This part of ISO 14592 specifies a method for evaluating the biodegradability of organic test compounds by aerobic microorganisms in natural waters by means of a continuous flow river model with attached biomass. This part of ISO 14592 is applicable to organic test compounds present in lower concentrations than those of natural carbon substrates also present in the system. Under these conditions, the test compounds serve as a secondary substrate and the kinetics for biodegradation would be expected to be first order (“non-growth” kinetics). This part of ISO 14592 is applicable to organic test compounds, which under the conditions of the test and at the chosen test concentration, are:  water soluble;  quantitatively detectable with appropriate analytical methods or available in radiolabelled form;  non-volatile from aqueous solution (e.g. Henry's law constant < 1 Pa⋅m3/mole);  not significantly adsorbed;  not photolyzed;  not inhibitory to the microorganisms of the test system. The test is not recommended for use as proof of ultimate biodegradability (mineralization) which is better assessed using other standardized tests (see ISO/TR 15462).