Modified ultrafiltration membrane to improve organic matter conversion in anaerobic digestion of kitchen waste: Performance and separation mechanism

The low organic matter conversion efficiency during the anaerobic digestion of kitchen waste is one of the major technical bottlenecks limiting its deep resource utilization. In this study, a polysulfone (PSF) membrane modified by ferric oxide hydrate (HFO)/carboxylated carbon nanotubes (CNTs) nanocomposite was applied to extract humic acid (HA) from biogas slurry produced during the anaerobic digestion of kitchen waste. The modified ultrafiltration membrane demonstrated an increase in water flux from M-0 (285.7 L m- 2 h-1) to 489.5 (L m- 2 h-1), with the HA rejection rate improving from 85 % to 98.6 %. The flux recovery ratio of M-4 increased by 30.4 %, while irreversible fouling was reduced by 35.3 %. The extended Derjaguin-Landau-Verwey-Overbeek theory was used to verify the fouling resistance of the membrane. Theoretical calculations revealed that electron cloud rearrangement can take place between HFO and CNTs, forming the heterojunction. The HFO/CNTs heterojunction exhibited stronger adsorption with PSF (Eads= -1.3658 eV) compared to CNT alone (Eads=0.2836 eV). Additionally, the electron flow was more abundant in the HFO/CNTs/PSF system, which further promoted the dispersion of nanoparticles in the casting solution. The enhancement of the HA rejection rate was primarily attributed to the modification of the membrane surface charge characteristics. The HA extraction ultrafiltration system was constructed based on real biogas slurry. The concentration of HA increased by 15-20 times compared to its concentration in the biogas slurry through the system. This study provided an effective method for the deep resource utilization of organic matter during anaerobic digestion and a valuable insight for the practical application of ultrafiltration membranes in engineering projects.