Abstract: The present invention provides a process for the regulation of the activa sludge step of a clarification plant by control of the decomposition efficiency of the activated sludge, wherein an activated sludge sample is incubated under precisely defined conditions, which are optimised with regard to the pH value, the temperature and the salt concentration, with the substrate or substrates of interest above the saturation range and, in the initial phase, the decrease of the substrate or the increase of a decomposition product is determined kinetically as a measure of the decomposition efficiency for this substrate and the measured value determined is used as regulating value for the control of the clarification plant.