Environmental Fate and Distribution of Chemicals Produced During Bleaching
There have been several studies which address fate and distribution of chemicals produced during bleaching, especially for the relatively well characterized dioxins and fuans (Muir et al., 1992; Buser et al., 1989). Allard et al. (1988); Carey et al. (1993); Neilsen et al. (1991); Passivirta et al. (1985) and others have investigated the fate of chlorophenolic substances. Swanson et al. (1993a) have summarized the results of a comprehensive study of the Grande Prairie pulp mill on the Wapiti/Smoky River system.
For persistent hydrophobic substances such as the dioxins, there is a good appreciation of transport and transformation and a fair capability of linking loadings to concentrations. Environmental degradation is very slow and, in some cases, half lives can be measured in years. Sediments are a repository of past releases and may continue to release contaminant back into the ecosystem long after loadings are reduced or eliminated. For highly substituted chlorophenols, biodegradation and biotransformation are key processes and it appears that these substances have half lives from days to months. They do bioconcentrate but to a lesser extent than the dioxins, presumably because of their lower Kow values and greater susceptibility to metabolism and/or excretion. High and low molecular mass AOX, which comprise the bulk of the organochlorines discharged, are hydrophilic. The dominant transport process for these compounds appears to be simple advective transport and attachment to solid surfaces (as occurs in the AOX test) with subsequent aerobic and anaerobic degradation. It is likely that, because of high molecular mass, they do not bioconcentrate and any toxic effects will be restricted to physical interference, such as with respiratory surfaces.
Current practice in North America requires treatment of bleached kraft mill effluent by processes such as activated sludge and aerated stabilization basins or lagoons. Several reports are available on plant performance and on laboratory-scale systems with removal efficiencies being determined for AOX, chlorophenols, chloroform, and hydrophobic compounds such as the dioxins, as well as toxicity reduction (Wilson et al., 1992). Hydrophobic organics partition strongly to biomass and are removed by sedimentation with possible subsequent mineralization by aerobic or anaerobic processes. The unmineralized portion may be retained in sludges and their ultimate fate is determined by sludge management practices. Chlorophenols are substantially biodegraded in aerated lagoons, and typical removals in activated sludge range from 50% to 90% and 80% to 90% in anaerobic systems. There are differences between phenols, guaiacols, catechols and vanillins and with chlorine number (i.e., di-, tri-) with lower chlorine number compounds being more readily degraded. AOX is typically removed with about 50% efficiency, probably by settling and subsequent mineralization/degradation in the sludge (Wilson et al., 1992).