The Role of Protozoa in the Activated-Sludge Process

synopsis. Ciliates are the dominant protozoa in activated sludge, and those which either attach themselves to or crawl over the surface of the sludge aore the most com? mon. Experimental work has demonstrated that ciliated protozoa are essential for the production of a good quaility effluent. Without ciliates, highly turbid, low quality effluents are obtained. Experimental evidence further suggests that the predatory activities of the ciliates upon the dispersed growths of bacteria are responsible for clarification and for the reduction of coliform numbers during the activated-sludge process. Mathematical models have been devised which take into account both the bacterial and the protozoan populations. The populations have been simulated on a computer and the iresults obtained in this way have helped to explain many of the observations that have been made upon the microbial populations found in full-scafle activated-sludge plants. Although we have known for many years that aerobic biological sewage-treatment plants contain animal and plant life from many phyla, the roles played by the various individual groups of organisms in the purification processes in many cases are not fully understood. The complete fauna and flora of percolating filters consist of a wide variety of organisms including not only microorganisms, such as bac? teria, fungi, algae, and protozoa, but also large populations of macro-invertebrate animals such as insects, arachnids, worms, crustacea, rotifers, and others. Life in activated-sludge plants, however, is less varied, and bacteria in the form of sludge floc are generally the dominant organisms. Fungi are comparatively rare, but they and the actinomycetes occasionally become dominant. Algae rarely become established, presumably owing to a lack of light. Roti? fers, nematode worms, and more rarely, oligochaete worms and chironomid larvae may be found.