Dynamic Membranes in Ultrafiltration and Reverse Osmosis

Abstract

In principle, the basis of the filtration process appears to be as clear as the filtrate one would sometimes like to achieve. One needs, (it would seem), only to select a porous matrix whose flow pathways are dimensionally compatible with desired level of selectivity and to provide the necessary driving force in the form of a pressure gradient. Of course, even for classical microfiltration of particulates this concept is an oversimplification, since conventional filters often display significant selectivity toward species five to ten times smaller than their nominal pore size rating. Thus it comes as no surprise to find similar effects carrying over into the realm of ultrafiltration and reverse osmosis, and for identical reasons. The conditions at the filter/solution boundary and the interaction between the solute being filtered and the membrane barrier often contain the key factors which determine the overall behavior of both volume flux and selectivity. When the solute is a particle of colloidal dimensions (whether organic or inorganic), these factors may create a relatively permanent change in the filtrative characteristics of the original membrane and one then speaks of “dynamic membrane” formation. Although the term “dynamically formed membrane” would be a more accurate description, the usage of “dynamic membrane” has become well estabilshed.