Deep Eutectic Solvents as PVDF-HFP Membrane Modifiers: Morphology, Performance, and Fouling Impact in Gas–Liquid Methane Recovery from Anaerobic Effluents

Dissolved methane (D-CH4) emissions from anaerobic effluents constitute an energy loss and a source of greenhouse gas emissions, and its recovery by polymeric membranes can be hindered by fouling. In this work, the long-term performance under realistic conditions of poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) membranes fabricated with dimethyl sulfoxide (DMSO) as a nontoxic solvent and deep eutectic solvents (DES) as membrane modifiers via non-solvent-induced phase separation was evaluated. An analysis based on Hansen solubility parameters showed the role of polymer–solvent–additive interaction in membrane formation. The moderate affinity between PVDF-HFP and DMSO promoted fast liquid–liquid demixing, yielding porous structures with low crystallinity. The addition of DES modified phase inversion kinetics, increasing porosity and enabling variations in pore size and crystalline phase composition. Choline chloride-based DES (ChCl:glycerol, ChCl:urea and ChCl:ethylene glycol) enhanced the formation of the β-phase fraction from 0.68 to 0.85 and reduced the pore size from 0.44 to 0.23–0.32 μm, whereas thymol:menthol DES promoted larger pores of 0.54 ± 0.01 μm and α-phase fraction, evidencing the key role of additive polarity in solid–liquid demixing. All fabricated membranes exhibited D-CH4 removal efficiency (RE) profiles fully comparable to those of commercial PVDF (≈36%), achieving a RE at 5 h in the range of 36.4–41.7%. Long-term tests using the anaerobic effluent revealed a progressive RE decline to ≈20% in both the pristine membrane and with ChCl:EG due to fouling, with the latter showing a less pronounced RE decline during the initial hours (<100 h), though the final RE at 300 h was similar. The performance decline was due to surface deposition of organic foulants (extracellular polymeric substances and fatty acids) and inorganic salts, and the ultrasonic-driven cleaning process with water and citric acid partially removed the fouling. Overall, this study demonstrates that nontoxic DMSO with DES addition could replace hazardous traditional solvents in membrane fabrication without compromising D-CH4 RE.