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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