Changes to environmental legislation are setting new challenges to both operators and regulators. Optimisation tools can help to turn subjective standards such as Best Practicable Environmental Option (BPEO) (HMIP, 1994 : Sharratt, 1995) into more objective ones. Mixed Integer Non Linear Programming (MINLP) is used in this work to optimise overall environmental performance for a group of plants. These plants discharge their final effluents to the same receiving body - a river. The optimisation considers not only the possible configurations within each plant but also the behaviour of the receiving body. A two-stage optimisation is used. Examples illustrate the features of the model.