Standard Operating Procedure - Ultrafiltration - Department of ...

are more difficult to degrease, to clean of fatty deposits than glass or stainless steel. The fattymolecules adhere to hydrophobic polymers stronger than to hydrophilic glass and need morechemical, physical or mechanical energy for removal. Cellulose acetate membranes are morehydrophilic and from this point of view easier to clean than the other membranes were it not forthe severe limitation of pH and temperature.An important consideration is also the viscosity of the soil or its changes of viscosity in contactwith cleaners. In membrane systems it is much more dramatic if a mistake is made in thisrespect than in most other equipment. The equipment can always be dismantled and handcleaned. One cannot do this with most membranes.A chemical reaction can occur between the soil and cleaner coagulation and precipitation ofprotein, for instance; polymerization and plastification in the case of the reaction of aldehydeswith peptines (polysaccharides used in jam manufacturing) by acids, etc. that can lead to serioustrouble. Laboratory studies in the case of previously unknown soil types are recommended, thecouncil of experienced personnel in membrane cleaning should be sought BEFORE fouling themembrane.MEMBRANE MATERIAL:A membrane surface is not as smooth as a microscopic scale as a polished metal surface. Underthe powerful microscope it looks like a sponge, which it is in a way. The pores of the activefiltrating layer are actually so small that even electron microscopes cannot “see” them, after allthey are of the size of the molecules to be filtered. Most membrane material is hydrophobic, notliking water very well. When an aqueous solution with high surface tension comes into contactwith a hydrophobic surface full of pores and holes, no wetting of the pore is achieved. Theaddition of surfactants will change this. The choice of surfactants depends on the chemicalcomposition of the membrane. They must be compatible so that no damage to the membranesurface occurs. This is where much research effort goes in developing cleaners for membranesystems.The polarization potential of the membrane material previously mentioned also has an influenceon the choice of cleaning procedures. It turns out that organic, polymeric membranes allow forhighest water and product fluxes after cleaning when the last cleaning stop is alkaline. An acidrinse often leads to decreased water fluxes. On mineral, metallic membranes it is the opposite.This is due to desorption of Calcium ions which not only are removed in acid phase bydissolution, but also due to the positive charge on metal at acid pH, they are repulsed from thesurface. Organic polymers become negatively charged at acid pH and will absorb a layer ofCalcium ions. The use of an alkaline last step is therefore recommended in this case.Metallic membranes can even be conditioned in their polarization for the specific type of productto be filtered, as the membrane will retain the polarization for quite a while. Charging itpositively for the filtration of acid whey will lead to improved filtering capacity, for instance.On polymeric membranes this does not work, as the polarization does not hold long to beeffective. Only chemical modification - attachment of specifically charged molecular groups -can achieve this.61