Removal of COD and color loads in bleached kraft pulp effluents using ozone

The efficiency of ozone in removing chemical oxygen demand (COD) and color loads in the effluents of a kraft pulp bleach plant was kinetically investigated. Effluents from the following bleaching stages were tested: 1) the first acidic and second alkaline stages of a chlorine-bleached hardwood kraft pulp line, 2) the sulfuric-acid treatment (A) as well as the first acidic and second alkaline stages in a chlorine-dioxide-based elemental-chlorinefree (ECF) bleached hardwood kraft pulp line, and 3) the first acidic and second alkaline stages in a chlorine-dioxidebased ECF bleached softwood kraft pulp line. To obtain the highest ozonation efficiency, the effluents of the A-stage and the first acid stages of the chlorine and ECF bleaching processes for hardwood kraft pulps should be treated with ozone in their original form. However, the alkaline effluents of these bleaching processes should be neutralized before ozonation. Compared to the hardwood kraft pulp effluents, ozone does not effectively remove COD and color loads from either acidic or alkaline effluents, with or without neutralization, of the ECF bleaching process on softwood kraft pulp. It was also found that, irrespective of wood species (hardwood or softwood) or effluent type (acidic or alkaline), ozonation enhances COD removal in subsequent biological treatment.