This study concerns the effects on microbial populations and treatment plant performance on the presence of Chromium and surfactants in incoming wastewater. The study considers two actual treatment plants in a northern Italian City, one treating mostly urban sewage, the second a mixture of urban and industrial wastewater. A comparative assessment of the two plants was performed in laboratory conditions, in order to understand the effects of these exogenous contaminants on the biological processes, and possibly define management and operational criteria for process recovery in the case of renewed actual contamination. Pilot processes of the two plants were run at first, considering the actual design and normal influent conditions of the facilities, then, under stress conditions, by adding the external contaminants at known concentrations to the feed, performing process functionality observations and toxicity tests, and determining the qualitative and quantitative characteristics of the activated sludge biomass. Considerations on processes performance and biocenosis modifications due to pollutants contents are drawn. A discussion on possible effects mitigation strategies is included.

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