A method treats process water containing metal ions, in connection with bleaching of lignocellulosic pulp. The method is characterized by the steps of a) adjusting the Mg2+ content of the process water to form a flocculating base for metals and an organic substance; b) supplying a carbonate source to the process water to precipiate calcium as calcium carbonate; c) increasing the pH of the process water by adding white liquor to precipipate metals such as metal hydroxides; d) adding a flocculating agent to the process water; and e) separating the precipitated, flocculated metal compounds from the process water by flotation.