Deep Eutectic Solvents Pretreatment Enhances Methane Production from Anaerobic Digestion of Waste Activated Sludge: Effectiveness Evaluation and Mechanism Elucidation

Abstract

In anaerobic digestion (AD) research, optimizing methane production from waste activated sludge (WAS) is a core focus. This study investigated the efficacy and mechanism of deep eutectic solvents (DES) as pretreatment agents for AD systems of WAS. Two DESs were developed, key findings included significant increases in methane production: 390% for choline chloride-urea (ChCl-Urea) and 540% for chloride-ethylene glycol (ChCl-EG). Results showed that ChCl-Urea mainly disrupted extracellular polymeric substances (EPS) structures, aiding in initial sludge solubilization during pretreatment. ChCl-EG, instead, induced sludge self-driven organic solubilization and enhanced hydrolysis and acidification during AD process. Based on the extent to which the two DESs promoted anaerobic digestion for methane production, the AD process can be divided into stage Ⅰ and stage Ⅱ. In stage Ⅰ, ChCl-EG promoted methanogenesis more significantly. In this stage, both DESs enriched acetotrophic methanogens-Methanosarcina, thereby promoting acetoclastic methanogenesis. Notably, ChCl-Urea particularly influenced polysaccharide-related metabolism, whereas ChCl-EG targeted protein-related metabolism. In stage Ⅱ, the promotion role of ChCl-Urea was dominant. In this stage, ChCl-Urea bolstered metabolism by methanogenic archaea, thereby directly boosting methanogenesis. Conversely, ChCl-EG promoted genetic information processing by acidogenic bacteria, indirectly amplifying methanogenesis. In essence, this study investigated the microbial mechanism of DES-enhanced sludge methanogenesis and provided a reference for future research.

Keywords: Waste activated sludge, deep eutectic solvent, Extracellular polymer substances, microbial community, Metagenomics