Vibration of hollow fibre membranes in submerged membrane bio-reactors (SMBRs)
Date of Issue2015
School of Civil and Environmental Engineering
Nanyang Environment and Water Research Institute
In this study, the improvement of fouling control of submerged hollow fibre membrane filtration with mechanical sinusoidal vibration was examined experimentally in the dead-end filtration of inorganic Bentonite and organic yeast suspensions, as well as mixed liquor collected from a local water reclamation plant. The hollow fibre membranes were submerged in the reactor and vibrated both longitudinally and transversely. Experiments with longitudinal vibration in both Bentonite and yeast suspensions showed that the membrane performance could be greatly improved when vibration frequency or vibration amplitude was increased beyond a threshold magnitude. In addition, small degree of fibre looseness could further reduce membrane fouling under vibration. A comparison of vibrating the hollow fibres with and without the holding frame was also carried out to determine the effects of turbulence generated by the vibrating holding frame used in the experimental setup. Particle Image Velocimetry (PIV) measurements were performed to quantify the associated turbulence inside the membrane reactors. It was confirmed that the turbulence generated by the vibrating frame was more apparent at a high vibration frequency, but its contribution to the fouling reduction was minor. In addition, the energy consumption for vibration was significantly less than aeration with a comparable fouling rate, which could be due to the fact that only the boundary fluid layers around the fibres are mobilized in vibration and thus the energy dissipation is much reduced. Experiments with transverse vibration indicated that membrane fouling could be further reduced as compared to longitudinal vibration, and could be attributed to the separation of boundary layers and generation of secondary flows. A small degree of fibre looseness was also found to further reduce the membrane fouling by the transverse vibration. Experiments with varying packing densities of membrane bundles with transverse vibration showed that at larger vibration amplitudes, a high packing density of fibres could be operated with little membrane fouling, which indicated that the secondary flows generated could overcome the strong permeate flux competition within the bundle. The inclusion of vibration relaxation suggested that a short relaxation time was more favourable in the half vibration/relaxation mode. PIV measurements indicated that vortices could be generated by transverse vibration. Finally, the measurement of energy consumption indicated that transverse vibration was more efficient and effective for fouling control than longitudinal vibration. The effect of transverse vibration was further investigated for both short and long operating time in mixed liquor. For the short duration operation, a short relaxation interval was also found to be more effective for fouling control with same total combined vibration/relaxation time for vibration relaxation operations in mixed liquor. More membrane fouling was induced by higher concentration of mixed liquor due to more foulant deposited on the membrane surface. Transverse vibration can be more effective for fouling control in mixed liquor than longitudinal vibration due to the secondary flows and vortices generation by transverse vibration. The effect of transverse vibration could be further intensified with a small amount of air sparging due to the turbulence enhancement by the moving bubbles. For the long duration operation, the high concentrations of mixed liquor suspended solids (MLSS) from the feed accumulated in the reactor over a long period, fouled the membrane quickly and also led to fibre breakage under transverse vibration. The carbohydrates and proteins in soluble microbial products (SMP) and extracellular polymeric substances (EPS) were observed to play significant roles on the membrane performance, among which the soluble carbohydrate was dominant in the membrane fouling. The operating duration of the membrane filtration was greatly increased at higher vibration frequencies due to the higher shear stresses induced by the transverse vibration, however the risk of fibre breakage was also increased. An average removal efficiency of over 90% was recorded for both the organics and nutrients in the reactor due to the microorganism biological process, which indicated that transverse vibration could be effectively applied in submerged membrane bioreactors (SMBRs) with real mixed liquor in industrial applications. Lastly, a Vibratory-Stirring (VS) membrane module was developed by including a specially designed stencil in the module that held the fibre bundles. The VS membrane module was found to be able to further reduce membrane fouling, which can be attributed to the turbulence generated in the neighbouring region of the stencil resulting in a recirculation flow around the module and scouring of the membrane surface. The energy consumption measurement indicated that the vibration of the VS membrane module induced only very small amount of energy consumption. Overall, the results from the present study confirmed that at moderate frequencies and amplitudes, hollow fibre membrane fouling in SMBRs could be reduced substantially by vibration due to the dynamic shear enhancement on the membrane surface.
DRNTU::Engineering::Environmental engineering::Water treatment