Activated sludge samples from wastewater treatment plants from potato starch and starch derivatives factory and from a municipal sewage treatment plant were analyzed with DNA probes specific for several filamentous bacteria. It was found that Haliscomenobacter hydrossis, Sphaerotilus natans, Thiothrix sp. and Eikelboom Type 021N were common in the activated sludges. Fluorescent in situ Hybridization (FISH) analysis could detect more types of sheathed bacteria and yielded a more accurate quantification of bacteria than conventional microscopy.

In a pilot and a full scale wastewater treatment plant (WWTP) clear correlations were found between the SVI and growth of a Sphaerotilus natans and a Thiothrix sp. Addition of chlorine to the bulking sludge resulted in an improved SVI of the sludge but only damaged filamentous cells outside the floc. Nitrification was measured with substrate depletion and FISH analysis. Signal interpretation of FISH analysis was demonstrated both manually and with automated image analysis.

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