Effect of EPS and its forms of aerobic granular sludge on sludge aggregation performance during granulation process based on XDLVO theory

Different forms of extracellular polymeric substances (EPS) play different roles in the formation process of aerobic granular sludge (AGS). This work focused on the contribution of loosely bound EPS (LB-EPS), tightly bound EPS (TB-EPS) and EPS to the aggregation between sludge cells during the start-up of aerobic granular sludge in a sequencing batch reactor. By analyzing the changes of sludge surface characteristics before and after the extraction of each layer of EPS, the contribution of LB-EPS, TB-EPS and EPS to the adhesion and aggregation of sludge cells in the granulation was calculated by surface thermodynamics and the extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory. The experimental results showed that AGS reactor basically completed the granulation process and kept stable operation within 40 d. In the process of sludge granulation, the effect of LB-EPS on the aggregation of sludge cells shifted from attraction to repulsion. TB-EPS could improve the surface zeta potential and hydrophobicity and show an attractive effect in the granulation process, which was conducive to the adhesion between sludge cells and was the main contributor to the formation of granules. Additionally, EPS played an apparently positive role in sludge flocculation and could promoted cell aggregation in the whole granulation process. [Display omitted] •The contribution of EPS and its forms to granulation were studied using XDLVO theory.•LB-EPS in the flocs and granules respectively showed repulsive and attractive force.•TB-EPS was the main component that caused sludge accumulation in EPS.•EPS always had an increasing positive effect on the granulation.