New insight into dewatering of digested sludge via polyacrylamide flocculation conditioning: Transformation from binary gel-like structure to gel-like structure at flocs level.

Digested sludge (DS) exhibits intricate gel-like properties, and its dewatering remains a challenge due to the unclear structural mechanisms. This study deciphered the variation of the binary gel-like structure in raw and polyacrylamide (PAM) conditioned DS. The results show that raw DS exhibits a typical binary gel-like structure. The average mesh size of the interaction network correlates negatively with normalized capillary suction time (CST) (r=-0.879, p<0.05), while structural tightly bound extracellular polymeric substances (TB-EPS) correlates strongly positively with normalized CST (r=0.907, p<0.05). The viscoelasticity was dominated by the gel-like structure at flocs level in raw DS. After PAM conditioning, the interaction network at supra-flocs level of DS was severely disrupted and failed to function as a decisive factor governing dewaterability. Analysis of intermolecular forces shows weakened electrostatic interactions and van der Waals forces disrupt the interaction network at supra-flocs level. The gel-like structure at flocs level remained a positive correlation (r=0.809, p<0.05) with normalized CST. The pivotal role of gel-like structure at floc-level was further evidenced by significant correlations between multiple indicators and normalized CST. Higher total and effective porosity correspond to better DS dewatering performance, while variations in equilibrium water content and bound water mobility directly control water separation efficiency. Microrheological tests reveal stress relaxation force of DS floc (90-130 pN) and the power-law exponent β link to normalized CST (r=0.984, p<0.05). This provides guidance for clarifying the vital role of gel-like structure at flocs level in the dimensional reduction of binary gel-like structures during PAM conditioning.