The global bleaching technology trend is a change from chlorine gas bleaching to ECF. This segment for kraft pulp has grown from a share of 5% in 1990 to a share of 50% in 1997. We believe that this trend will continue and the chlorine gas will eventually be totally phased out.The development of the lignin reduction technology of the unbleached pulp like extended cooking and more effective oxygen delignification will continue and technologies for bleach plant effluent closure will be a reality within the nearest years.A comparative laboratory characterization study was carried out on effluents from ECF bleached, conventional and oxygen predelignified, softwood and hardwood pulps. Combined and separate streams were characterized and selected effluents were exposed to biological treatment. These studies were carried out both prior to and after biological treatment. The characterization of the main COD components ; lignin, carbohydrates, methanol, low molecular acids and extractives corresponded well with the calculation of the total organic composition.Since approximately 50% to 70% of the total bleaching effluent COD and colour were found in the alkaline filtrates, the concept of recirculationg those filtrates to the recovery system offers a great potential for reducing the discharge of, not only the total COD, but also its specific components, into receiving water bodies.