ASSESSING THE ENVIRONMENTAL-IMPACT OF WASTEWATERS

The conventional approach for assessing the environmental impact of wa stewaters uses a set of global physicochemical and ecotoxicological pa rameters and is well adapted to the vast majority of wastewaters. When some chemicals may be particularly harmful for the environment, a spe cific approach based on a comparison between the predicted environment al concentration (PEC) and the predicted no-effect concentration (PNEC ) may be used. The four steps of extrapolation required for PNEC evalu ation are discussed and the importance of the interspecies extrapolati on is highlighted. It may also be useful to use an integrated approach relating the characteristics of the wastewater to that of specific co mpounds. For physicochemical parameters, a simple addition is adequate , whereas, for ecotoxicity, the problem is more complex. The toxicity of a mixture of compounds acting by the same mechanism is often descri bed by the concentration addition model. Although this model is very u seful for practical applications owing to its simplicity, a statistica l evaluation of its performance indicates that it slightly overpredict s the toxicity of mixtures. A new model derived from the statistically sound ''independence action'' principle and based on a precise mathem atical description of the dose-response relationship is proposed. Appl ications of this model to mixtures tested with algae demonstrate the a ccuracy of this model with the experimental data. (C) 1998 Academic Pr ess.