Some Factors Influencing the Distribution of Certain Protozoa in Biological Filters

1. The purity of a medium, as measured by the amount of reducing material present in the solution, and the food supply, are two of the principal factors influencing the distribution of Protozoa in sewage filters. 2. The Protozoa considered occur throughout a wide range of pH values, but the optima for different species are different. 3. Where chemical compounds added to the solution affect the protozoan population adversely, it may be due either to the formation of deleterious oxidation products, or to the development of a bacterial flora which is inimical to the Protozoa.