Abstract: A method of purifying waste water, for example waste water from a coking plant, comprises supplying waste water to be purified to the upper part of a stripping column comprising a plurality of substantially horizontal plates disposed above one another, supplying vapor to the lower part of the column, and supplying a strong base to the column laterally at one or more levels, wherein the water to be stripped is passed downwardly from the top of the column across a number of successive plates sufficient to permit the pH of the water to decrease to a value below 7, and wherein the base is supplied at a level or levels lower than that at which the pH has decreased to the above value.