Membrane characterisation: the need for a standard

Biologists have been aware for at least 200 years that membranes in living creatures set up and maintain concentration differences between different regions of an organism, without which life could not occur. The search for synthetic membranes able to bring about useful separations in industry was a conscious attempt to mimic nature; early successes were limited, and industrial membrane technology has been a major development over the last 10 to 20 years. Now there are at least a dozen widely differing processes that use membranes to bring about separations. When the liquid to be treated contains colloidal particles or aggregated molecular structures larger than about 0.01 μm in size, it is appropriate to think of it as a suspension rather than a solution. Such large particles undergo relatively minor Brownian motion and sediment appreciably, if rather slowly, under gravity. In order to separate them from the suspending liquid, which is usually a true solution and may contain molecular solutes, it is appropriate to think of filtration as an alternative to, for example, centrifugation. Conventional filters made from compacted fibres or powders are available for removing particulates smaller taken about 10 μm from liquid feeds. For this purpose, polymer technologists have developed techniques to cast films using a combination of solvent and precipitants that contain controllably small pores of fairly narrow size range. These ‘membrane microfilters' are used extensively for the filtration of bacteria and colloidal particles and are coming up against increasing competition from a rapidly growing range of ceramic microfilters. Methods of characterising the membranes are diverse and no standard exists between different manufacturers for the measurement of properties such as pore size and size distribution, pore shape, asymmetry, permeability and wettability. These properties, together with measured permeate flux decline and rejection data, are the ones that most affect the potential suitability of a membrane to a particular industrial application.