A quantitative study of the population structure of activated sludge is an important
component of biological wastewater treatment control. However, in the studying of live samples
of the activated sludge, some complications arise, in particular, associated with the relatively
short time of the subsample suitability. A subsample is the part of the sample that is placed on a
glass slide and in which organisms are counted. The issue of optimization of counts of organisms
with large amplitude of population density is considered. The results of counting ciliated
protozoa in activated sludge were described. The samples were counted in 45 sub-samples of 25
µl each. An average of 10 counts was required to achieve high reliability in determining
population densities with more than or equal to 1 specimen per 25 µl in sub-samples. For small
population densities (less one specimen per 25 µl) of free-swimming, crawling, and sessile
ciliates, 30 counts are necessary. When the density of colonial protozoan populations is
established, the number of counts should be increased to 40, especially when colonies with
significant differences in the number of zooids are found
