Changes in kraft bioeffluent quality were evaluated as a function of increased chlorine dioxide (ClO2) substitution, and ultrafiltration (UF) membrane and heterogeneous photocatalysis (HP) treatment of extraction (E-stage) wastewater prior to aerobic biological treatment. An increase from 10% to 50% ClO2 substitution in the bleaching of softwood produced a more readily biodegradable waste and an improved bioeffluent quality, especially with respect to toxicity and absorbable organic chloride (AOX). All UF-HP treatments of E-stage wastewater from 10% ClO2 substitution resulted in significant improvements in aerobic bioeffluent with regard to all measured parameters. The best physical/chemical product never corresponded to the best final bioeffluent, indicating some important modification of pollutant biodegradability during the membrane/photocatalysis process. Enhanced treatment of E-stage wastewater from 50% ClO2 substitution also resulted in significant improvements in aerobic bioeffluent with regard to total organic carbon (TOC), chemical oxygen demand (COD), and color, moderate improvement of toxicity, but no substantial reduction in AOX.

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