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 char acteristics of the D and (EO) stage effluents has been investigated. E ffluents from OD(EO) laboratory bleaching of softwood kraft pub were c haracterized by fractionation and analysis of the fractions. The efflu ents from the sequence with the D stage begun at pH 2 (D-pH 2 bleachin g) 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 kapp a number and higher total organic carbon (TOC) in the effluents than t he D-pH 2 case. D-pH 4 bleaching also gave lower chlorine-to-carbon ra tios in the effluents from both eht D and (EO) stages. The ether-extra ctable fraction of the D-pH 4 effluent and its phenolic subfraction, w hich contain compounds of environmental interests, exhibited lower chl orine-to-carbon ratios than those from the D-pH 2 effluents. However, since the percentage of total TOC within these fractions was higher, o verall AOX levels were nearly equal. Because of the greater delignific ation at D-pH 4 conditions, environmental and economic benefits may be realized by a reduction of the applied ClO2 charge.