Effects of cadmium on the performance and microbiology of laboratory-scale lagoons treating domestic sewage

Two experiments were performed to assess the impact of cadmium on the sewage lagoon wastewater treatment process. For each one, three laboratory-scale pilot plants with one tank receiving the same raw effluent were used; one plant served as control and the other two were contaminated once only with cadmium. In the first study, the effects of a shock load of two concentrations of cadmium chloride (60 and 300 micrograms/l) on the plant performance, microbial populations (protists and bacteria) and enzyme activities were determined. Initially, most of the performance parameters were affected concentration-dependently. A reduction in the protist population density and some influence on the total bacterial population were observed, and the potential enzymatic activities were also modified. A second experiment with a lower cadmium concentration (30 micrograms/l), supplied as chloride or sulphate, still perturbed most of the parameters studied, and the effects of the two cadmium salts were identical.