A detailed chemical analysis of the RO feed water is an absolute necessity for identifying potential foulants. This should include a measurement of the hardness (calcium and magnesium), barium, strontium, alkalinity, pH, and chlorine. The data from the chemical analysis can be used by the RO equipment designers to determine the optimum membrane array that will both minimize the tendency of scale and deposit formation and maximize the recovery and flux rate.

For example, the Langelier Stability Index (LSI), a measure of the calcium carbonate scaling tendency of the water, is computed from the water analysis to determine the maximum permissible concentration of dissolved minerals in the reject stream before scale deposition becomes a problem. Because of the number of variables that must be considered, these calculations are difficult to do with pencil and paper. Fortunately, the membranes manufacturers have developed computer programs that make these computations fast and easy to perform where the user can project the performance of membranes at actual feed conditions.

Although a water analysis is helpful in predicting the tendency of dissolved minerals to cause problems in the RO System, it does not always forecast the fouling tendency of colloids and other finely dispersed suspended solids. The Silt Density Index (SDI) is a useful tool for quantifying the fouling tendency of the feed water. This test is conducted by filtering a sample through a 0.45 micron (µm) filter and measuring the time required to collect a unit volume of filtrate. An index number is calculated from this data. Traditionally, a SDI value of less than 3.0 is desirable for RO feed waters. The SDI measurement has certain limitations in that it does not model the cross flow design of an RO membrane.