Scale-up of microfiltration systems: fouling phenomena and Vmax analysis

Proper sizing and scale-up of normal flow filtration devices requires an understanding of the effects of membrane fouling on system capacity. The Vmax test is often used to accelerate testing and reduce the required process volume, but the underlying assumption that fouling occurs by uniform constriction of cylindrical membrane pores is rarely met in practice. We have examined the validity of the Vmax model and have compared the results with predictions of a new model that accounts for fouling due to both pore blockage and cake formation. The Vmax analysis significantly overestimates the system capacity for proteins like bovine serum albumin that foul primarily by pore blockage, but it underestimates the capacity for lysozyme which fouls primarily by cake formation. In contrast, the pore blockage–cake filtration model provides a much better description of membrane fouling, leading to more accurate sizing and scale-up of normal flow filtration devices.

Although the Vmax method provides a simple and quick approach for sizing normal flow filtration devices, the underlying assumption that fouling occurs by the uniform constriction of cylindrical membrane pores is rarely met in practice. The net result is that the Vmax analysis can lead to significant errors in the evaluation of the membrane capacity. For proteins like BSA that foul primarily by a pore blockage mechanism, the Vmax analysis overestimates the system capacity, particularly when flux decay data are obtained over very short filtration runs. In contrast, the Vmax analysis underestimates the system capacity for proteins like lysozyme that foul mainly by cake filtration. The combined pore blockage and cake filtration model provides a very accurate description of the flux decay data, leading to much better estimates of the system capacity.