Beyond charge neutralization: Self-skeletization of inorganic coagulants reinforces pore networks for enhanced sludge...

Coagulation is one of the most widely applied conditioning techniques for sludge dewatering. Traditionally, charge neutralization and colloidal destabilization have been regarded as the main mechanisms of sludge dewatering via coagulation. However, this study demonstrates that the improvements in dewatering performance and sludge-cake structure achieved by coagulation extend well beyond these conventional mechanisms. Polymeric aluminum chloride (PAC) and polymeric ferric sulfate (PFS) were chosen as representative inorganic coagulants and compared with polyacrylamide (PAM) as a flexible flocculant and fly ash (FA) as a rigid skeleton builder. PAC and PFS conditioning reduced the sludge cake water content (Wc) to 63.92% and 65.58%, respectively, outperforming PAM and approaching the FA benchmark (Wc = 57.24%). To investigate the mechanisms underlying this performance, multi-scale coupled characterisation was conducted, revealing a previously overlooked "skeleton-like" structural function of inorganic coagulants. Beyond optimising surface charge and floc size, most Al³⁺/Fe³⁺ was rapidly immobilised within the developing filter cake during the early stage of coagulation, undergoing in-situ hydrolysis to form micro-scale frameworks. These frameworks increased pore coordination numbers by 80% (PAC) and 89% (PFS) relative to raw sludge (RS), narrowed throat-diameter dispersion to 1.2 µm, raised effective porosity by two orders of magnitude, and simultaneously lowered tortuosity. Consequently, absolute permeability became 3.5-4.4 times higher than RS, the compressibility coefficient dropped by 63-69%, and Young's modulus nearly doubled, again outperforming PAM and approaching FA. Moreover, strong correlations observed between permeability and Wc, as well as between compressibility and Wc, underline the synergistic importance of hydraulic connectivity and mechanical stability for efficient dewatering. These findings highlight a novel self-skeletization mechanism of coagulant conditioning in sludge dewatering, thereby offering new mechanistic insight for optimising coagulation strategies in sludge management. In practice, this reduces reliance on high-dosage rigid skeleton builders and enables further exploitation of coagulants' skeleton-like function through operating-parameter optimisation and targeted formulation tuning.