The modified fouling index, a method of determining the fouling characteristics of water

Abstract 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 μ) 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.