A small number of viable options are presently available for the bleaching of wood pulp (delignification and brightening) during the manufacture of pulp and paper. The use of chlorine dioxide (ClO2) in the bleaching process has largely superceded the earlier use of Cl2, or elemental chlorine, in the majority of North American pulp mills: It was discovered over a decade ago that the previous widespread use of Cl2 contributed to the production and subsequent release to the environment of specific forms of potentially deleterious and highly persistent chlorinated organic contaminants, such as polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans.
The environmental risks and benefits associated with the virtual elimination of Cl2 in pulp and paper manufacture, and substitution during bleaching with ClO2 have been formally examined by many scientists; see for example papers by Axegård et al.(1993), Berry et al.(1991), Carey et al.(1993), Luthe et al.(1992), Munkittrick et al.(1996), Södergren (1989), Solomon et al.(1993), and Tana and Lehtinien (1996). In particular, Solomon et al.(1993) conducted a review and risk assessment of the chemistry and biological impacts associated with a 70% to 100% substitution of elemental chlorine with ClO2 at the request of the Alliance of Environmental Technologies.
Major conclusions from that review are as follows:
The reaction of lignin with ClO2 occurs by different chemical processes than the reaction of Cl2 with lignin. Substitution of Cl2 by ClO2 results in great reduction in production of highly chlorinated organic byproducts, (five to ten-fold less) and even greater reductions in dioxins and furans. This has been widely observed in effluent monitoring data.
Much of the chlorinated organic material released in effluent from bleach kraft pulp mills, and which constitutes the major portion of 'Adsorbable Organohalogens' (AOX) or 'Extractable Organohalogens' (EOX), is water soluble with very limited lipophilicity, and has little if any tendency to bioaccumulate in aquatic food webs. A clear distinction is required, therefore, between different kinds of organochlorines.
Toxicity test results for compounds found in final (treated) effluent from pulp mill employing a high degree of ClO2 substitution do not lead to the expectation of toxicity in receiving waters typical of North American mill operations.
The weight of evidence overwhelmingly indicated that a high rate of substitution of Cl2 by ClO2 will result in reductions in the quantities of organochlorines produced, degree of chlorination in individual molecules, degree of environmental persistence, potential for bioaccumulation and food web transfer, potential toxicity, and adverse ecological effects.
Finally, the Panel concluded that, pulp mills employing 100% ClO2 substitution, employing secondary treatment, and discharging into receiving waters with dilution factors typical of most North American mills present an insignificant risk to the environment from organochlorine compounds.
In the same review (Solomon et al.,1993), a number of areas of uncertainty based on the state of knowledge up to 1993 were also identified. In particular, it was concluded that -
More knowledge was required of the variability in concentrations of organochlorines in biologically treated effluents from mills using high chlorine dioxide substitution, and of whole effluent toxicity (either as acute lethality or measured through sub-acute effects).
The substances responsible for MFO induction and plasma steroid reduction in fish exposed to biologically treated pulp mill effluents from mills with and without any form of bleaching had not been identified.
The risks from substances present in effluents produced from mills using 'chlorine free' bleaching techniques had not been formally examined.
More identification and quantification were required of organochlorine compounds in effluent from mills using high chlorine dioxide substitution.
The sublethal responses in fish at the population level required further investigation.
Following completion of the 1993 review (Solomon et al.,1993), some interest groups have expressed the concern that replacement of Cl2 by ClO2 in the bleaching of pulp -- while responsible for a reduction in the production rate of organochlorine byproducts -- has not curtailed their production completely. Nor, according to some, can bioaccumulation of organochlorines from effluent for pulp mills using 100% ClO2 substitution be ruled out. One of the major points of contention between the most polarized of the factions is whether 100% ClO2 substitution has resulted in 'only marginal environmental improvements', or whether such substitution is sufficient, and when coupled with effective secondary treatment of effluent represents an example of the 'best available technology' for pulp and paper production.
Four years have elapsed since the 1993 review -- a period in which the actual monitoring data for pulp mills employing 100% ClO2 substitution has more than doubled. Considerable additional research has been completed on the fate and effects of pulp and paper mill effluent. This review builds on the content of the previous report (Solomon et al.,1993). It critically evaluates the previous conclusions, and addresses the key questions above along with other issues that have recently been identified as of potential importance. Like the previous review, this report is focused on the effects of pulp mill effluents in the aquatic ecosystem (including possible effects on wildlife via bioaccumulation through the food chain). It specifically excludes air emissions and other environmental impacts external to the mill, such as those associated with production of wood and the impacts of products exported from the mill.