWTP effluents in northern China. The results indicated a significant improvement in water quality. Significant seasonal and spatial variations of organics removal were observed in the Wu River CW with a higher COD removal efficiency of 64–66% in summer and fall. Obvious seasonal and spatial variations of CH4and CO2emissions were also found with the average CH4and CO2emission rates of 3.78–35.54 mg m−2d−1and 610.78–8992.71 mg m−2d−1, respectively, while the higher CH4and CO2emission flux was obtained in spring and summer. Seasonal air temperatures and inflow COD loading rates significantly affected organics removal and CH4emission, but they appeared to have a weak influence on CO2emission. Overall, this study suggested that large-scale Wu River CW might be a potential source of GHG, but considering the sustainability of the multi-stage CW, the inflow COD loading rate of 1.8–2.0 g m−2d−1and temperature of 15–20 °C may be the suitable condition for achieving the higher organics removal efficiency and lower greenhouse gases (GHG) emission in polishing the domestic WWTP effluent. The obtained knowledge of the carbon dynamics in large-scale Wu River CW will be helpful for understanding the carbon cycles, but also can provide useful field experience for the design, operation and management of multi-stage CW treatments.Graphical abstractDisplay OmittedHighlights•A large-scale multi-stage CW for WWTP effluents was operated in northern China.•Organics removal and emissions of CH4and CO2varied seasonally and spatially.•COD removal efficiency ranged from 17% to 66% and was higher in summer and fall.•Highest CH4and CO2emissions of 35 and 8992 mg m−2d−1were in spring and summer.•COD removal and CH4emission increased when temperature and inflow loading rose.