Method of purifying waste water

In a method of purifying waste water containing volatile and stable ammonium compounds and phenolic compounds in a stripping column comprising an upper part and a lower part, each part including a plurality of substantially horizontal plates disposed above one another, at least some of said plates being perforated plates, said method comprising supplying initially basic waste water to be purified to the top of said upper part, supplying stripping vapor to the bottom of said lower part and supplying a strong base laterally to the column at at least one level, the strong base reacting with the stable ammonium compounds to form ammonia vapors, the improvement comprising additionally supplying stripping vapor to the bottom of said upper part, controlling the flow of waste water downwardly from the top of the bottom of said upper part such that in a region at the bottom of said upper part the waste water becomes acidic without addition of an acid to the stripping column, supplying the strong base to a first zone which is at least one level lower than the acidic region, preventing the ammonia vapors formed in said first zone from contacting the waste water in the acidic region by conveying the ammonia vapors directly to a second zone located in said upper part in which the waste water is still basic and presents a content of volatile ammonia in equilibrium with the ammonia content of the ammonia vapors resulting from the stripping of the stable ammonium compounds located in said first zone.