Experimental and CFD simulation investigations into fouling reduction by gas-liquid two-phase flow for submerged flat sheet membranes

摘要

Submerged flat sheet membranes are mostly used in membrane bioreactors forwastewater treatment. The major problems for these modules are concentrationpolarization and subsequent fouling. By using gas-liquid two-phase flow, these problems can be ameliorated. This thesis aimed to optimize the use of gas-liquid two-phase flow as a cleaning mechanism for submerged flat sheet membrane. The effect of various hydrodynamic factors such as airflow rate, nozzle size, nozzle geometry, intermittent bubbling, intermittent filtration, channel gap width, feed concentration and membrane baffles were investigated for model feed materials (yeast suspensions and mixed liquor from activated sludge plants). Insights into mechanisms by which two-phase flow reducesfouling for submerged flat sheet membranes were obtained by using Computational FluidDynamics.Experiments conducted showed that an optimal airflow rate exists beyond which nofurther flux enhancement was achieved. Fouling reduction increased with nozzle size atconstant airflow. Nozzles of equal surface area but different geometries performeddifferently in terms of fouling reduction. Bubble size distribution analyses revealed thatthe percentage of larger bubbles and bubble rise velocities increased with the airflow rateand nozzle size. Thus the results of this study suggest that the effectiveness of two-phaseflow depends on the bubble size. CFD simulations revealed that average shear stress onthe membrane increased with airflow rate and bubble size and further indicated that anoptimal bubble size possible exists. Using intermittent filtration as an operating strategywas found to be more beneficial than continuous filtration. This study also showed theimportance of the size of the gap between the submerged flat sheet membranes.Increasing the gap from 7 mm to 14 mm resulted in an increase in fouling by about 40%based on the rate of increase in suction pressure (dTMP/dt).Finally, this is the first study which investigated the effect of baffles in improving airdistribution across a submerged flat sheet membrane. It was found that baffles decreasedthe rate of fouling at least by a factor of 3.0 based on the dTMP/dt data.