CROSS-FLOW MICROFILTRATION WITH HIGH-FREQUENCY REVERSE FILTRATION

A primary method of reducing membrane fouling during cross-flow microfiltration is periodic reverse filtration. This in situ method of cleaning the membrane forces clear fluid in the reverse direction through the membrane and readjusts the particle or solute accumulation on the retentate side of the membrane. This work focuses on the design of a high-frequency, reverse-filtration strategy to maximize the flux for washed yeast suspensions through 0.2-mu m cellulose acetate flat sheet membranes. Several experiments were conducted with reverse-filtration times ranging from 0.5-4 s and forward-filtration times ranging from 1-40 s. For every back-filtration time, there exists an optimum forward-filtration time that gives the maximum global average flux. The optimum average flux increases with decreasing back filtration times and feed concentrations, but shows little dependence on cross-flow velocity and reverse filtration transmembrane pressure. The optimum flux with rapid backflushing is 20 to 30 times higher than the long-term flux in the absence of backflushing. A theory presented assumes that cake formation during forward filtration follows dead-end filtration theory and the cake is instantly removed during reverse filtration. The measured average flux per cycle follows the trends predicted by the theory, but the measured values exceed the predictions, presumably due to brief delays in cake removal and cake formation at the start of reverse and forward filtration, respectively, during each cycle.