Influence of turbulent mixing on the composition of extracellular polymeric substances (EPS) and aggregate size of aerated activated sludge

The effects of turbulent mixing on the composition of extracellular polymeric substances (EPS) and aggregate size of activated sludge were investigated at varying aeration intensities (40, 80, 100, 120, 140 and 160 L h−1; representing a turbulent dissipation rate increased from 0.017 to 0.078 m2 s−3). The average aggregate size of the inoculated sludge was 48.6 μm, and reached a final average aggregate size of 120, 160, 256, 164, 139 and 102 μm, respectively, after being mixed under the aeration intensities. The content of humic acids and proteins in loosely bound and tightly bound EPS (LB-EPS and TB-EPS, respectively) first increased and then decreased with increasing aeration intensity. During the last 21 days of the experiments, the aggregate size was found positively related to the content of LB-EPS and TB-EPS, suggesting that large aggregates may decrease the dissolution rate of LB-EPS and TB-EPS. The aggregate size was negatively related to the LB-EPS content in every reactor in the time series, which suggested that increasing LB-EPS content weakened cell attachment. We found a critical turbulence intensity of 0.038 m2 s−3, below which the aggregation was limited by the turbulence available to promote the collision and adhesion between sludge cells. Increasing turbulence intensity may decrease LB-EPS, which prevents cell attachment. Turbulence intensity above the critical turbulence intensity resulted in higher shear forces that caused the breakdown of aggregates.