Forward osmosis-integrated granular anaerobic MBR: enhancing filterability and reducing mass transfer limitations

Submerged forward osmosis (FO) systems integrated with anaerobic membrane bioreactors (AnMBRs) are subject to membrane fouling and mass transfer limitations, which are strongly influenced by operational conditions. This study systematically investigates how critical operational parameters, including draw solution (DS) type, gas sparging and granular sludge fractions, influence FO filtration performance, with the aim of understanding and overcoming the current limitations of FO-integrated AnMBRs. The results reveal that the choice of DS significantly influences both water flux and reverse solute flux (RSF). For example, sodium acetate showed a lower specific RSF and reduced salinity accumulation in the reactor compared to sodium chloride. Continuous gas sparging at an optimized rate of 0.25 m³/m²h effectively mitigated external concentration polarization (ECP) and fouling during filtration tests. In contrast, intermittent gas sparging led to fluctuations in water flux due to rapid ECP development, underscoring the need for stable hydrodynamic conditions. Additionally, the size of the sludge granules was crucial for controlling fouling. Larger granules (greater than 0.63 mm) showed a lower potential for fouling. Three-dimensional excitation-emission fluorescence spectroscopy revealed that proteins accumulate on the membrane surface and significantly contribute to FO membrane fouling. However, employing sieved granular sludge and applying gas sparging reduced the extent of foulant accumulation. Simple membrane flushing with deionized water was effective in restoring initial flux, confirming the reversibility of fouling. These findings provide valuable insights for optimizing FO-integrated AnMBRs by balancing operational conditions for long-term performance. © The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature 2025.