This Test Guideline describes two simulations tests used in the aerobic sewage treatment.Activated Sludge Units are designed to determine the elimination and the primary and/or ultimate biodegradation of water-soluble organic compounds by aerobic micro-organisms in a continuously operated test system simulating the activated sludge process. Two continuously operated test units are run in parallel under identical conditions. Normally the mean hydraulic retention time is 6 h and the mean sludge age is 6 to 10 days. Sludge is wasted by one of two methods, the test substance is normally added at a concentration of between 10 mg/l dissolved organic carbon (DOC) and 20 mg/l DOC, to the influent of only one of the units. The second unit is used as a control. The DOC, preferably, or chemical oxygen demand (COD) is determined, together with the concentration of the test substance by specific analysis, in the effluent from the unit receiving the test substance.In Biofilms, synthetic or domestic sewage, and the test substance, in admixture or alone, are applied to the internal surface of a slowly rotating inclined tube. A layer of microorganisms is built up on the internal surface. Effluent from the tube is collected and either settled and/or filtered before analysis for DOC and/or the test substance by a specific method. Control units are operated in parallel under the same conditions.The difference between the concentrations of DOC/COD in the effluent from the test and control units is assumed to be due to the test substance. This difference is compared with the concentration of the added test substance to calculate the elimination of the test substance. Biodegradation may normally be distinguished from bio-adsorption by careful examination of the elimination-time curve.