The assessment of Dutch waste water discharges or effluents is focused on the reduction of specific pollutants or substances. Many effluents in the Netherlands are of a complex nature though. Because of several limitations, an assessment of complex effluents is not possible by merely using a chemical specific approach. Examples are lack of analytical techniques, lack of ecotoxicological data and unknown combination processes. Therefore an additional approach is needed. The Whole Effluent Toxicity or WET approach has been introduced to overcome the mentioned limitations. It focuses on acute toxicity. In 1994 a pilot study was initiated in order to try out, adjust and evaluate the WET approach. The aims were to gain more insight in the occurrence of acute toxicity in complex effluents, and to try and prove the additional value of the approach. The study was performed with 17 different industrial effluents, consisting of 6 industrial branches. All effluents were tested with four different organisms. The results showed acute toxicity for at least one organism in 15 out of 17 effluents. In 8 cases the toxicity of the effluents could not be explained by using physical and chemical data. The results clearly show that the use of acute toxicity tests produces valuable, and especially additional, information.

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