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As part of the EU-sponsored MACOBS (Monitoring and Control of Bulking Sludge through Molecular Probe Assays) project, the size and the composition of the filamentous population in 73 industrial activated sludge plants, were investigated. The inventory included plants in Denmark, Germany, Italy and The Netherlands. Large filamentous populations were present in about 60% of the plants, which means that bulking of activated sludge, which is no longer a serious problem in modern domestic nutrient removal plants, still occurs frequently in industrial treatment plants. The filamentous micro-organisms present were identified/described by "traditional" methods, based on their morphological features and several staining techniques, and by the application of molecular probes (FISH: Fluorescent In Situ Hybridisation). Nine species specific probes were available when the project started, nine new probes have been developed during the project. Fifty nine different filamentous species were observed in the samples, including ca. 40 species that have not been described so far. The unknown species are denoted as Type Ind-1, 2, 3, etc. Probes are now available for the identification of about 17 species. Further research aimed at developing additional probes will be necessary in order to establish which species distinguished provisionally are identical, viz. morphotypes of the same filamentous organism.

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The diversity of filamentous bacteria present in industrial wastewater treatment plants was analysed by a combination of classical and molecular-biological approaches. Many unknown filamentous bacteria were observed in about 80 screened activated sludge samples from different industries with sometimes severe bulking sludge problems. A special focus was paid to filaments which resembled "Nostocoida limicola", a filamentous bacterium which was found to be present in many WWTPs. These filamentous bacteria are hardly cultivable and only one strain was obtained and maintained in co-culture with a yeast. The 16S rRNA sequences of several other "Nostocoida limicola"-like filamentous bacteria from different sludge samples were obtained by micromanipulation and different molecular-biological methods. The sequences were phylogenetically analyzed and specific molecular probes were developed and applied. The results clearly demonstrate that "Nostocoida limicola"-like filaments from industrial WWTPs are different from all other "Nostocoida limicola" types investigated so far. Our strains are affiliated to the alpha-subclass of Proteobacteria.

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