In situ hybridization (FISH) of two fluorescent oligonucleotide probes, TNI and 21N, directed against 16S rRNA fragments of Thiothrix nivea and type 021N filamentous bacteria, was used to study activated sludge samples of various origins. Comparison of the results obtained by this technique with morphological data enabled us to evaluate the sensitivity and specificity of the probes, as well as to propose practical classification criteria, in an effort to correlate the identification and ecology of these two microorganisms. We applied the two oligonucleotide probes to the study of Thiothrix sp. in pilot and laboratory batch reactors fed a substrate that was moderately deficient in rapidly available phosphorus. We monitored the growth dynamics of Thiothrix sp. in the pilot reactor, using various hydraulic configurations and after certain perturbations, such as transient substrate overloads. The results provided data which led us to question the importance of the length of time oxygen-deficient sludges spend in clarifier tanks and we discuss the effects of both moderate reductions in oxygen and of very intermittent feedings on the occurrence of Thiothrix sp. growth peaks. The identification and quantification of these filamentous bacteria by size class, made possible by in situ hybridization, are shown to be powerful tools for the early detection and evaluation of episodes of proliferation of filamentous bacteria, revealing them to be much more sensitive indicators than the sludge volume index (SVI).

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