Study of (ir)reversible fouling in MBRs under various operating conditions using new on-line fouling sensor

► The MBR-VFM is an on-line method to measure (ir)reversible fouling in MBRs. ► It was validated in lab-scale MBRs under different combinations of HRT–SRT. ► The MBR-VFM was found a valuable tool to monitor and better understand fouling. ► (Ir)reversible fouling decreased with increasing HRT due to lower foulant concentrations. ► The effect of SRT was limited to reversible fouling and dependent on HRT. In this study, a new fouling sensor was validated under different conditions of hydraulic (HRT) and sludge retention time (SRT). The MBR-VFM (membrane bioreactor-VITO Fouling Measurement) allows the simultaneous determination of the physically reversible and irreversible fouling potential of a mixed liquor during a single crossflow filtration test. In accordance with the on-line filtration behavior, the measured reversible and irreversible fouling propensities differed significantly between MBRs operated at different combinations of HRT-SRT. Moreover, a significant negative correlation was found between the on-line permeability and the reversible and irreversible fouling propensity measured by the MBR-VFM. This corresponded to observations made on membrane recovery after physical or chemical cleaning actions. Higher reversible and irreversible fouling were observed at lower HRT, presumably as a consequence of increased concentrations of foulants present. The effect of SRT was much smaller and restricted to the reversible fouling component. Possible explanations for the increased fouling at prolonged SRT are the higher sludge and colloid concentration and the smaller floc size. It can be concluded that the MBR-VFM is a useful tool to monitor fluctuations in a mixed liquor’s (ir)reversible fouling potential and can contribute to a deeper understanding of the occurring fouling phenomena.