Silt Density Index - The RO Gatekeeper

The Silt Density Index is most frequently used to determine fouling potential prior to RO filtration. You can think of SDI as a bouncer, keeping the riff-raff out of the RO feed water. The higher the number, the greater the likelihood of fouling. The maximum SDI number allowed depends on the type of RO membrane being used; most manufacturers recommend a maximum SDI of 4 or 5. SDI is found by calculating the rate at which a membrane filter is plugged. ASTM standard D4189-07 defines that the nominal filter for this application is a white hydrophilic MCE membrane filter, with 0.45 ?m pore size and a 47 mm diameter. The reason this particular membrane is used is that it is more susceptible to plugging from colloidal material than from hard particles such as sand, therefore giving a better indication of the factors that might plug an RO membrane down the line. Other measures that can be derived from the SDI include the plugging factor and the Modified Fouling Index (MFI). The plugging factor expresses the level of suspended solids as a percentage of the measured SDI value to the maximum SDI value, so a 100% plugging factor would indicate that your membrane is completely plugged. The MFI incorporates cake filtration theory into its calculation of fouling potential. Since this formula is more complex than SDI, it is not as frequently used in the field. SDI can be determined

manually

or

automatically

with a measurement kit. Got any tips or experiences measuring SDI? Let us know in the comments!