This International Standard specifies a screening test method for the determination of the degree of adsorption of substances on to the activated sludge or primary sludge in a waste water treatment plant. The conditions described in this International Standard normally correspond to the optimum conditions for the adsorption to occur at the chosen activated-sludge concentration and water hardness during the test period. The method applies to substances for which an analytical method with sufficient accuracy is available and which, under the conditions of the test and at the test concentration used, a) are water-soluble; b) or, if only slightly water-soluble, allow sufficiently stable suspensions, dispersions or emulsions to be prepared; c) are not significantly removed from the test solution during the test by known abiotic processes such as stripping or foaming; d) do not deflocculate activated sludge; e) are not readily biodegradable (for a discussion of biodegradability, see ISO/TR 15462). An important parameter that can influence the reliability of the test results is the stability of the test compound during the test. If no information on the stability is available, it is recommended that this be checked before the test. If any transformation (e.g. due to hydrolysis) is observed, it is recommended that the degree of adsorption of the transformation products be determined, if possible. Since biodegradability of the test compound may also lead to an incorrect assessment of the degree of adsorption, it is recommended that the biodegradability be investigated in advance using standard biodegradation tests which are preferably based on oxygen consumption or on carbon dioxide production and in which adsorption has no influence on the test result. If biodegradation cannot be excluded, sterilized sludge may be used (see Clause 7). There is generally no need to carry out adsorption tests on substances which are readily biodegradable as they are sufficiently removed biologically in waste water treatment plants. Substances which are easily adsorbed on activated sludge in waste water treatment plants are preferably removed by adsorbing them in sludge digesters and degrading them anaerobically. For such substances, high adsorption may be a reason for carrying out anaerobic biodegradation tests. An overview of standardized biodegradation tests is given in ISO/TR 15462. The test compound concentrations used in this method are usually very low and therefore no negative effects are to be expected on the capacity of the activated sludge to adsorb even toxic test compounds. When there is any doubt, it is recommended that microscopic investigations of the flocs and suitable toxicity tests such as that specified in ISO 8192 be carried out.