Microfiltration and ultrafiltration with high frequency backpulsing

Microfiltration and ultrafiltration are pressure-driven membrane processes in which suspended colloids and particles are retained by membranes. Particles filtered often foul the membrane by blocking the membrane pores and/or forming a cake layer on the membrane surface. Many studies have shown that hydrodynamic methods such as using baffles to generate turbulance, dean vortices, pulsated flow and air sparging are able to reduce fouling. Backpulsing, which is an in-situ method for cleaning the membrane by periodically reversing the transmembrane pressure, is another effective technique for reducing fouling phenomenon in membranes. When the transmembrane pressure is reversed, permeate liquid is forced back through the membrane to the feed side. This dislodges deposited foulants, which are then carried out of the membrane module by the tangential flow of retentate. The effectiveness of the rapid backpulsing technique to prevent membrane fouling has been reported by several groups who have obtained flux enhancements of up to 30-fold., despite performing the backpulsing experiments at relatively low frequencies. Recently, it has been reported that backpulsing at extremely high frequency (about 10 Hz), in combination with the silicon nitride microsieve, has brought about extremely high fluxes. Using various kinds of synthetic solutions, the influence of the high frequency backpulsing technique on common membrane filtration phenomena such as concentration polarisation, pore blocking and cake formation is investigated in this thesis. The rapid backpulses for the above-mentioned investigations are generated by a system called Dynamic Crossflow Pulse (DCP). This technology has its limitation when it is used to create backpulses with conventional ceramic membranes. In this case, a system that is based on backpulsing piston is used to generate the rapid backpulses. Similarly, various model solutions are used to study the effect of high frequency backpulsing technique on fouling and concentration polarisation. In addition, a comparison between the microfiltration with microsieve and conventional microfiltration membrane, as well as with ultrafiltration membrane, is discussed. The effect of the rapid backpulsing on concentration polarization will also be studied theoretically using a numerical simulation model. Lastly, the potential of performing crossflow membrane filtration with the high frequency backpulsing is presented.