The colloidal index (CI) (Silt Density Index—SDI) is an important integral indicator of the quality of water prepared for reverse osmosis plants (ROP). To date, this is the only indicator that allows us to assess the likelihood of the membrane unit’s contamination with colloids and suspended solids contained in the source water. It is shown that the new method for measuring the colloidal index, in contrast to the standard one, makes it possible to eliminate the contribution of nonlinearities to the description of water quality and it has a smaller error in a wide range of concentrations of suspended particles, which provides a more accurate prediction of potential contamination of reverse osmosis plants. Based on the differential form of the CI, a generalization of the standard technique for measuring the colloidal index in the linear region of decreasing the water flow rate using more accurate measurement and automation methods is presented. For waters with a low content of suspended and colloidal particles, the water flow through the microfilter changes slowly; therefore, to notice a drop in its performance, a considerable observation time is required (approximately 15 min). Attempts to measure CI on water with a high content of colloidal particles and suspended solids show their practical impossibility. The data of experimental studies are presented, confirming the correctness and advantages of the methodology for measuring the colloidal index demonstrated in the article. Comparison of methods for calculating the generalized colloidal index presented in the article and the modified fouling index (MFI) is performed. It is shown that the colloidal index is proportional to the concentration of suspended particles, and the MFI additionally contains a normalization factor inversely proportional to the filtrate velocity. In practice, the results presented in the article are likely to make it possible to calculate the load on reverse osmosis plants and to reveal the correlation of the frequency of chemical purification. In the course of the experiments, the phenomenon of an initial sharp drop in the flow rate of the filtrate through the filter was discovered, which is still awaiting its explanation.