Protozoa in biological sewage-treatment processes—I. A survey of the protozoan fauna of British percolating filters and activated-sludge plants

Abstract

Samples of effluent from 52 percolating filters and of mixed liquor from 56 activated-sludge plants were examined microscopically for the presence of protozoa. Ciliated protozoa were generally the dominant protozoa present, but flagellated protozoa and amoebae (both naked and thecate forms) were also sometimes observed. In all, 53 and 67 species of ciliate were identified in percolating filters and activated-sludge plants respectively. A complete species list of these organisms is given, together with information concerning their frequency and relative abundance. All percolating-filter samples contained ciliated protozoa, but 3 activated-sludge samples did not. Activated-sludge plants which delivered effluents of high quality contained a wide variety of ciliated protozoa in large numbers. Plants which delivered turbid low-quality effluents either did not contain ciliated protozoa or contained only a few species in small numbers. No real evidence was found to suggest that any particular component of industrial effluents had, by itself, any harmful effects upon the protozoan populations of activated-sludge plants.