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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 are toxic to most microorganisms at certain concentrations. These toxicants are common pollutants of sewage, particularly where there is industrial waste input. The protozoa community is a complex assemblage of interacting organisms, often including species that are sensitive, resistant or intermediate in their tolerance to pollutants. Many studies conducted on contaminated activated sludge and mixed liquor have revealed changes in the dynamics of the protozoa community. Tests on the acute toxicity of pollutants on ciliates have revealed that these microorganisms are useful bioindicators for evaluating the toxicity of waters polluted by different concentrations of metals.

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