Non-target organisms in the environment are exposed simultaneously to a multitude of substances from various sources. In the aquatic environment, effluents of wastewater treatment plants (WWTPs) represent a key source for co-incidental mixtures of chemicals from various uses. The aim of the present project was to explore the consideration of mixtures of chemicals released via WWTPs in an Environmental Risk Assessment. Different organisms were exposed in chronic toxicity tests to single substances and various mixtures of them. The results show that the concept of concentration addition (CA) predicts mixture toxicity in effluents sufficiently and for a prospective risk assessment the use of a mixture assessment factor (MAF) could be helpful.