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Abstract
Submerged flat sheet membranes are mostly used in membrane bioreactors for
wastewater treatment. The major problems for these modules are concentration
polarization 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 reduces
fouling for submerged flat sheet membranes were obtained by using Computational Fluid
Dynamics.
Experiments conducted showed that an optimal airflow rate exists beyond which no
further flux enhancement was achieved. Fouling reduction increased with nozzle size at
constant airflow. Nozzles of equal surface area but different geometries performed
differently in terms of fouling reduction. Bubble size distribution analyses revealed that
the percentage of larger bubbles and bubble rise velocities increased with the airflow rate
and nozzle size. Thus the results of this study suggest that the effectiveness of two-phase
flow depends on the bubble size. CFD simulations revealed that average shear stress on
the membrane increased with airflow rate and bubble size and further indicated that an
optimal bubble size possible exists. Using intermittent filtration as an operating strategy
was found to be more beneficial than continuous filtration. This study also showed the
importance 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 air
distribution across a submerged flat sheet membrane. It was found that baffles decreased
the rate of fouling at least by a factor of 3.0 based on the dTMP/dt data.