Abstract: A method of treating residual acid from production of chlorine dioxide is described. The method is characterized in that a compound containing iron is added to the residual acid to react therewith and form a product, which contains iron in trivalent form. As iron compound preferably ferrous sulphate is added, which under oxidation reacts with the residual acid to form ferric salt. Preferably sodium chlorate, sodium hypochlorite, hydrogen peroxide or an oxygen-containing gas is added as oxidant.

Abstract: A method of treating residual acid from production of chlorine dioxide is described. The method is characterized in that a compound containing iron is added to the residual acid to react therewith and form a product, which contains iron in trivalent form. As iron compound preferably ferrous sulphate is added, which under oxidation reacts with the residual acid to form ferric salt. Preferably sodium chlorate, sodium hypochlorite, hydrogen peroxide or an oxygen-containing gas is added as oxidant.

Abstract: 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.