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This International Standard specifies a method for the determination of the elimination and the biodegradability of organic compounds by aerobic micro-organisms. The conditions described simulate a waste-water treatment plant. Two test systems can be used: activated sludge plants or porous pots. The tests can optionally be performed under conditions of nitrification and denitrification (Annex A) and coupling of the units (Annex B). The method applies to organic compounds which, under the conditions of the test, are a) soluble in tap water at the test concentration and not expected to be transformed to insoluble metabolites if biodegradation, in addition to elimination, is determined; b) poorly water-soluble, but which are satisfactorily dispersible in water and allow detection with suitable analytical means (e.g. organic carbon measurements); c) non-volatile, or which have a negligible vapour pressure under the test conditions; d) not inhibitory to the test micro-organisms at the concentration chosen for the test. Inhibitory effects can be determined by using a suitable test method (e.g. ISO 8192[15] or ISO 15522[27]). Compounds inhibitory at concentrations used in this test may be tested at concentrations less than their EC20 value, followed by higher practical concentrations after a period of acclimatization. The method can also be used to measure the biodegradation and elimination of dissolved organic compounds in waste water (also called “test compound” in the method). If more or different information is required to predict the behaviour of test compounds or waste water in a treatment plant, other degradation tests may be performed. For appropriate use of this method and for alternative biodegradation methods, see ISO/TR 15462 and for general information on biotesting, see ISO 5667-16.