What is SDI?

Suspended solids and colloidal materials in feed water are one of the biggest problems in reverse osmosis systems.

Although most systems have some pretreatment, including 5-micron prefilters, these fine particles are responsible for the fouling of reverse osmosis membranes.

IIn order to have some measure of the degree of this fouling problem, a concept called Silt Density Index is used. Here a 0.45 micron filter is exposed to the feed water under pressure and filtration rates are calculated.

An SDI of less than 5 is considered acceptable for the reverse osmosis systems. This means that at values of SDI of less than 5, the membranes should foul at a very low rate. Even though the concept works most of the time, there are exceptions when a lower SDI (less than 3) is desirable due to the nature of the suspended solids in that feed water.

To perform the test, a new filter is placed in the filter housing, and the operator times and records how long it takes for the water flow to fill a pre-set volume in a graduated cylinder.  The water is then allowed to run through the filter for a predetermined amount of time, and then the amount of time it takes to fill the same volume is recorded again.  This indicates amount of 'clogging' that has occurred during the elapsed time, and the SDI of the water can be calculated from these figures.   Click here for the detailed test procedure.