Silt density indices (SDI), percent plugging factor (%PF): their relation to actual foulant deposition

The SDI has been increasingly used by the membrane industry as a design and operational parameter and limit. Although widely used, the physical meaning of the test and the correlation of the calculated plugging factors with the tendency of membrane surfaces to foul (with consequent performance loss) are not well understood. In this paper, the following is offered for consideration: a simplified mathematical hypothesis that shows the relationship between the measured SDI test value and the corresponding amount of foulant deposited on the test filter disc. This relationship is confirmed by the ability to predict SDI values on successively diluted feed samples, leading in turn to calculation of the relative foulant weights deposited on the test filter discs. The data show that increases in measured SDI values correspond to geometric increases in the amount of foulant deposited. As illustrated in Fig. 1, for each unit increase in the SDI, as measured and calculated, the amount of foulant increases geometricallly. For example, for each SDI value from 1 to 5, the amount of foulant approximately doubles with each unit increase in SDI. Putting it another way, there is more total foulant deposited between SDI values of 4 and 5 than there is between SDI values of 1–4 combined. Further, between SDI values of 5 and 6, the amount of deposited foulant triples. Therefore, even infrequent excursions above a reasonably set SDI value can be quite harmful and every effort should be made to minimize or eliminate them; they need not and should not be tolerated. And, especially, in view of the geometric relationship described, the fallacious practice of reporting linearly averaged SDI values should be recognized and dispensed with at once.