Fouling tests in reverse osmosis

A measure of the fouling tendency can be obtained by performing fouling tests, which can be carried out in an apparams similar to that given in figure VII - 27. Through the use of such an apparatus the flux decline can be measured as a function of time under constant pressure, i.e. the cumulative volume will be measured as a ftinction of time. All types of solution can be used for this test, e.g. tap water seawater and also solutions of suspensions or emulsions. Many parameters have been advanced to describe fouling phenomena [28,29]  

This equation is similar as eq. VII -73, only here the membrane resistance has been taken into account. A plot of t as a function of V should give a straight line after an initial linear section. The slope of this line is defined as the MFI (see figure Vn - 28). 

However there are also some drawbacks since the MFI values are only qualitative and should not be overstressed. Furthermore, MFI expeiiments are dead-end experiments whereas membrane filtration in practice is carried out in a cross-flow mode. Also it is assumed that the cake resistance is independent of the pressure, which is not the case in general. Finally, the MFI method is based on cake filtration whereas also other factors contribute to fouling. Nevertheless, these methods are useful as a first estimate. 

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