Flow-through laboratory bioassays were conducted with a domestic secondary sewage effluent that had been disinfected by chlorination, by chlorination followed by dechlorination, and by ozonation. Effluent without disinfection served as a control. Disinfection with chlorine and ozone generally maintained the effluent at total coliform levels of less than 1,000 per 100 ml. Lake Superior water served as the diluent source for the experiments. Short-term exposures were conducted with 13 species (seven fish and six invertebrates), and long-term (generation) tests were performed with three species (one fish and two invertebrates). In both series of tests the chlorinated effluent was lethal at appreciably lower concentrations than any of the other three effluent treatments. Fish were more sensitive than the invertebrates to the chlorinated effluent in 7-day tests. Residual ozone rapidly decreased in the treated effluent and was not measureable in the test tanks. When special short-term test procedures and shorter retention times for the ozonated effluent were used, measured residual ozone was about as lethal to fathead minnows as residual chlorine. The highest mean total residual chlorine concentrations having no long-term adverse effect on fathead minnows, amphipods, and Daphnia were 14, 12, and 2-4 micrograms/l, respectively. No daphnids survived at approximately 10 micrograms/l mean total residual chlorine, a concentration that corresponds to a chlorinated sewage concentration of about 2.5%.

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