INFLUENCE OF SHEAR RATE ON THE FLUX DURING ULTRAFILTRATION OF COLLOIDAL SUBSTANCES

The shear rate at the surface of a membrane has a distinct influence o n the membrane flux in systems containing colloidal substances. If the shear rate is decreased, flux decreases since the thickness of the bo undary layer of retained solutes at the membrane surface then increase s. When low molecular solutes are used, flux increases again when the shear rate is increased. However, if large solutes are used, polymeric and colloidal substances, for example, it sometimes happens that the flux cannot be increased to its original value again by increasing the shear rate: the flux decline is regarded as irreversible. Experiments , performed in a new type of turbulence-promoting module, which illust rate the influence of shear rate on flux when treating solutions conta ining colloidal substances, are presented in this paper. It is also sh own that a flux decrease which can not be restored by increasing the s hear rate, may be completely restored by a temporary interruption of t he permeate flow.