Aquatic toxicity of point-source discharges are examined with the MICROTOX system using luminescent bacteria, Photobacterium phosphoreum. Samples include treated wastewaters from industrial dischargers, municipal sewage treatment plants and community sewage treatment plants, all of which eventually flow into Tokyo Bay. Among 34 samples from industry, 11.7% exhibit high toxicity (EC50 =< 0.2), 17.6% exhibit low toxicity (0.2 < EC50 =< 1), and 70.7% were non- or only slightly toxic (1 < EC50). Aquatic toxicity of treated wastewater is mostly due to the chlorination process. Results suggest that 96% of the total toxicity load of all point-source discharges is caused by chlorination. The toxicity assessment with luminescent bacteria is shown to be effective and of practical use, since it facilitates the evaluation of the total toxicity which cannot be well embodied in the current regulation systems by specific indices.

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