Simultaneously Enhanced Dewaterability and Biopolymer Release of Sludge by Natural Deep Eutectic Solvents: Performance, Mechanisms, and Insights of Theoretical Calculations

The dewatering and resource recovery of waste-activated sludge (WAS) seemed to be two contradictory processes due to the high moisture content induced by the extracellular polymer substances, which mainly consist of proteins and polysaccharides. Natural deep eutectic solvents (NADESs) were proposed to synergistically enhance dewaterability and biopolymer release of WAS. It showed that NADES with low-temperature pretreatment can significantly improve the dewaterability and release of valuable resources. With the addition of 1.0 mL of NADES combined with 60 °C thermal treatment for 120 min, the capillary suction time (CST) of sludge significantly decreased from 75.9 ± 7.5 to 30.5 ± 1.6 s, and 11.83 mg/L of the total amino acids (AAs) was obtained. The protein and polysaccharide contents were increased to 876.48 and 213.42 mg/L, respectively. This study indicated that NADES combined with low-temperature pretreatment is a green, cost-effective, and environmentally friendly method to improve sludge dewaterability and resource recovery simultaneously. The theoretical calculation results suggested that the generation of NADES···AAs aggregates is an exothermic reaction and proceeds nonspontaneously. A good correlation is also found between the theoretically calculated free-energy change and the concentrations of AAs. On the basis of this study, the biopolymers from WAS can be recovered as value-added resources, enabling a recycling concept within the circular economy vision. The theoretical calculation can be used to guide future experiments for screening highly selective and effective NADES toward resource recovery from WAS.