ABSTRACT
A large number of chemicals are formed in the course of converting wood to bleached pulp. Given the complexity and diversity of modern kraft mills and the transition state of the industry in response to environmental concerns, it is impossible to completely predict the chemical composition of effluents. This review uses selected examples of what has been learned about the formation of chemicals in kraft pulping and bleaching to illustrate factors to be considered in understanding sources of chemicals and in identifying new chemicals potentially related to emerging environmental concerns. The basic characteristics of wood, including extractives, lignin and polysaccarides, are the starting materials for most compounds formed in the process. Kraft pulping removes substantial portions of lignin, forming a wide array of by-products. Polysaccarides and extractives are also modified by the process. Basic differences in the chemistry of chlorine and chlorine dioxide result in significant differences in the types of by-products formed. Although many compounds are formed in the pulping and bleaching process, not all end up in the effluent. The chemical composition of the major discharges to the sewers (including debarking, black liquor, condensates, causticizing bleach plant wastewaters) varies significantly. Biological treatment further modifies the composition. Although removal is generally determined by comparing influent and effluent concentrations, it does not account for potential transformations. One example is the transformation of resin acids to either oxidized forms or to aromatized structures such as retene, depending on the conditions in the treatment system.
