This Test Guideline describes a method to assess effects of a test chemical on the
phagocytotic activity of activated sludge containing protozoan organisms under defined
conditions in the presence of different concentrations of the test chemical. The principle
of biological sewage-treatment plants (STP) is to transform the organic matter of
incoming waste-water in microbial biomass, which in turn is separated from the liquid
yielding a purified effluent. The purpose of the test is to provide a means to record
effects of test chemicals on ciliated protozoa in sewage treatment plants, which due
to their grazing on bacteria considerably contribute to the functioning of STPs.