Abstract

Membrane bioreactors (MBRs) have emerged as a leading technology for high-quality wastewater treatment by combining biological degradation with membrane filtration. However, their long-term performance is critically hindered by membrane fouling, which increases energy consumption, reduces permeate flux, and necessitates frequent cleaning or replacement. This study presents a chemical engineering perspective on the fouling mechanisms that dominate during prolonged MBR operation. It classifies fouling into reversible and irreversible forms, including organic, inorganic, particulate, and biofouling, and emphasizes the role of foulant–membrane interactions at the molecular level. The formation and evolution of the fouling layer are examined through mass transfer limitations, adsorption dynamics, and physicochemical interactions such as electrostatic attraction, van der Waals forces, and hydrogen bonding. Special attention is given to extracellular polymeric substances (EPS) and soluble microbial products (SMP) as key contributors to gel layer development and pore blockage. The study explores how operational parameters such as hydraulic retention time (HRT), sludge retention time (SRT), aeration intensity, and flux loading affect fouling rates and compositions. Advanced characterization techniques including Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and atomic force microscopy (AFM) are highlighted for diagnosing fouling morphology and chemistry. Additionally, modeling approaches based on mass transport equations, resistance-in-series frameworks, and fouling kinetics are discussed to provide predictive insights into membrane lifespan and cleaning cycles. Strategies for fouling mitigation are evaluated from a process design standpoint, including dynamic membrane operation, membrane surface modification, and chemically enhanced backwashing. This chemical engineering approach reveals the interdependence between biological processes and physical transport phenomena in fouling behavior. By understanding the mechanistic basis of long-term fouling, the study informs the design of more resilient and energy-efficient MBR systems. Ultimately, the findings contribute to optimizing membrane selection, operational regimes, and maintenance protocols, ensuring the sustainable deployment of MBRs in both municipal and industrial wastewater treatment applications.