Clemens Fritzmann, Matthias Hausmann, Wim Doyen, Matthias Wessling, Thomas Melin:
Spacers in MBR!? Towards low air sparging in flat sheet membrane-bioreactors
In: AMS6/IMSTEC 2010, Sydney, Australia, 22-26.11.2012
Submerged membrane bio-reactor (MBR) systems suffer from severe fouling due to the high solid content and measures to consistently clean the membrane and to minimize fouling during operation must be taken. In all submerged membrane systems, air sparging is used for removal of a fouling layer to guarantee stable operation of the MBR. Rising bubbles result in higher shear stress and further initiate fluid cross flow as well as movement of the membranes, which contribute to the removal of foulants. It can be concluded that air sparging is essential for stable operation of a MBR. However, air sparging largely contributes to the overall process costs of submerged membrane bioreactors and about 50% of the total energy consumption follows directly from membrane aeration. This high energy consumption is a bottleneck of current MBR systems and is a main cause for the limited spread of the MBR technology. Therefore measures must be taken to further reduce the necessity for air sparging aiming at a more energy efficient overall process. Spacers are applied in various membrane processes such as reverse osmosis or ultrafiltration and largely contribute to efficient operation, since the spacer induced hydrodynamics largely increases mass transfer, while at the same time fouling is reduced. However, the application of membrane spacers to the treatment of fluids with high solid loads is currently prohibited, since the spacer filaments provide obstacles in the flow field leading to local stagnant flows and a high risk of channel blockage. In this work, a new type of membrane spacer is introduced that is designed for the conditions found in filtration of high solid loads. The spacer geometry displays no flow obstacles perpendicular to the main flow, which strongly reduces the risk of channel blockage. In addition, no line contacts of spacers and membrane are found. This hinders precipitation of larger agglomerates in the vicinity of the contact regions between spacer filaments and the membrane, which is found when current net spacers are applied. By use of statistical design experiments the influence of several module and design parameters such as cross flow velocity, spacer geometry and air sparging rate on critical flux and fouling rate is quantified and compared. Significant increase in critical flux was observed in the experiments when applying the new membrane spacer and the application of spacers proved to be efficient even at low cross flow velocities. It thus can be clearly shown that application of a spacer as developed in this work might be a solution towards energy efficient operation of membrane bioreactors.