Axial features in the fouling of hollow-fibre membranes

The use of a hollow-fibre membrane for microfiltration introduces axial features which have important implications for both module design and fouling behaviour. The rate and mechanism of fouling of these fibres depend on their length and diameter. In this paper, the effect of fibre length on fouling behaviour is investigated. Axial features play a key role in the initial stages of fouling, but become less significant as permeate flowrates decline substantially. The rate of fouling is initially insensitive to fibre length, but as fouling progresses, longer fibres foul more slowly. After substantial fouling, the rate of fouling becomes dependent on membrane surface area, as might be expected from a planar membrane. A simple model is presented which isolates the contribution to fouling due to axial features arising from hollow-fibre geometry. This model predicts the experimentally-observed dependence of fouling behaviour on fibre dimensions under conditions where axial features are significant. Although, properties of the feed material (such as particle size, size distribution, cake permeability and compressibility) and transport phenomena (such as inertial lift and back-diffusion) may be key contributors to membrane fouling, the decline in permeate flowrate is dominated by axial features until substantial fouling has occurred.