Molecular dynamic modeling of EPS and inorganic/organic flocculants during sludge dual conditioning

Extracellular polymeric substances (EPS) are the key components determining the dewatering behavior of wastewater sludge. However, current technical optimization of sludge conditioning for dewatering is limited by the poor understanding of the conditioner-EPS interactions at molecular levels. Herein, a combination of molecular dynamic (MD) simulations, dewaterability assessment and EPS characterization was used to reveal the sludge dewatering mechanisms using dual conditioning processes (prevalent inorganic (poly aluminum chloride (PAC)) and organic (poly dimethyl diallyl ammonium chloride (PDDA)). Results suggested that PAC and PDDA bridged the biopolymers mainly through electrostatic interactions, promoting the agglomeration of biopolymers and reducing their contact probability with water molecules. Water molecules were tightly bound to EPS mainly through hydrogen bonding with polar oxygen-containing functional groups. The adsorption of PAC and PDDA on hydrophilic components reduced the molecular polarity of biopolymers and altered the conformation of water molecules in the hydration shell, resulting in a decreased hydration capacity of EPS and the release of bound water, and sludge dewaterability was improved. PAC was found to be more effective than PDDA in disrupting the hydrogen bonding between water molecules and EPS, especially the protein β-sheet structure inside the molecular clusters with its high charge strength and diffusivity. Sludge bound water decreased by 73.16 % after PAC conditioning. In addition, PDDA exhibited superior agglomeration ability to biopolymers and promoted the electrostatic interaction between PAC and polar groups during dual conditioning. The strength and hydrophobicity of EPS molecular clusters were thus enhanced, and the conditioning efficiency was improved. This study offers molecular-level insights into the coagulation treatment process of sludge and provides theoretical references for process optimization and new conditioner development.