Improved permeate flux by flocculation of biological feeds: comparison between theory and experiment

Deposition of the rejected species on the membrane surface during tangential flow microfiltration is often observed. This cake layer can lead to lower permeate fluxes and partial rejection of dissolved solutes. Flocculation of the feed stream prior to microfiltration has been shown to increase the volume-based modal particle size which in turn leads to increased permeate fluxes. Relating the properties of the flocculated feed stream to the permeate flux is important if the effectiveness of a flocculant is to be assessed. Experimental observations reported here indicate that flocculation of yeast and CHO cell feed streams result in a self-similar particle size distribution. Further, though flocculation leads to an increase in the volume-based modal particle size, the number-based modal particle size decreases. Therefore after flocculation there is a large fraction of smaller flocs. Observations of deposition of flocculated feed streams during microfiltration indicate that these smaller particles preferentially deposit on the membrane surface. Consequently the expected permeate flux based on the volume-based average particle size after flocculation is significantly higher than observed. Further rearrangement of the deposited particles in the cake during microfiltration often leads to a reduced permeability of the cake resulting in lower permeate fluxes.