Impact of extracellular polymeric substances on the settlement ability of aerobic granular sludge

Instability of aerobic granular sludge (AGS), which is mainly caused by filamentous outgrowth, is a bottleneck in applying this technology to treat wastewater. In order to reduce the effect of filamentous outgrowth on settlement ability of AGS, the role of the distribution of extracellular polymeric substances (EPS) in settlement ability was investigated in this study. Chemical oxygen demand (COD) of influent increased gradually from 1000 mg L(-1) to 2000 mg L(-1) to 4000 mg L(-1) as organic loading rate (OLR) changed from 2 to 4 to 8 kg COD m(-3) d(-1) in the synthetic influent. The relationship between settlement ability and EPS was investigated. The sharp increase in loosely bound EPS (LB-EPS) content reduced the settlement ability, whereas the highest content of tightly bound EPS (TB-EPS) was observed in the sludge with best settlement ability. The TB-EPS plays an important role in maintaining the matrix structure of AGS. Abundance of LB-EPS did not favour the settlement ability of AGS. These results would provide useful information for improvement of stability of AGS.