AOX reduction of kraft bleach plant effluent by chemical pretreatment—Pilot-scale trials

Laboratory tests indicate that the adsorbable organic halogen (AOX) in bleached kraft pulp mill effluent can be reduced by about 30% by an alkaline sulphide chemical treatment. Pilot-scale trials, consisting of chemical pretreatment of effluent with green liquor followed by air activated sludge biological treatment, were conducted to evaluate this chemical treatment process. The chemical pretreatment reduced the AOX in the acid effluent by 50%, thus reducing the AOX in the influent to the biological treatment plant by 30%, in agreement with the laboratory tests. The performance of the biological treatment was not adversely affected by the chemical pretreatment in terms of COD, BOD and AOX removal. The percent AOX reduction by combined chemical and biological treatment ranged from 53 to 59% depending on the operating conditions of the biological treatment plant. The chemical pretreatment significantly enhanced the overall treatment versus biological treatment alone.