Unraveling the mechanism of fouling mitigation in AGS-MBR system: From AGS properties to foulant interactions.

Aerobic granular sludge (AGS) has demonstrated a lower fouling propensity than floc sludge in membrane bioreactors (MBRs) due to various hypotheses, including differences in particle size and the efficacy of physical scouring. However, controversy exists regarding the dominant cause of this lower fouling. Therefore, in this work, we systematically investigated the contribution of four potential mechanisms of AGS on membrane fouling alleviation in MBRs: 1) loosening cake layer; 2) scouring of the membrane surface; 3) regulating soluble microbial product (SMP) secretion; and 4) changing the rheology of the bulk solution. Our results showed that, regardless of granular size range, AGS hardly caused cake fouling due to its low hydraulic resistances (<0.8 × 10 m) and limited accumulation on the membrane surface. Scouring by AGS was ineffective in reducing the thickness and hydraulic resistance of the fouling layer compared with granular activated carbon, a commonly used scouring material for MBRs. Furthermore, liquid chromatography-organic carbon detection-organic nitrogen detection (LC-OCD-OND) results indicated that the lower fouling was related to reduced SMP secretion by AGS, with an optimal particle size (800-1000 μm) at which SMP secretion was minimized. AGS with this optimal particle size secreted over 54 % less high-molecular-weight SMP compared to floc sludge. As granule size further increased, SMP secretion increased due to biomass decay and cell lysis resulting from substrate transfer limitations in granules. Moreover, compared to floc sludge, granular sludge bulk solution exhibited lower viscosity, particularly in the 450-1000 μm size range. This enhanced rheological behavior could potentially improve shear stress induced by aeration, thereby mitigating membrane fouling. These findings emphasize that the indirect effects of AGS, including reduced SMP secretion and improved rheological properties, played a crucial role in the lower membrane fouling in AGS-MBRs, while direct effects such as loosening cake layer and the scouring effect played minor roles.