CROSS-FLOW MICROFILTRATION WITH HIGH-FREQUENCY REVERSE FILTRATION

A primary method of reducing membrane fouling during cross-flow microf iltration is periodic reverse filtration. This in situ method of clean ing the membrane forces clear fluid in the reverse direction through t he membrane and readjusts the particle or solute accumulation on the r etentate side of the membrane. This work focuses on the design of a hi gh-frequency, reverse-filtration strategy to maximize the flux for was hed yeast suspensions through 0.2-mu m cellulose acetate flat sheet me mbranes. Several experiments were conducted with reverse-filtration ti mes ranging from 0.5-4 s and forward-filtration times ranging from 1-4 0 s. For every back-filtration time, there exists an optimum forward-f iltration 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 an d reverse filtration transmembrane pressure. The optimum flux with rap id backflushing is 20 to 30 times higher than the long-term flux in th e absence of backflushing. A theory presented assumes that cake format ion during forward filtration follows dead-end filtration theory and t he cake is instantly removed during reverse filtration. The measured a verage flux per cycle follows the trends predicted by the theory, but the measured values exceed the predictions, presumably due to brief de lays in cake removal and cake formation at the start of reverse and fo rward filtration, respectively, during each cycle.