New insights into silt density index and modified fouling index measurements

Abstract

In order to investigate the accuracy and reproducibility of the parameters SDI (silt density index) and MFI (modified fouling index), tests in diverse conditions were performed: with three different types of filter holder, two microfilter pore sizes, with and without permeate spacer and with two foulants (alginate as organic foulant and silica flour as the particulate). Additionally the effect of pressure on fouling indices was analysed. It was concluded, that there is a need to define more boundary conditions in the ASTM standard (exact type of filter holder, exact type of membrane, start conditions) for the SDI to achieve a parameter with reliable and comparable values. But in comparison to MFI, SDI seems to be a more robust parameter for a use in practice pertaining to the influence of a feed pressure level and the effective membrane area. The MFI gives more insights in acting mechanisms and offers advantages for research. It shows a correlation to a concentration of particulate or organic foulants. Like the SDI, it is necessary to define and standardize exact boundary conditions for the MFI tests (classified by the use of MF, UF or NF filter media).

Assessing membrane fouling potential of humic acid using flow field-flow fractionation

“…Membrane fouling is considered an inevitable consequence of filtration processes [5,6] and, therefore, it is important to qualitatively and quantitatively assess the properties of the feed water in relation to the performance of the membrane. Silt density index (SDI) and modified fouling index (MFI) are commonly used as qualitative indicators for the feed water; however, it has several shortcomings [7][8][9][10]. Firstly, SDI/MFI standard tests are conducted using microfiltration MF membrane (pore size of 0.45 m) and, therefore, do not accurately account for the fouling potential of foulant * Corresponding author.…”

Particulate fouling

“…As mentioned in the ASTM standard and also reported in the literature (Salinas Rodriguez et al, 2019, ASTM D4189 -14, 2014, Rachman et al, 2013, Alhadidi et al, 2011c, Nahrstedt and Camargo Schmale, 2008, the SDI value will vary with: material of the membrane, origin of the filter membrane (manufacturer), membrane porosity, and even filters in the same production batch. This suggests that SDI values obtained using filters from different membrane manufacturers are not comparable.…”

“…The limitations of the SDI test are well documented and include (Salinas Rodriguez et al, 2019, Rachman et al, 2013Alhadidi et al, 2011c, Alhadidi et al, 2011a, Nahrstedt and Camargo Schmale, 2008, Boerlage, 2007, Schippers and Verdouw, 1980):…”

The Modified Fouling Index Ultrafiltration constant flux for assessing particulate/colloidal fouling of RO systems

“…Nahrstedt and Camargo Schmale [21] studied the effect of the membrane holder in SDI and MFI results. They tested three filter holders: Millipore inline 47 mm (external diameter), Sartorius SM 47 mm (external diameter), and Sartorius SM 25 mm (external diameter).…”

“…However, increasingly the value of this test to predict the rate of fouling in RO systems due to particle deposition is being questioned. The limitations of the SDI test are well documented [1,2,5,21,23,29,33] and include: no correction for test water temperature; the result is heavily dependent on the test membrane permeability; not applicable for testing high fouling feed water e.g., raw water -ASTM recommends that turbidity should be b 1 NTU;…”