Impact of granular sludge size on membrane fouling in partial nitrification-anammox systems: Insights from the extended Derjaguin-Landau-Verwey-Overbeek theory and partial Least Squares Regression modelling.

Partial nitrification-anammox (PN/A) granular sludge integrated with membrane bioreactors (MBRs) offers efficient wastewater treatment, but membrane fouling remains challenging. This study investigated the impact of PN/A granules size (>700 μm with sludge I, 200-700 μm with sludge II, <200 μm with sludge III) on membrane fouling propensity, focusing on stratified extracellular polymeric substances-EPSs (soluble EPS, SEPS; loosely bound-EPS, LB-EPS; tightly bound-EPS, TB-EPS). Sludge I exhibited higher protein (PN) and lower polysaccharide (PS) contents in SEPS and LB-EPS, with PN/PS ratios of 3.83 and 4.48, respectively. Besides, more humic acid-like substances were presented in SEPS of sludge I. Sludge II and III showed higher PN and lower PS contents in TB-EPS, with PN/PS ratios of 2.83 and 3.17. Candidatus Kuenenia dominated sludge I with a relative abundance of 34 %, followed by sludge II at 11 %. Membrane filtration tests confirmed that size-dependent membrane fouling was correlated with stratified EPS composition. TB-EPS caused the most severe flux decline, and its impact increased with size. SEPS induced less severe flux decline and LB-EPS caused the mildest flux decline. The extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) analysis revealed stronger adhesion resistance for LB-EPS of sludge I than sludge II, with values of 34.15 and 15.42 × 104 kT/g MLSS, respectively. Partial Least Squares Regression modelling identified two orthogonal fouling pathways: molecular adhesion pathway dominated by acid-base interactions and EPS components, and physical deposition pathway governed by size and sludge concentration. This study provided insights for optimizing PN/A sludge characteristics to control membrane fouling in PN/A-MBR systems.