Standard Operating Procedure - Ultrafiltration - Department of ...

Metal ions, such as high amounts of iron or manganese are detrimental. Their effect iscompounded when higher levels of silicate are present. The membranes might start to becomebrownish, blackish, and fluxes go down because of the silicate film that is deposited on thesurface; whereas, iron and manganese might still be removable with certain chemical agentsand/or acids (membrane specs permitting), silicates are almost impossible to remove.Hydrofluoric acid usually does it, but you might as well throw the membrane out. Operationsucceeded, patient died!High salt content of the chloride type is not recommended either. This is less for the benefit ofthe membranes, than for the supporting piping, tanks etc. The combination of chlorides withacids causes pitting corrosion and should be avoided!Water hardness can be a big problem. Many plants use water softeners, ion exchangers,permeate water off their RO plant or boiler water for cleaning. The recommended water is thepermeate water. Any water other than permeate may cause irreversible fouling. Check all watersupplies.CHEMISTRY:For membrane cleaning there are basically three types:• Alkaline cleaners with various types of buffer systems classifying them according to the pHrange they generate;• Neutral cleaners with or without enzymes;• Acid cleaners.Most cleaning can be done with a good alkaline product. Most types of soil that we encounterrequire - or are best removed with alkalinity. Protein, most any kind of protein, is more easilyremoved at high pH values. (figure 10.) At these pH values the protein also is slowlyhydrolyzed, making it more soluble. As neutral values are approached, the solubility decreases,and at pH 4-5 many proteins can even be precipitated, they become quite insoluble and difficultto remove.FAT:Under normal conditions of cleaning (time, temperature and alkalinity usually encountered), notmuch hydrolysis (called saponification, the making of soap) occurs. The higher the alkalinity,the higher the temperature, the more fat will be hydrolyzed, leading to salts of fatty acids. IfCalcium ions are around and not enough chelating power is in the cleaner (case of commoditycaustic), then Calcium soaps, insoluble deposits will form which can clog up filters and alsomembranes. This should be avoided. Alkalinity based on caustic along, pH, is not sufficient. Itis difficult to control the pH; no buffer system is there; no detergency. There simply is no soilcarrying capacity present. Builders are needed! Sodium or potassium silicate, a favorite builder63