The sewage consists of gas condensates from coal-gasifying plants and/or coal chemical plants and contains the anions SO.sub.4.sup.--, SCN.sup.-, NO.sub.3.sup.-, Cl.sup.- and F.sup.- in a total of at least 2 mval/l and contains organic matter corresponding to a chemical oxygen demand of at least 1000 mg/l. The sewage is passed through a biological purification stage (8), and a succeeding fine purification stage (13, 17, 18). In an anion exchanger (19), strong anions are exchanged with hydrogen carbonate ions. The water leaving the anion exchange stage has an alkalinity of at least 2 mval/l and is passed at least in part through a cation exchanger (26) before the water is recycled to the sewage (line 5).
The water which has left the anion exchanger (19) may be used as cooling water in a cooling tower (21) before or after the cation exchanger (26). Organic acids are used for regeneration in the cation exchanger (26) and the regeneration eluate is added to the sewage which is to be treated in the biological purification stage (8).