REMOVAL OF AOX FROM BLEACH PLANT EFFLUENTS BY ALKALINE-HYDROLYSIS

Removal of organic chlorine from bleach plant effluents using alkaline hydrolysis was studied with the objective of applying this technique for AOX control at bleached kraft mills. Simple mixing of chlorination and caustic extraction (hardwood) effluents produced AOX reduction of up to 25%. Adjustment of pH of softwood chlorination effluents to 11.0 with NaOH and Ca(OH)2 resulted in AOX reduction of 54% and 67%, respectively. When using weak black liquor as neutralizing agent alone in the 4-11 final pH range, it gave AOX removals of 30-45%. The AOX removal could also be increased substantially (up to 80%) when the treatment was performed at increased temperature (150-degrees-C) and pressure (475 kPa) with NaOH. Under these conditions, the AOX appeared to be removed by conversion to inorganic chloride.