In the case of direct dosing of powdered activated carbon (PAC) into the aeration tank for further trace substance elimination, the biological reactor simultaneously serves as a contact space for the adsorptive processes on PAC. Our investigations within the KomPhoS project show that a specific PAC dose of 2.5 mg PAC/mg DOC is required for 80 % elimination of nine selected trace substances. Here, we demonstrated for the first time that in the activated sludge system, especially the sorption of hydrophobic organic compounds is a competitor for the desired trace compound adsorption. The manuscript is available free of charge until 10.12.2022 at the following link: https://authors.elsevier.com/a/1fyIl7taLYPnHq.