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Microfauna, comprising protists and little metazoan, has proved to be a useful tool for assessing the occurrence of pollution in wastewater treatment systems, namely on activated-sludge plants. In the present work, the response of the microfauna communities of activated sludge to three toxicants—copper, zinc, and cycloheximide—was studied by means of a series of assays using a bench-scale plant. Along with the community descriptive parameters such as density, taxonomic richness, and the use of biological indexes—the Shannon-Wiener Index and the Sludge Biotic Index—several operating parameters were determined in order to allow for the comparison and possible correlations between the biological and the physical–chemical parameters. The results emphasize the ability of activated sludge communities, both bacteria and microfauna, to survive and to react to toxicants. High concentrations of copper and zinc (20 and 50 mg/L) prevented the satisfactory plant efficiency and the healthy state of the microfauna, including its survival. Cycloheximide did not have important and lasting effects below 5 mg/L. Among physical–chemical parameters, removal of soluble chemical oxygen demand seems to be the only one that presents patterns revealing a cause–effect relation along toxicological assays. Biological parameters were much more sensitive and coherent. The Sludge Biotic Index was, by far, the best tool in detecting intoxicant effects in the microfauna communities. Considering all parameters studied, copper was more toxic than zinc. Cycloheximide, in the range of concentration tested, was less toxic than these metals. The work highlights the role of microfauna as an indicator of toxicants entrance, besides the well-documented indicator value of plant operation conditions and efficiency.Fundação para a Ciência e a Tecnologia (FCT

Microfauna as Indicator of Copper, Zinc, and Cycloheximide in Activated Sludge Processes