A method of purifying sewage is described. The method relates to purifying sewage in sewage systems which comprise a toilet, by adding a precipitant for precipitation of phosphates and for binding organic substances. The method is characterized in that the precipitant is selected from the group consisting of iron salts and aluminum salts, such as sulphates, chlorides and nitrates and is adapted to be flushed with water in the toilet bowl when flushing. In the method, the sewage from the toilet is preferably conducted to a sludge separator through an outlet pipe which leads to the lower part of the sludge separator below its outlet.