Abstract: An improved process for the treatment of aqueous effluents containing strong anions such as sulphate, nitrate, chlorides and fluorides wherein the aqueous effluent is subjected to a biological purification process is described. According to the process, aqueous effluent such as those obtained by condensation of gases present in a coal gasification plant and containing these anions and others in a combined concentration of at least 2 mval/l and organic contaminants in a concentration of at least 1000 mg/l are subjected to the biological purification followed by separation of water from solids of the effluent. Thereafter, the water is subjected to anion exchange in an anion exchanger against hydrogen carbonate ions and at least part of the so-treated water is recycled to the aqueous effluent so as to be present therewith during the biological purification. By such a process the biological purification can be conducted at an optimum pH such as a pH in the range of 6 to 8. CO.sub.3 /HCO.sub.

Abstract: 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).