The influences of shear stress on Extracellular Polymeric Substances of activated sludge

Abstract

Shear stress is one of the key factors in aggregation and detachment of microbial flocs, which will influence the production of Extracellular Polymeric Substances (EPS) as well. In this study, the components and composition of activated sludge EPS, including loosely bound EPS (LB-EPS) and tightly bound EPS (TB-EPS), were analyzed under different shear stresses. To perform this, sequencing batch reactors were set up separately with various shear stresses (represented by superficial upflow air velocity). The results show that shear stress significantly affects EPS production in the SBRs. When the superficial upflow air velocity increased from 2.3 to 9.1 m/h, the contents of total EPS, TB-EPS, and LB-EPS first increased and then decreased. The highest total EPS (70.94 mg COD/gVSS) and LB-EPS (27.96 mg COD/gVSS) contents were obtained under superficial upflow air velocity of 5.7 m/h, while the maximum TB-EPS (45.03 mg COD/gVSS) content was attained when superficial upflow air velocity rose up to 6.8 m/h. The contents of polysaccharides, proteins and humic substances in EPS showed similar trends with an increase in the shear stress. Compared with the variation of protein and humic substance, the variation of polysaccharide was extremely dramatic in this study, especially in LB-EPS. It was also found the contents of EPS (TB-EPS, LB-EPS and total EPS) have a significant linear correlation with the sludge volume index (SVI). The highest correlation coefficient (R2 = 0.9522) showed the closest correlation between LB-EPS contents and SVI. These findings indicate that LB-EPS and polysaccharides rather than TB-EPS and proteins play important roles for the settling properties of activated sludge under various shear stresses.