New insights into filamentous sludge bulking: The potential role of extracellular polymeric substances in sludge bulking in the activated sludge process

The control of filamentous sludge bulking has been regarded as an important issue in the activated sludge process due to there is still a lack of understanding of the bulking mechanisms. In this study, changes in the extracellular polymeric substances (EPS) and metabolic profile of bulking sludge based on the proteomics level was investigated to reveal the potential role of EPS in deteriorating sludge floc stability and structure during filamentous bulking. The results showed that the EPS content gradually decreased from 210.23 mg/g volatile suspended solids (VSS) to 131.34 mg/g VSS during sludge bulking. The protein (PN) content of the EPS significantly decreased from 173.33 mg/g VSS to 95.42 mg/g VSS during sludge bulking. However, a gradual increase in polysaccharides (PS) was observed. Bacterial aggregation was hindered by the changes in the EPS and its components. The excessive proliferation of filamentous bacteria had a significant effect on the molecular functions of the extracellular PN and metabolic pathways of the EPS. The proteins associated with the hydrophobic amino acid synthesis decreased, whereas the proteins associated with the hydrophilic amino acid synthesis increased during sludge bulking. Electric repulsion was the key factor affecting the aggregation and flocculation ability of the bacteria during sludge bulking. The changes in the EPS and its components induced by the excessive proliferation of filamentous bacteria resulted in a loose floc structure and poor settling performance during sludge bulking. These findings provide new insights into sludge bulking during the activated sludge process.