Biological wastewater treatment is a process of increasing importance in a world with an ever-increasing human population. Wastewater treatment facilities are designed to maintain the high density and activity levels of those microorganisms that carry out the various purification processes. Protozoa are one of the most common components in these man-made ecosystems and play an important role in wastewater purification processes. Protozoa are responsible for improving the quality of the effluent, maintaining the density of dispersed bacterial populations by predation. Studies of the relationships between protozoa and physicochemical and operational parameters have revealed that the species structure of these communities is an indicator of plant efficiency. The Sludge Biotic Index (SBI), an index based on the structure and abundance of the microfauna inhabiting the activated sludge and mixed liquor, has been devised to monitor activated-sludge plant performance. Heavy metals and other pollutants ar...

Protozoa in wastewater treatment processes: A minireview

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

The microbial ecology of the activated sludge process

Samples of mixed-liquor from 39 activated-sludge plants and of biological film from 49 rotating discs were examined microscopically for the presence of Ciliated Protozoa. 45 and 47 species of Ciliates were identified in activated-sludge plants and rotating discs, respectively.

The structure of Ciliated Protozoa communities in biological sewage-treatment plants

Seasonal recording of the Protozoa in various sewage disposal units has shown that, together with changes correlated with the degree of purification of the sewage, fluctuations occur as a result of the interplay of seasonal non-specific factors and local specific factors. The incidence and abundance of Protozoa in each disposal plant would seem to be controlled by sewage ‘strength’ and characteristic local drainage. Fluctuations of the population, however, are limited by different factors in different seasons.



In the bacteria bed investigations a ‘weak’ sewage shows a decrease of a generally rich fauna during the periods of sloughing, a ‘strong’ acid sewage during sloughing shows an increase in an otherwise restricted fauna and where the sewage is medium in ‘strength’ and supports an abundant insect population the fluctuations of the Protozoa are dominated by the more active insect populations of summer and by temperature in winter. In this last-named environment a significant inverse correlation between ciliate and insect frequency throughout the summer indicates the thoroughness with which the insect larvae devour the film; this may explain the summer reductions in certain groups of Protozoa.



In activated sludge channels conditions favour a uniform population which may vary in composition according to the purity of the sewage. Flocculation is most pronounced when protozoan populations comprise a restricted variety of ciliates and would seem to depend on a protozoan fauna since cultures of bacteria obtained from sewage did not flocculate unless Protozoa, free from or contaminated with bacteria, were added.



The protozoan populations of the two systems of disposal are compared in discussion.

A. N. Barker

Papers

The protozoan fauna of sewage disposal plants.