A systematic analysis of fouling evolution and irreversibility behaviors of MBR supernatant hydrophilic/hydrophobic fractions during microfiltration

Membrane fouling remains a challenge for membrane bioreactor (MBR) application to wastewater treatment. The hydrophilic/hydrophobic organics present in MBR sludge supernatant form an important part of foulant. Their effects on fouling evolution and irreversibility were systematically investigated in this study. The organics, as fractionated into hydrophilic substances (HIS), hydrophobic acids (HOA), hydrophobic bases (HOB), and hydrophobic neutrals (HON), were subjected to fouling experiments using six microfiltration membranes covering the size range of 0.1–0.45 µm with varied hydrophobicity. Modeling methods were applied to quantitatively characterize the dynamic fouling process as consisting of the initial pore blocking and subsequent gel layer stages. The results demonstrated the importance of foulant concentration to fouling evolution: the apparent fouling rate on a same membrane followed the order HIS>HOA⪢HOB>HON, whereas the order became HOA>HIS with normalization of concentration to give the intrinsic propensity for fouling evolution (across the initial stage) as well as the specific resistance (for the gel layer stage). Foulant–membrane interactions (i.e. hydrophobic adsorption and size exclusion) and foulant–foulant interaction (polymer–metal–polymer complexation) were identified as key mechanisms for fouling at the initial pore blocking stage and the gel layer stage respectively, with the foulant properties of hydrophobicity, molecular size and carboxylic complexing groups closely involved in the interactions. Additionally, membrane properties were also important factors affecting the fouling behaviors of the hydrophilic/hydrophobic fractions, as suggested by the combined impact of membrane hydrophobicity and pore size on fouling evolution propensity, and the decisive role of membrane hydrophobicity in fouling irreversibility.