Department Process Engineering

During filtration of natural water through membranes, fouling of membranes can not be avoided. In practical applications, complex operation schemes including flushing and cleaning cycles are applied in order to limit the fouling effects. However, the fundamental mechanisms of membrane fouling by natural organic compounds (NOM) are still only partially understood. A main problem in obtaining this understanding is the fact that the composition of natural waters is highly complex. In order to reveal the contribution of the individual compounds, synthetic media were used containing well-defined surrogates of the most important fouling components: biopolymers and humic acids. Also relevant inorganic water compounds were spiked to the model media, including Calcium and inorganic particles. The results have shown that, in contradiction to the general opinion, turbidity (caused by inorganic particles) is not a strong fouling component. Inorganic particles do not result in a large flux decrease and are easily removable from the membrane by flushing. However, elevated concentrations of inorganic particles in natural waters are often accompanied by high NOM concentrations. Especially humic acids appear to adsorb relatively strongly to the membrane and lead to membrane fouling that is not hydraulically reversible. Furthermore, it was shown that biopolymers (mainly polysaccharides) are also an important fouling component. Similar amounts of alginate resulted even in a larger flux decline than humic acids, especially in the presence of Calcium. In contrary to humic acids however, the major part of alginate fouling could be reversed hydraulically. The combination of humic acids, polysaccharides and Calcium leads to the most detrimental form of membrane fouling, both with respect to flux decline and to reversibility.