Effects of Washer Efficiency on Characteristics of Effluents from D(EPO)D Bleaching of a Softwood Kraft Pulp, 1998 Pulping Conference Proceedings

This paper describes the results of a systematic study of AOX, BOD, and COD released by bleaching southern pine kraft pulp in the D0(EPO)D1 sequence at a kappa factor of 0.20 and a D1 stage ClO2 charge of 1.2%. Measurements of all three parameters were made at three different levels of carryover of D0 stage filtrate into the (EPO) stage and three different levels of carryover of (EPO) stage effluent into the D1 stage, and the entire experiment was duplicated.

The combined AOX contributions of the D0 and (EPO) stages amounted to 11.6% of the chlorine atoms applied in the D0 stage, while the contribution of the D1 stage amounted to 2.7% of the chlorine atoms applied in the D1 stage. The combined AOX contributions of all three stages decreased when the D0 stage carryover was increased from 0 to 10% and was unaffected by (EPO) stage carryover. With perfect washing between stages, COD contributions to the combined bleach plant filtrates by the D0, (EPO) and D1 stages were, respectively, 14.1, 39.8, and 2.9 kg/odumt. The combined COD contributions of all three stages was relatively unaffected by a low level (5%) of D0 stage carryover, but a higher level (10%) resulted in an overall COD increase of 2 kg/odumt. Increasing (EPO) stage carryover from zero to 30% decreased total COD by 3 kg/t. With perfect washing between stages, BOD contributions to the combined bleach plant filtrates by the D0, (EPO), and D1 stages were, respectively, 3.8, 7.0, and 2.2 kg/odumt. No significant effect of carryover levels on the total BOD release could be detected.