Unrevealing the role of in-situ Fe(II)/S₂O₈^2- oxidation in sludge solid–liquid separation and membrane fouling behaviors of membrane bioreactor (MBR)

Membrane bioreactor (MBR) has received continuous attention in waste activated sludge treatment, however, membrane fouling still remains a big and inevitable challenge to its wide applications. Ferrous-persulfate (Fe(II)/S₂O₈^2–) oxidation possesses the outstanding oxidizing capability and high-efficiency in degrading refractory organics and enhancing sludge solubilization. In this study, in-situ Fe(II)/S₂O₈^2- oxidation was coupled for pretreating sludge, and the effect on the subsequent filtration performance and membrane fouling behaviors in MBR was investigated. Fe(II)/S₂O₈^2- oxidation had an outstanding effect on enhancing filterability of mixed sludge liquor and alleviating membrane fouling. Under the optimal pretreatment condition of 1.5/1.2 mmol-Fe(II)-S₂O₈^2-/g-VS, the membrane flux of reactor reached 20.6 mL/s/m², and a 46.6% decrease in trans-membrane pressure (TMP) was obtained compared to the raw sludge without pretreatment. Correlation analysis reveal that polysaccharides in extracellular polymeric substances was the decisive factor affecting the membrane fouling behavior. The strong oxidizing ability of in-situ Fe(II)/S₂O₈^2- pretreatment could effectively break the glutinous and pilotaxitic biopolymer matrix deposited on the membrane surface, and rupture the hydrophilic bonds of proteins and polysaccharides. The high molecular weight biopolymers were brought down, with the water bounded inside the flocs released into liquid phase. Iron-mediated coagulation was also conductive to the re-aggregation of the fine particles and subsequent membrane fouling control. For raw sludge, the predominated fouling mechanisms were intermediate pore blocking and cake layer formation, while complete pore blocking and standard pore blocking occurred more frequently for pretreated sludge. This work provides a promising approach based on Fe(II)/S₂O₈^2- oxidation processes for efficient abatement of membrane fouling and sludge treatment.