The project aims on precise investigation of the residual organic load, remaining in the paper mills effluents despite of downstream biological treatment steps. Furthermore a new method for determining of the residual organic load is scheduled to be elaborated that allows a practical relevant determination and typical characterisation of treatment plant performance. This method is based on standardized test procedures for determining the biological degradation of organic singles substances. Background / Problem area: The paper making process is associated with intensive water use, since water represents one of its essential production materials. Amount and quality of this resource are basic factors determining potential production capacities and product qualities. However, the ecologically favourable reduction in fresh water consumption achieved in previous years caused a rise in process- and waste water concentrations. The residual organic load is of increasing importance to the further optimisation of water systems, which also includes the integration of biological treatment methods. Until now there has been no specific method which ensures the precise determination of residual loads (residual COD) in papermaking process water and effluents. Objective / Research results: The aim of the project was to develop a method to determine the refractory organic load remaining in papermaking effluents after their biological treatment. This should allow a practice-oriented determination of effluent residual loads and representatively describe the plant operation. The Zahn-Wellens test (static test, organic single component test method) and SCAS test (semi-continuous test) were selected as methodical platforms to determine the aerobic biodegradability. The Zahn-Wellens test is easy to perform and has a shorter duration, while the SCAS test permits the adaptation of active biomass by the daily exchange of tested effluents and corresponds therefore more to the continuous processes in industrial practice. The investigations were accomplished with real media from effluent treatment plants (inflow, activated sludge) and the determined residual loads were compared with corresponding reference values of the plants. The two test methods yielded comparable results, which is why the easier static test principle was used as the basis for method development. Based on the investigation of effluents from 7 treatment plants using different operating principles, varying relevant test parameters, the following optimised test conditions were specified: initial COD concentration: 1000-2000 mg/l; dilution: smaller than 1.5; activated sludge concentration: 1-2 gTS/l; pH: 6.5-7.5; temperature: 20-25 degrees. The test has to be carried out as a repeat determination. In addition a blank value and a reference value are planned for control purposes. The computation of organic residual load, however, will be carried out without blank corrections.