Treatability of kraft pulp bleaching wastewater by biochemical and photocatalytic oxidation

In this study the biological treatability and TiO2 photocatalyzed oxidation characteristics of sulfate pulp bleaching effluents were investigated. The original wastes had a low biodegradability as determined by BOD5/COD ratios. Biological treatment was conducted in a batch activated sludge reactor. The nonbiodegradable fraction amounted to 60 % of the initial COD and compounds specific to these wastes were not removed during biotreatment. In order to enhance the biodegradability of these wastes, mixed raw effluent, C/E-H stage effluent, D/E-D stage effluent and biologically pretreated wastes were subjected to TiO2 photocatalyzed oxidation. Photocatalytic oxidation led to an increase in the BOD5/COD ratio of DIE-H stage raw and biologically mated wastewater, while chloride formation was observed in both cases in a five-hour reaction period in the presence of 1 g l(-1) TiO2 and 15x10(-3) M H2O2 at pH=6.5, The specific absorption values (A(272nm)/COD5, A(254nm)/COD5, A(346nm)/COD5, A(436nm)/COD5) exhibited parallel decreases during photocatalytic oxidation which indicate that oxidation reactions were nonspecific with respect to the organics present in these wastes. As a conclusion it can be suggested that biological treatment should be placed before the photocatalytic oxidation method. Even in this scheme, the application of photocatalytic oxidation could only then be favored when the COD and chloride concentrations in the wastewater:were below certain values or when wastewater was diluted. (C) 1999 IAWQ Published by Elsevier Science Ltd. All rights reserved.