Specific component comparison of extracellular polymeric substances (EPS) in flocs and granular sludge using EEM and SDS-PAGE

• Flocs, aerobic and anaerobic granular sludge were selected for specific EPS comparison.
• Protein (PN) contents in LB-EPS and TB-EPS of granular sludge were higher than that of flocs.
• PN significantly contributed to the formation of granular sludge via surface charge adjustment.
• Proteins with high molecular weight favored the sludge granulation.
• Aromatic protein-like and tryptophan protein-like substances were more abundant in the granular sludge.

Extracellular polymeric substances (EPS) plays an important role in the formation of bioaggregates such as flocs, biofilm and granular sludge. However, the role of their specific components in sludge flocculation and granulation is still unclear. Three sludge samples including the flocs, aerobic and anaerobic granular sludge were investigated in this study and the specific components in different EPS structures of loosely bound-EPS (LB-EPS) and tightly bound-EPS (TB-EPS) were analyzed. Results showed that the protein (PN) contents in LB-EPS and TB-EPS of the aerobic and anaerobic granular sludge were 33.6 ± 9.7 and 96.8 ± 11.9, 27.1 ± 2.8 and 61.6 ± 4.2 mg g−1 VSS, respectively, which were both higher than the flocs of 8.5 ± 1.5 and 43.1 ± 2.7 mg g−1 VSS. But the polysaccharide (PS) contents in the three sludges were all about 30 mg g−1 VSS. The analysis of sludge surface charge indicated that they had a linear correlation with the PN content, which implied that PN significantly contributed to the formation of granular sludge. The results of sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) demonstrated that the molecular weight of PN in flocs was mainly distributed in 14.3–66.2 kDa, while it was 20.1–97.4 kDa in the granular sludge, which indicated that the proteins with high molecular weight favors the sludge granulation. According to the three-dimensional fluorescence (EEM) results, the aromatic protein-like and tryptophan protein-like substances were more abundant in the granular sludge than that in flocs, suggesting they are the key components in the structural stability of granular sludge.