Choline chloride pretreatment on volatile fatty acids promotion from sludge anaerobic fermentation: In-situ deep eutectic solvents-like formation for EPS disintegration and associated microbial functional profiles upregulation

Slow solubilization/hydrolysis efficiency has been widely recognized as a limiting factor for volatile fatty acids (VFAs) production from waste activated sludge (WAS) via anaerobic fermentation. This study developed an innovative choline chloride (ChCl)-based approach for WAS treatment. Accordingly, the VFAs yield was increased by 2.5-623.3%, which was attributed to the concurrent acceleration of WAS solubilization, hydrolysis, and acidification. Fourier transform infrared spectrometer (FTIR) and molecular docking analysis showed that ChCl can attack and disrupt extracellular polymeric substances (EPS) by forming hydrogen bonds with organics occurred in WAS (i.e. proteins). More importantly, the generated VFAs in turn interacted with ChCl for in-situ deep eutectic solvents (DES)-like formation to significantly improve the organics dissolution of WAS for more bioavailable substrates associated with VFAs biosynthesis, and such effects were intensified with the VFAs level. Besides, ChCl exhibited "screening-effects" to enrich the hydrolytic and acidogenic anaerobes in WAS fermentation systems, while reduced VFAs-consumers. Meanwhile, the microbial metabolic profiles shift to favor the protein metabolism, and the genetic traits associated with extracellular and intracellular substrates metabolism, membrane transport, and fatty acid biosynthesis were all upregulated. Noteworthily, the functional hydrolytic and acidogenic species maintained their microbial activity for efficient VFAs production via quorum sensing (QS) and two-component systems (TCS) to counteract ChCl stress. The ranking efficiency product (REP) indices revealed that ChCl pretreatment showed a higher sustainable footprint (84.0%) over other traditional pretreatment approaches for WAS fermentation in view of both economic and environmental benefits. This work provides an innovative niche for efficient VFAs production from WAS with the sustainable application.