The conventional approach for assessing the environmental impact of wastewaters uses a set of global physicochemical and ecotoxicological parameters and is well adapted to the vast majority of wastewaters. When some chemicals may be particularly harmful for the environment, a specific approach based on a comparison between the predicted environmental concentration (PEC) and the predicted no-effect concentration (PNEC) may be used. The four steps of extrapolation required for PNEC evaluation are discussed and the importance of the interspecies extrapolation is highlighted. It may also be useful to use an integrated approach relating the characteristics of the wastewater to that of specific compounds. 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 described by the concentration addition model. Although this model is very useful for practical applications owing to its simplicity, a statistical evaluation of its performance indicates that it slightly over-predicts the toxicity of mixtures. A new model derived from the statistically sound "independence action" principle and based on a precise mathematical description of the dose-response relationship is proposed. Applications of this model to mixtures tested with algae demonstrate the accuracy of this model with the experimental data.