The effect of D stage pH on effluent quality: Characterization of effluent fractions from OD(EO) bleaching of softwood kraft pulp

The effect of the initial pH of ClO2 delignification stage on the characteristics of the D and (EO) stage effluents has been investigated. Effluents from OD(EO) laboratory bleaching of softwood kraft pub were characterized by fractionation and analysis of the fractions. The effluents from the sequence with the D stage begun at pH 2 (D-pH 2 bleaching) were compared to the same sequence with the D stage begun at pH 4 (D-pH 4 bleaching). D-pH 4 bleaching resulted in a lower extracted kappa number and higher total organic carbon (TOC) in the effluents than the D-pH 2 case. D-pH 4 bleaching also gave lower chlorine-to-carbon ratios in the effluents from both eht D and (EO) stages. The ether-extractable fraction of the D-pH 4 effluent and its phenolic subfraction, which contain compounds of environmental interests, exhibited lower chlorine-to-carbon ratios than those from the D-pH 2 effluents. However, since the percentage of total TOC within these fractions was higher, overall AOX levels were nearly equal. Because of the greater delignification at D-pH 4 conditions, environmental and economic benefits may be realized by a reduction of the applied ClO2 charge.