Shear enhancing hydrodynamics for low pressure membrane processes
Date of Issue2014
School of Civil and Environmental Engineering
Singapore Membrane Technology Centre
Low pressure membrane applications, such as microfiltration (MF) and ultrafiltration (UF), have been widely employed in wastewater treatment systems including membrane bioreactors (MBRs). The main problem of these applications is that the operating life period of the membrane is greatly reduced due to the rapid fouling rate, resulting in frequent cleaning of the membrane and consequently increasing the plant maintenance and operating cost. Dynamic shear-enhancing approaches, including vibrating hollow fibres membrane modules, have been proven to be effective to reduce concentration polarization and membrane fouling. However, to date, the effects of the parameters of vibrating system on the fouling have not been well studied comprehensively. In order to obtain a better understanding of the behavior of supermicron particles in the shear flows, the particle deposition in a simple crossflow microfiltration (CFMF) channel was first investigated theoretically and experimentally. Direct observation through the membrane (DOTM) was employed to determine the local critical flux in the channel. The mass transfer of the particles in the channel was simulated using the finite difference method. The results showed that a critical modified Peclet number (Pecrit ) can be introduced as a generalized (not depending on the hydrodynamics) criterion for the onset of particle deposition rather than the critical flux. The effects of vibration parameters, namely frequency and amplitude, and geometrical parameters (such as fibre radius and bundle packing density) on the wall shear rate at the membrane surface were studied both analytically and numerically for longitudinal vibrating fibres. The CFD results showed a good agreement with the analytical solution results. For transverse oscillating fibres, in addition to the shear flow around the membrane fibres, some secondary flows also exist. CFD simulations were performed to analyse the secondary flows induced by transverse oscillating membrane fibres. In addition, the hydrodynamics of different fibre layouts, such as bundle and curtain of membrane fibres, were studied and compared. For validation purpose, the CFD simulation results were compared with experimental data acquired using the technique of Particle Image Velocimetry (PIV) in the laboratory with different Reynolds numbers. The computational results generally showed a good agreement with the experimental observations. The effect of different anti-fouling techniques (such as aeration and vibration) on the internal fouling of the membranes was assessed using different methods, including liquid displacement porometry (LDP), evapoporometry (EP) and field-emission scanning electron microscopy (FESEM). The results suggested that using vibration might have adverse effects on the performance of the membrane with internal fouling. Finally, the behavior of accumulated particles near vibrating surfaces was also evaluated using PIV. The results showed that in order to prevent the cake formation on the membrane surface, shear rate (induced by vibration) and washing flows (generated by aeration) are both needed.
DRNTU::Engineering::Environmental engineering::Water treatment