Suspended solids and colloidal materials in feed water are one of the biggest problems in reverse osmosis systems. Even though most systems have some pretreatment including 5 micron prefilters, these fine particles are responsible for fouling of reverse osmosis membranes.
In order to have some measure of the degree of this fouling problem, a concept called Silt Density Index (SDI Testers) 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.
