Influent salinity affects substrate selection in surface flow constructed wetlands

Conducted in pilot-scale surface flow constructed wetlands (SFCWs), this study’s objective is to identify the effect of influent salinity on substrate selection. Compared with gravel and sand SFCWs, soil SFCWs performed similarly to worse and similarly to better at low and high salinities, respectively, in removal efficiencies (REs) of salt, total nitrogen (TN), total phosphorous (TP) and chemical oxygen demand (COD). Soil generally increased macrophyte growth (especially at high salinity) in dry biomass, leaf chlorophyll concentration, root activity and root catalase and superoxide dismutase activities. The decrease of bacterial diversity in rhizosphere may be caused by high salinity, while soil improved rhizosphere bacterial community stability at varying salinities more than gravel or sand. At high salinity, soil support of macrophytes and rhizosphere microorganism increased pollutant REs in SFCWs or at least offset relatively poor biofilm attachment to soil. This highlights the necessity of the varying substrate selection in SFCWs with influent salinities for both increasing pollutant REs and reducing input cost.