SILT DENSITY INDEXES (SDI), PERCENT PLUGGING FACTOR (PERCENT-PF) - THEIR RELATION TO ACTUAL FOULANT DEPOSITION

The SDI has been increasingly used by the membrane industry as a desig n and operational parameter and limit. Although widely used, the physi cal meaning of the test and the correlation of the calculated plugging factors with the tendency of membrane surfaces to foul (with conseque nt performance loss) are not well understood. In this paper, the follo wing is offered for consideration: a simplified mathematical hypothesi s 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 o n succcessively diluted feed samples, leading in turn to calculation o f the relative foulant weights deposited on the test filter discs. The data show that increases in measured SDI values correspond to geometr ic increases in the amount of foulant deposited. As illustrated in Fig . 1, for each unit increase in the SDI, as measured and calculated, th e amount of foulant increases geometricallly. For example, for each SD I 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, t he amount of deposited foulant triples. Therefore, even infrequent exc ursions above a reasonably set SDI value can be quite harmful and ever y effort should be made to minimize or eliminate them; they need not a nd 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.