Destruction of sludge floc and extracellular polymeric substances structure by natural deep eutectic solvent with thermal treatment assistant for synergistic promotion of dewaterability and protein recovery from waste-activated sludge

The significant hydrophilicity of extracellular polymeric substances (EPS) in waste-activated sludge is a major obstacle to effective dewaterability. This research proposed a natural deep eutectic solvent (NADES), synthesized with choline chloride (ChCl) as the hydrogen bond donor (HBD) and citric acid (CA) as the hydrogen bond acceptor (HBA). When combined with thermal treatment, NADES effectively enhanced the dewaterability and protein recovery efficiency of sludge. The capillary suction time (CST) of the sludge cake dropped from 28.2 s to 21.1 s, and the water content (Wc) reduced from 70.49 % to 57.49 % after adding 1.5 mL of NADES and heating to 80 degrees C for one hour. Simultaneously, the protein concentration in the treated sludge increased to 315.2 mg/L. These characterizations by particle size, zeta potential, scanning electron microscopy (SEM), three-dimensional excitation emission matrix (3D-EEM), and X-ray photoelectron spectroscopy (XPS) indicated that the alterations in the physicochemical properties of the sludge and the compositions of EPS were due to the neutralization of the negative charges on the sludge surface as well as the degradation of EPS. Density functional theory (DFT) calculations further reveal that NADES enhances protein solubility and reduces the activation energy for peptide hydrolysis. This study demonstrates that using NADES in conjunction with thermal treatment is an eco-friendly approach to enhancing the dewaterability of sludge and the efficiency of protein recovery.