Novel nanofiltration composite membrane with a sandwich-structure of polyvinyl alcohol interlayer and Fe3+-tannic acid polyamide layer for carbon source recovery

Most wastewater treatment plants (WWTPs) encounter the problem of insufficient carbon sources. Particularly, the removal of organic pollutants is severely affected by carbon limitation and thus requires the use of external carbon sources in the secondary biological treatment process, which considerably increases operating costs. Even after the secondary treatment of high-concentration organic wastewater, the effluent still contains various organic substrates, such as low molecular weight organic acids and carbohydrates, etc., which can be used as carbon sources in the process of nitrogen and phosphorus removal. Effective separation and recovery of organic carbon sources in high-concentration organic wastewater are crucial to realize the resource utilization of organic carbon sources. In this study, a novel sandwich-structure thin-film composite (TFC) nanofiltration membrane was synthesized via chemically bonding polyvinyl alcohol (PVA) to form an interlayer and surface modification by Fe 3+ and tannic acid (TA) chelating coordination on the polyamide (PA) layer. The design promoted the deposition of metal polyphenol networks on the PA layer, exposing more chelating sites while reducing the hydraulic resistance generated by deposition. The interlayer improved the permeability of the membrane, and the deposition of Fe–TA complexes enhanced membrane separation efficiency. Compared with the TFC-Fe membrane, the resulting membrane (PVA-TFC-Fe) showed a 56.42 % increase in permeability and enhanced rejection of inorganic salts and small-molecule organic carbon sources. Compared with the retention of the TFC-control membrane, that of PVA-TFC-Fe for small-molecule carbon sources, namely, acetic acid, propionic acid, and butyric acid, increased by 19.05 %, 17.34 %, and 15.67 %, respectively, and exhibited long-term operational stability.