Dead-end filtration experiments on model dispersions: comparison of VFM data and the Kozeny-Carman model

Permeate flux decline is a major item when considering pressure driven membrane filtration. The decline is a result of the deposition of (dispersed) feed materials as a layer on the membrane surface. This means loss of production of permeate or, when applying a higher compensating pressure, loss of energy. A number of characterization techniques such as SDI (Silt Density Index) and MFI (Modified Fouling Index) are available to study such effects by using a dead-end filtration set-up. A more recent technique, VFM (Vito Fouling Measurement), is described in [1,2]. VFM is a pragmatic characterization method which presents dead end flux decline results as a graph or as a table formatted “multi value index”. To the author's opinion it is very difficult to sequeeze the complex permeate flux decline behaviour of a real feed into a one number “model” such as SDI or MFI without losing crucial information or even missing a crucial zone within the complete set of data. In this paper it is demonstrated that, even for a simple situation of a model dispersion of ceramic powders in water, it is improbable to achieve a correct mathematical description of the hydraulic conditions during cake formation on a membrane surface. Therefore, a universally applicable mathematical model which predicts in an accurate way the flux decline, for a real feed with a complex composition, seems impracticable. This argues in favour of the experimental approach, such as the VFM.