Indicator organisms provide information about the quality of
activated sludge from biological wastewater treatment plants and can indicate both negative and positive conditions for the sludge. As a result, indicator organisms have a very narrow tolerance spectrum and only occur under the specific, i.e. unambiguous and thus assignable, conditions that are ideal for them.
A typical example of indicator organisms for a high sludge load and high
BOD inflow values are ciliate animals of the genus
Opercularia, which often occur together with
Vorticella microstoma.
Particularly clear indicator organisms for unfavorable activated sludge conditions and the resulting poor sludge properties are ciliate animals of the genus
Plagiopyla, Metopus and
Caenomorpha, as they are easy to identify under the microscope due to their characteristic morphology.
On the other hand, identifying good activated sludge conditions using indicator organisms is much more difficult. But here, too, there are organisms whose presence and frequency indicate good drainage quality such as
Zoothamnium sp., Thuricola kellicotina, Vorticella campanula. « back 
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