Roles of carbon source-derived extracellular polymeric substances in solids accumulation and nutrient removal in horizontal subsurface flow constructed wetlands

The sustainable operation of horizontal subsurface flow constructed wetlands (HSSF CWs) is challenging due to the accumulation of organic and inorganic solids in substrate interspaces. In this study, wetland microcosms with (EC-CWs) and without (C-CWs) external carbon source were monitored for 120 days to investigate the impact of extracellular polymeric substances (EPS) on inorganic particle settlement and nutrient removal of HSSF CWs. EPS flocculation aggravated the solids accumulation in the inlet zone of the EC-CWs, and ion bridging through EPS and sweep flocculation were the main flocculation mechanisms. EPS increased the hydraulic resistance of the accumulated solids, which almost halved the hydraulic retention time (HRT) of the EC-CWs. Compared with the C-CWs, the media storage of total phosphorus (TP) in the EC-CWs was improved by 70.18 +/- 2.74%, which was mainly due to the increased adsorption sites introduced by EPS. RDA analysis showed that solids accumulation in the HSSF CWs was positively correlated with the TP removal, but the process had a minor influence on organic matter and nitrogen removal. Combined analyses revealed that the accumulation of EPS and inorganic particle not only caused HRT reduction but also introduced abundant adsorption sites. The present results would promote the understanding on role of EPS in sustainable operation of HSSF CWs, and further maintain the stable nutrient removal performance.