Separation of humic substances and inorganic salts from biogas slurry by a novel SiO2 interlayer-based loose nanofiltration membrane

Membrane separation has become more attractive for sustainable treatment and resource recovery from biogas slurry. For humic substances (HS) in biogas slurry, conventional methods such as nanofiltration or ultrafiltration face limitations in effectively separating HS from inorganic salts and recovering HS. To address this point, we developed a novel SiO₂-interlayered loose nanofiltration (iLNF) membrane for separation of HA/salt from biogas slurry for the first time. The introduction of in-situ generated SiO₂ interlayer reduced the selective layer thickness and cross-linking degree, while enhancing membrane surface hydrophilicity and electronegativity. More importantly, the optimized iLNF2 membrane exhibited a high water flux of 147.19 LMH·bar −1 , excellent HA rejection of 98.07 % and low mixed salts retention of 5.07 %. Even under elevated salt concentrations such as 30 mM, iLNF2 maintained robust HA rejection (98.17 %) and efficient mixed salts permeation (97.31 %). Additionally, it also demonstrated good long-term stability and good antifouling properties. This work provides a promising strategy for sustainable HS recovery from biogas slurry by LNF technology. Importantly, LNF as pretreatment can further reduce membrane fouling in subsequent nanofiltration (NF) or reverse osmosis (RO) processes, thereby enhancing the overall efficiency and sustainability of biogas slurry treatment.