Microfiltration with silicon nitride microsieves and high frequency backpulsing

Silicon nitride microsieves are microfiltration membranes manufactured with techniques commonly used in the semi-conductor industry for wafer production. These membranes are characterised by the uniform pore size and the high porosity. Furthermore, due to their extremely thin selective layer and the relative open support structure, the clean water fluxes are usually very much higher (an order of magnitude of at least 100 times higher) than the conventional microfiltration. This extremely high fluxes causes the microsieves to be extremely susceptible to fouling and flux decline during filtration. As such, a new operating technique, i.e. high frequency backpulsing is introduced to keep the microsieve surface cleaned and to maintain the high fluxes. This paper discussed the mechanism used to create backpulses at very high frequency and the impact of such high frequency backpulses on particulate and nonparticulate fouling of the microsieves were studied with different model solutions.

A new concept of using a rotating shut-off valve, i.e. the Dynamic Cross Flow Pulse technology from FluXXion B.V., to create backpulses at very high frequencies has been discussed. The impact of high frequency backpulsing on fouling control produced using the DCP technology has been studied with feed comprising only particulate foulants (latex suspensions), only nonparticulate foulants (Ca(OH)2) as well as yeast suspension, which is a mixture of yeast cells and other colloidal foulants. Backpulsing has shown to be very effective in removing the cake layer formed by the depositions of foulants which are larger than the pore size. However, backpulsing has its limitation in preventing internal fouling, i.e. pore clogging and pore constriction. For colloidal foulants as seen in the experiment with Ca(OH)2, rapid backpulsing with shallow amplitude is incapable of removing the cake layer. Probably in this case, the backpulses merely lift the cake layer off the membrane surface temporarily and stronger backpulses would be required to keep the surface clean. The results have provided some insights on the future development of microsieves. It is important to ensure that microsieves with appropriate pore size are used in order to achieve the high fluxes, i.e. the majority of the foulants should be larger than the pores so that they cannot flow through the pores and foul the microsieves at the rear side during backpulsing. This also points to the need to manufacture microsieves with very small pore size. However, it is important to note that smaller pore size will result in lower fluxes. Hence, there is an optimium between the pore size and flux which is dependent on the composition of the feed. The results have also shown the importance of the design of microsieve. Fouling at the rear side of the selective layer can be minimised if there is also a tangential flow in the support structure to carry out the foulants that are deposited, as in the case at the feed side. High frequency backpulsing has proven in this work to be an effective means to prevent external fouling of the microsieves. However, it is also important to highlight that the efficiency of backpulsing is also influenced by feed characteristics. Solute-solute interactions, as shown in the case of yeast filtration, can determine the stability of fluxes. However, membrane fouling is also known to be affected by factors such as operating parameters, membrane material properties as well as the feed characteristics. Stronger backpulsing may be more effective in reducing fouling, but this will result in an increased permeate loss which in turn affect fluxes, while weak backpulsing is insufficient to blow the fouling layer off the microsieve surfaces.