Abstract: A method for eliminating detrimental substances in a process liquid. The process liquid has a first pH and, in addition to said detrimental substances, comprises metal ions and suspended material. Carbon dioxide is added to the process liquid in order to bring the process liquid to a second pH, lower than the first pH. Thereby, the metal ions comprise hydroxide builders, wherein the carbon dioxide (CO2) is added in an amount which causes the second pH to be between 8 and 6.5, so that the carbon dioxide primarily forms bicarbonate ions (HCO3−) having a pH-buffering effect and the metal ions primarily form metal hydroxides. Thereafter, the detrimental substances coagulate or agglomerate with the metal hydroxides and are adhered to the suspended material so that they become inactive. The method is advantageously used in connection with pulp and/or paper making processes.