AbstractSummary: A method for the estimation of the abundance of protozoa and small metazoa in activated sludge is described. 5 × 10 μl of activated sludge are counted in vivo microscopically using a Thoma‐Counting‐Chamber and a magnification of 100: 1. The efficiency of this counting method was tested by “recovery” experiments. 46–100 of the ciliates (mean 85 ,n= 165 countings) were recovered. A positive correlation between body size and percentage of recovery could be found. Preliminary experiments showed that the method is suitable for nematodes (recovery 95 ,n= 10), rotators (recovery 84 ,n= 10) and testate amoebae (recovery 78 ,n= 10), too. The described method is independent of the sludge‐structure, is easy to handle and is an acceptable compromise between time spent and necessary degre
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