Transverse vibration as a fouling limitation strategy in membrane bioreactors

Abstract

Membrane fouling is a key factor that significantly contributes to the performance of membrane bioreactors (MBR). Aeration is commonly applied in MBRs to control the rate of fouling. In applications where aeration is not a feasible option, such as in anaerobic membrane bioreactors (AnMBRs), limiting fouling could be a challenge. Transverse membrane vibration could potentially improve the performance of such systems by promoting shear enhancement at the membrane surface and reducing the rate of fouling. In this thesis, the impact of transverse vibration on fouling limitation was studied using model solutions (yeast, bentonite, alginate) and complex feed (secondary effluent) from an AnMBR.

During the fundamental study of filtration with model solutions, it was found that transverse vibration has great potential in limiting cake formation. However, the impact of it on internal pore fouling was found to be limited. It was estimated that the power required for the membrane displacement was similar in magnitude to that utilized by aeration to limit fouling.

Further improvement in the performance of the transverse vibration was studied by optimizing operating parameters (frequency and displacement). Although increasing both frequency and displacement further reduced the rate of fouling, it was found that the improvement was dependent on the resulting shear rate rather than the individual parameter. The impact of fiber arrangement and distance between fibers was also studied using a high concentration of yeast. It was found that during filtration with vibration, the performance of the system was not as sensitive to the close vicinity of the fibers, even at high feed concentrations. As reported in literature, the vortices generated by the fibers in close proximity interact, hence potentially increasing the shear rate and providing better fouling limitation.

As reported in literature with MBR feed, a two stage fouling phenomenon has also been observed with transverse vibration aided filtration of supernatant from an AnMBR. Investigation into fouling mechanisms during vibration showed that the initial stage of a slow gradual increase in transmembrane pressure due to internal fouling was prolonged. Critical flux was also found to remain constant when the MLSS concentration of the feed was increased during filtration with transverse vibration. The performance of the system when using transverse vibration was found to be better than that observed with crossflow velocity (CFV) or gas sparging under similar flow conditions. It was also found that the fouling which occurred during filtration with vibration was more reversible than that incurred with CFV and gas sparging. Filtration performance with transverse vibration was further improved when coupled with either periodic backwash or relaxation. However, periodic backwash during transverse vibration aided filtration increased the irreversible fouling incurred.

Structural loads imposed on the fiber due to vibration were also studied using simulations. It was found that the stresses imposed on the membrane based on the conditions used in this study, were much lower than that required for fiber failure.