Why does organics dissolution reach "equilibrium-limited dissolution" during sludge thermal hydrolysis: The...

Thermal hydrolysis is constrained by a "equilibrium-limited dissolution" phenomenon, where organics yield plateaus despite increasing thermal energy input. A critical unresolved question is whether this limit arises from hydrolysis completion or from competing antagonistic mechanisms. To answer this, this study elucidates the cause of this equilibrium-limited dissolution phenomenon by revealing the dynamic equilibrium between hydrolysis, repolymerization, and solid phase adsorption. Although high temperature drives the initial degradation of macromolecules, we identified two opposing forces that limit the ultimate dissolution efficiency. First, the soluble intermediates from protein and polysaccharide hydrolysis reorganize and precipitate, resulting in a dissolution loss rate of up to 6.00 % ± 0.99 %. Specifically, proteins unfold to expose hydrophobic regions and reorganize into highly ordered, β-sheet-rich insoluble aggregates; polysaccharides precipitate via carboxyl group protonation and cross-linking, forming hydrophobic aggregates. The key lies in the double-edged nature of the Maillard reaction: initial glycosylation introduces hydrophilic hydroxyl groups that aid dissolution, but the subsequent pathway shifts toward generating macromolecular, hydrophobic nitrogen-containing heterocyclic compounds and melanoidins, which results in precipitation. Second, the sludge solid phase actively intercepts dissolved organics, with an inhibition rate reaching up to 16.61 % ± 1.23 %. Following thermal hydrolysis, the sludge solid phase possesses an increased specific surface area and abundant surface functional groups (e.g., OH, NH₂, C=O, COO⁻), which facilitate the adsorption of dissolved organics through mechanisms such as hydrogen bonding, hydrophobic interactions, and Ca²⁺ bridging. Notably, the solid phase selectively adsorbs Maillard reaction intermediates, thereby reducing hydrolysate browning. In contrast to these limiting factors, we also discovered that lipids play a unique role as in situ surfactants, promoting solubilization through hydrolysis and emulsification, although their contribution to SCOD was limited (0.11 % ± 0.01 %).This study systematically demonstrates that breaking through the current equilibrium-limited dissolution threshold hinges not on merely increasing energy input, but on precisely regulating the competitive balance between hydrolysis production and repolymerization/solid phase adsorption. This provides a crucial theoretical basis for enhancing sludge resource recovery efficiency.