The Wupperverband operates 14 sewage treatment plants in the catchment area of the Wupper river. New treatment plants, which were designed for extended nutrient removal, have been equipped with on-line analysers. To find the lay-out data, to control operation and as a support for the start-up phases of new treatment plants, a mobile container for on-line analysis of COD, BOD, NH{in4} and NO{in3} is in use. Several types of analysers were tested with different types of sewage and at different sampling points (influent, effluent primary sedimentation, effluent denitrification, effluent nitrification and final effluent). This enables us to compare and to assess the tested machines. As a result of these comparisons and for operational reasons the photometric principle was chosen for further application. Adequate maintenance with trained staff provided, especially of sampling preparation units, all tested photometric analysers could be used for control and automation purposes because of their reliable measuring values.