In Denmark, as well as in a few other countries, the regulatory practice for granting permits for discharge of industrial waste water is based on evaluation of the potential effects of both the individual chemical substances in the waste water and the complex waste water itself by establishing “predicted-no-effect-concentrations” (PNECs). The present study was initiated in order to establish an extrapolation method for deriving PNECs for complex mixtures. The toxicity of 5 complex mixtures comprising from 5 to 20 different substances was tested with 5 different species/test systems: the green alga Rhaphidocellis subcapitata, daphnia (Daphnia magna), zebra fish (Brachydanio rerio), duckweed (Lemna minor) and Microtox (Vibrio fischeri). The results of the tests showed that the differences in sensitivity of the test species were considerably lower (5-12 times) than was the case for the individual substances present in the complex mixtures. Thus, based on the differences in sensitivity of the species, application factors for complex mixtures are proposed. Moreover, the results showed that the joint toxicity of the complex mixtures could be described by assuming that the toxicity of the individual substances in the mixtures was more or less additive.

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