A method for treating a contaminated liquid in a reaction vessel provided with at least two reaction spaces situated one above the other, wherein the liquid for treating is carried through the reaction vessel from the bottom to the top, as well as through a filter material arranged in at least a higher located space, wherein one or more reagents is added to the liquid flow at least in a reaction space under the filter material, whereby it is possible in simple manner to filter out reaction products or other contamination products. It is also possible to capture gaseous reaction products, this such that the liquid is cleaned efficiently.