Composition and molecular structure analysis of hydrophilic/hydrophobic extracellular polymeric substances (EPS) with impacts on sludge dewaterability

Extracellular polymeric substances (EPS) have a significant effect on the dewaterability of waste-activated sludge (WAS); however, the role of polarity distribution (hydrophilic/hydrophobic characteristics) of EPS in sludge dewatering remains unclear. In this study, the relationship between the characteristics of EPS hydrophilic/hydrophobic fractions (composition and molecular structure) and sludge physicochemical properties of 8 different sludge samples was investigated, and a structural equation model was constructed to describe the potential mechanisms of key EPS parameters on sludge dewatering. Experimental results showed that the variation in sludge EPS polarity led to significant changes in the surface hydrophilicity/hydrophobicity and structure compactness, which affected the sludge dewaterability. Specifically, the increase in the content of tyrosine-like proteins, especially those related to hydrophobic acid fraction (HPO-A), played a key role in increasing the surface hydrophobicity of sludge and promoting its dewaterability. In contrast, humic acids in hydrophilic EPS (HPI) could interact with hydrophobic proteins to form dense biopolymers (characterized by high nonprotonated nitrogen (N_nonpr) content), resulting in a more stable and compact structure of the EPS matrix (characterized by low random coil content) and deterioration of sludge dewaterability. This study provides a new perspective for the exploration of the relationship between hydrophilic/hydrophobic biopolymers and sludge dewaterability.