Molecular choreography of sludge extracellular polymeric substances-From biomolecule identification to energetics and assembly dynamics.

Extracellular polymeric substances (EPS) shape the immediate environment for microbial survival and biofilm formation. Dynamic agglomeration of EPS dominates the formation kinetics and structural properties of activated sludge flocs as a consequence of biopolymer interactions across the wastewater treatment process. Current partial understanding and imprecise modeling of the structure hinder the comprehensive elucidation of the dynamic reorganization of clusters as component interactions change, causing a gap in the fundamental knowledge of EPS generation and functions. Here, biopolymer models of aerobic activated sludge and anaerobic digestion sludge (ADS) were constructed through molecular screening, and the dynamic landscape of EPS multicomponent clusters was then captured by an extensive set of molecular dynamics simulations. Biopolymer chains are assembled hierarchically driven by interactions between polar functional groups and stabilized by hydrogen bonding and van der Waals forces after several substates to obtain the final conformation. Electrostatic repulsion induced by carboxylic groups causes the rugged energy landscape of the process. Biopolymer molecular arrangement governed by polar interactions determines the nonuniform distribution of functional groups and characteristic regions, resulting in the microscopic heterogeneity of EPS clusters. The structure of alpha-helices enhances protein aggregation efficacy by facilitating more polar interactions compared with other residues. Meanwhile, the flexible branched structure and amphiphilic unit improve the energetic contribution of polysaccharides to EPS structural stabilization. Higher humic substance and carboxyl groups content primarily weaken the structural strength of ADS EPS. In general, this study proposes a powerful approach for investigating the molecular choreography within EPS, utilizing atomic simulations based on solved structures to explore the contribution of specific biopolymer features to structural energetics, providing theoretical insights to guide EPS-engineered regulation in wastewater treatment processes.