The low gas-to-liquid mass transfer rate is one of the main challenges in syngas biomethanation. In this work, a new concept of the floating membrane system with high gas hold-up was introduced in order to enhance the mass transfer rate of the process. In addition, the effect of the inoculum-to-syngas ratio was investigated. The experiments were conducted at 55 °C with an anaerobic mixed culture in both batch and continuous modes. According to the results from the continuous experiments, the H₂ and CO conversion rates in the floating membrane bioreactor were approximately 38% and 28% higher in comparison to the free (suspended) cell bioreactors. The doubling of the thickness of the membrane bed resulted in an increase of the conversion rates of H₂ and CO by approximately 6% and 12%, respectively. The highest H₂ and CO consumption rates and CH₄ production rate recorded were approximately 22 mmol/(L·d), 50 mmol/(L·d), and 34.41 mmol/(L·d), respectively, obtained at the highest inoculum-to-syngas ratio of 0.2 g/mL. To conclude, the use of the floating membrane system enhanced the syngas biomethanation rates, while a thicker membrane bed resulted in even higher syngas conversion rates. Moreover, the increase of the inoculum-to-syngas ratio of up to 0.2 g/mL favored the syngas conversion.

Biofouling remains a critical challenge in thin-film composite reverse osmosis (TFC-RO) membranes, leading to severe flux decline during long-term operation. This study presents a comparative evaluation of unmodified and CuO-enhanced TFC-RO membranes immobilized with a partially purified lactonase (PPL) enzyme as a quorum-quenching strategy. TFC-RO membranes were fabricated via interfacial polymerization using optimized 18 wt% PES supports, and CuO nanoparticles (0.01 wt%) were incorporated into the polyamide layer to improve surface properties. The PPL enzyme from Bacillus sp. T5 exhibited optimal activity at 25 °C and pH 7 and was immobilized with efficiencies of 86.0% and 90.2% for unmodified and CuO-enhanced membranes, respectively. CuO incorporation reduced the contact angle from 75.8° to 59.7° and decreased surface roughness by ~38–39%, while maintaining high salt rejection (~97%). Under low-pressure operation, CuO-enhanced membranes showed improved water permeability. Biofouling tests using Escherichia coli revealed lower flux decline for enzyme-immobilized membranes (42.6%) compared to unmodified membranes (51.6%). Notably, CuO-enhanced biocatalytic membranes exhibited the most stable performance, maintaining a post-biofouling flux of ~5.6 L m⁻² h⁻¹ and a salt rejection of 95.1%. SEM, FTIR, and confocal microscopy confirmed reduced biofilm coverage and more homogeneous surface morphology. Overall, this study demonstrates that combining CuO nanoparticles with immobilized PPL enzyme offers an effective dual-function antifouling strategy for TFC-RO membranes.