In many drinking water treatment facilities with an activated carbon (AC) purification step, polar and hydrophilic molecules are substances of concern. Many of the current test methods for AC do not really reflect AC's adsorption capacity for these substances. To develop a fast and economically reasonable parameter that characterizes AC's ability to remove these compounds, three substances, namely diclofenac, nitrobenzene, and diatrizoate were investigated as test substances. A test set of different AC samples was created by reactivating an exhausted AC in three different ways. Additional AC samples were obtained by sampling different depths of a full-scale AC filter at regular intervals during the operation time of the filter. Several adsorption tests were performed with the three test substances, including adsorption isotherms in the concentration range mg/L, single-point adsorption tests in the concentration range μg/L, and pilot-scale fixed bed filter experiments. The results suggest that diatrizoate allows for a sensitive comparison of different AC samples. A characteristic figure, the ‘diatrizoate number’ was developed. This diatrizoate number allows for a sensitive, cheap, and quick comparison of different ACs. A similar survey with special focus on problematic substances present on site is recommended for other water treatment facilities.