Enhanced microfiltration of yeast by flocculation

Flocculation of yeast cells prior to microfiltration leads to improved permeate flux. A number of commercially available cationic polyacrylamides from Cytec Industries (Stamford, CT, USA) were tested. These flocculants had different molecular weights and charge densities. The optimum flocculant dose and stirring conditions were determined for each polymer. The particle size distribution before flocculation and after flocculation and microfiltration was determined. The permeate flux depended strongly on the particle size distribution. Very small micron and submicron sized particles plug the pores of the membrane leading to decreased permeate flux. Very large flocs, 500 µm or larger plug the lumen of the hollow fibres again leading to a decrease in permeate flux. The optimum particle size is about 100–200 µm. The permeate flux increased with increasing molecular weight and charge density (percentage of monomer units that are charged) of the flocculant.

The results obtained here show that for flocculated yeast suspensions, the absorbance of the supernatant after centrifugation may be used to determine the optimum flocculant dose. Further, the larger the model polymer size, the higher the permeate flux during microfiltration providing very large flocs which could plug the lumens of the hollow fibres, are not formed. For 1 mm ID hollow fibres, the modal particle size should be around 100–200 µm. Small sub micron sized particles will plug the membrane pores leading to a decrease in the permeate flux. Large flocs 500 µm or more will plug the lumens of the hollow fibres again leading to a decrease in permeate flux. Measuring the particle size distribution will give valuable information on the expected performance of a given flocculant. Increasing the charge density and molecular weight of a flocculant will lead to an increase in the permeate flux during microfiltration.