HYDRODYNAMIC AND COLLOIDAL INTERACTIONS EFFECTS ON THE REJECTION OF APARTICLE LARGER THAN A PORE IN MICROFILTRATION AND ULTRAFILTRATION MEMBRANES

The application of membrane separation processes, such as microfiltrat ion and ultrafiltration, is one of the most important developments in chemical engineering in recent years. Membrane fouling is the most imp ortant problem which restricts application of membrane processes. Rece ntly, it has been demonstrated that colloidal and hydrodynamic interac tions govern membrane fouling and they can be manipulated by choice of processing conditions, for example, pH, ionic strength and applied pr essure. The paper presents a quantification of both colloidal (electro static and van der Waals) and hydrodynamic effects to identify conditi ons for the operation of such processes with much greater efficiency. In particular, the hydrodynamic and colloidal forces on a charged sphe rical particle slightly larger than a pore at various distances from a charged cylindrical pore in a charged planar surface have been calcul ated. In the absence of electrostatic interactions, filtration of such particles can result in a catastrophic loss in flux as they can plug pores highly effectively. The rejection of the particle at a membrane pore is described in terms of a balance between the hydrodynamic force which is driving the particle towards the membrane and the colloidal forces between the charged particle and the charged membrane surface. A Galerkin finite element scheme combined with automatic mesh refineme nt and error estimation strategy has been used to provide numerical so lutions of the non-linear Poisson-Boltzmann equation for electrostatic interactions and of the Navier-Stokes equation for hydrodynamic inter actions. The results show that under the conditions covered by the cal culations, which correspond to those occurring in practice, the electr ostatic interactions can play a crucial role in controlling the approa ch of such a particle to a pore. The calculations have a number of imp ortant consequences for membrane separation processes. Firstly, the qu antification of the operating conditions which allow separation withou t the particles coming into intimate contact with the membrane-potenti ally non-fouling conditions. Secondly, a demonstration that the manufa cture of membranes with a high surface potential would be very benefic ial to the efficient operation of such processes. (C) 1998 Elsevier Sc ience Ltd. All rights reserved.