Effects of substrate combinations on greenhouse gas emissions and wastewater treatment performance in vertical subsurface flow constructed wetlands

Climate warming would be generally influenced by the quantity and component of greenhouse gases during the carbon and nitrogen removal in constructed wetlands (CWs). In this study, the impacts of the combinations by walnut shell (organic-rich substrate improving denitrification), manganese (Mn) ore (electron-exchange substrate transferring CH4 to CO2) and activated alumina (phosphorus-adsorption substrate) on global warming potentials (GWPs) were investigated to propose a strategy for both mitigating GWPs and performing well in wastewater treatment. During hybrid CWs, the highest COD and TP removal efficiencies were respectively 89.4% and 98.1% in CWs with the substrate combination of Mn ore and activated alumina, and the highest TN removal efficiency was in CWs with walnut shell and Mn ore. The substrate combination of walnut shell and Mn ore would simultaneously strengthen the nitrification and denitrification process resulting in the lowest N2O flux in CWs. The combinations of Mn ore with walnut shell or activated alumina could significantly decrease the GWPs through promoting the conversion of CH4 to CO2 due to the better redox potential environment favorable for methanotrophs (pmoA) instead of methanogenesis (mcrA) provided by Mn ore. This study provided a feasible way to mitigate climate warming during wastewater treatment in CWs by using substrate combination of Mn ore and walnut shell.