The current methods used to determine the fouling characteristics of reverse osmosis feed water e .g . Silt Density Index and Silting Index show important disadvantages . The main problem of these methods is, that no linear relationship exists between the index and the concentration of colloidal and suspended matter . The newly developed Modified Fouling index, however, is proportional to the concentration of colloidal and suspended matter . The existence of this linear relationship has been proved by measuring standard colloidal formazine solutions .
The Modified Fouling index which has been derived from the Silting Index and the Silt Density Index is based on the occurrence of gelfiltration . Numerous measurements have shown that the filtration of water containing colloidal matter through a membrane filter (0,45 p) is controlled by the following mechanisms : in the beginning blocking filtration, after a short time gelfiltration without compaction, followed by gelfiltration with compaction . The mathematical description of gelfiltration without compaction is simple and applicable for the determination of an index related to the nature and concentration of colloids . Measurements carried out on water types from different sources have indicated that the Modified Fouling Index may be a very useful tool for explaining and predicting the rate at which fouling of reverse osmosis membranes occurs .