Biofouling of ultra- and nanofiltration membranes for drinking water treatment characterized by fluorescence in situ hybridization (FISH)

Fouling layer formation in membrane processes and the role of bacteria in it were investigated using rRNA-targeted fluorescently labelled oligonucleotides. In a laboratory-scale flat-channel test unit, nano- and ultrafiltration of an oligotrophic reservoir water were performed. The bacterial composition of the primary fouling layer showed a dominance of the (-subclass of proteobacteria. The mature fouling layer was dominated by bacteria of the "- and $-subclass, which was similar to the population structure of the raw water. The results indicate that in particular (-proteobacteria have attachment mechanisms to form the primary biofilm, which is then colonized by other bacteria. It could be shown that bacterial activity was high enough to obtain good fluorescence signals, though staining of active cells by a tetrazolium salt failed. Inorganic precipitation played a minor role in the fouling layer formation, while natural organic matter made up its main part. Scanning electron microscopy revealed both layer and pore fouling for the ultrafiltration membrane due to organic deposition.

This work showed that fluorescence in situ hybridization is a suitable method for the inves- tigation of biofouling in membrane processes for drinking water treatment. Similar attachment behaviour for the different subclasses of the proteobacteria was found as already observed in drinking water biofilms. It was shown that the (-subclass of proteobacteria established the primary biofilm on both UF and NF membranes. No influence of the membrane surface properties was found. The bacterial composition of the mature fouling layer was similar to the composition in the raw water, indicating that growth of bacteria played a minor role in the biofouling process. Further investigations should aim at a more detailed population analysis with regard to first attaching bacteria, different pre-treatment processes, the role of pathogens in the fouling layer and the behaviour of different bacterial species during backwashing or disinfection procedures. The post-treatment of potentially contaminated backwashing water is also of high interest.