Molecular-scale mechanism of extracellular protein over-flocculation for sewage sludge dewatering by polyacrylamide

Flocculation is one of the most common treatment processes for sludge dewatering, representing the last line of solid-liquid separation for sludge dewatering. However, the macroscopic and descriptive theories of polyacrylamide (PAM) -based over-flocculation have limited the optimization of its performance in the dewatering of sewage sludge, whose water is typically trapped within a three-dimensional gel matrix governed by extracellular polymeric substances (EPS). This study focuses on loosely bound EPS (LB-EPS) to uncover molecular-level mechanism of excessive PAM dosing. PAM overdosing initially increases both small and large molecular fractions of PN, but subsequently promotes their decrease. we have also extended this PN behavior to the functional group level, highlighting the fundamental importance of CO, NH and OH groups on PN-PN interactions and the PN hydrophobicity during over-flocculation. Drawing upon the spectral and structural characteristics of PN, this study has elucidated PN behavior during over-flocculation process and offers valuable theoretical insights to guide future applications of flocculation in practical settings.