The Role of Protozoa in the Activated-Sludge Process

Ciliates are the dominant protozoa in activated sludge, andthose which either attach themselves to or crawl over the surfaceof the sludge are the most common. Experimental work has demonstratedthat ciliated protozoa are essential for the production of agood quality effluent. Without ciliates, highly turbid, lowquality effluents are obtained. Experimental evidence furthersuggests that the predatory activities of the ciliates uponthe dispersed growths of bacteria are responsible for clarificationand for the reduction of coliform numbers during the activated-sludgeprocess. Mathematical models have been devised which take intoaccount both the bacterial and the protozoan populations. Thepopulations have been simulated on a computer and the resultsobtained in this way have helped to explain many of the observationsthat have been made upon the microbial populations found infull-scale activated-sludge plants.