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. In order to measure the degree of this fouling problem, a concept called Silt Density Index is used. Here filtration rates are calculated by exposing a 0.45-micron filter to the feed water under pressure.