Rapid municipal sludge dewatering via choline chloride–oxalic acid deep eutectic solvent coupled with microwave heating

Municipal sludge dewatering, a persistent bottleneck in wastewater treatment, is fundamentally constrained by the highly hydrated extracellular polymeric substance (EPS) network, yet conventional conditioners rely on metal salts or synthetic polymers that leave secondary residues. Herein, a rapid, metal- and polymer-free sludge-conditioning process is reported that couples a choline chloride-oxalic acid deep eutectic solvent (ChCl-OA DES) with microwave heating. At optimised conditions (2 wt% DES, microwave irradiation at 90 °C for 1 min), the centrifugal dewatering efficiency reached 84.05%, a 43.86 percentage-point increase over untreated raw sludge (40.19%). ChCl-OA outperformed ChCl-AlCl3 DES because oxalic acid drives proton donation and Ca 2+ /Mg 2+ chelation without external metal input. Multiscale characterisation shows that the DES chemically loosens the EPS network, while microwave irradiation rapidly generates interconnected porous channels, redistributes flocs (particle-size Span from 6.94 to 3.32), and decomposes proteinaceous EPS (soluble microbial product fraction from 31.63% to 13.33%). Crucially, the zeta potential recovers to near-raw values (−16.55 mV), indicating that dewatering is governed by EPS reconstruction rather than charge neutralisation. This DES–microwave synergy provides a green, process-intensified sludge-conditioning route that avoids the metal residues and polymer demand of conventional conditioning, supporting efficient sludge treatment in municipal wastewater engineering.