This review and subsequent report were conducted at the request of the Alliance for Environmental Technology by a Scientific Review Panel specifically convened for the purpose. The task put to the Panel was to review and summarize the known or plausible impacts and, where possible, quantify the risks to aquatic organisms and ecosystems associated with the discharge of treated effluent from pulp and paper mills using 100% chlorine dioxide substitution (ECF). Also included were several other issues related to the environmental impacts of pulp production and bleaching. This was done with special reference new information that had become available since the last review which was conducted on data available up to 1993. In this process, the panel used a weight of evidence approach and followed an approach that was similar to the risk assessment frameworks developed by the U.S. Environmental Protection Agency and Environment Canada.
The risk assessment completed in 1993 concluded that "mills bleaching with high chlorine dioxide substitution (100%), employing secondary treatment and with receiving water dilutions typical of most mills in North America, present an insignificant risk to the environment from organochlorine compounds." The authors of the 1993 risk assessment identified several gaps in our knowledge of the fate and effects of pulp mill effluent from mills: For example; (i) the substances responsible for liver mixed function oxidase (MFO) enzyme induction and reductions in serum concentrations of reproductive steroid hormones in fish exposed to biologically treated pulp mill effluents from mills with and without any form of bleaching had not been identified; (ii) the sublethal responses in fish at the population level required further investigation, and (iii) more identification and quantification were required of organochlorine compounds in effluent from mills using high chlorine dioxide substitution.
In this review, the panel identified two major questions that have formed the basis for report. These are:
and, what are the specific risks, if any, contributed by the use of ClO2 as opposed to other factors in pulp and paper production?
With regard to the second, and narrower of these two questions, and in confirmation of the conclusions of the 1993 review and assessment, the conclusions of the Panel with respect to chlorinated substances are straightforward. The clear weight of the evidence is that bleaching with 100% ClO2 substitution (ECF bleaching) produces chlorinated substances, such as mono- and di-substituted chlorophenols. These are similar in composition and structure to naturally occurring chlorinated substances, and, as opposed to compounds with three, four, or more chlorine atoms in the molecule, are invariably less persistent and less bioaccumulative. The environmental effects, persistence, and modes of degradation in the environment of these chlorinated substances are well understood. Exposure concentrations of chlorinated substances detected in mill effluent continue to be generally low and do not suggest that acute or chronic effects will result from their presence. This risk assessment for chlorinated substances produced as a direct result of bleaching using ClO2 reconfirms the earlier conclusion that the chlorinated substances produced as a direct result of bleaching using ClO2, and subjected to secondary biological treatment, present a negligible (insignificant) environmental risk to aquatic ecosystems.
The previous review noted declines in the concentrations of 2,3,7,8-substituted dioxins and furans in pulp mill effluent from mills using ECF bleaching as well as in organisms in the receiving environment. The continuation of this trend clearly reflects marked decreases in environmental inputs resulting from improvements in technology, including the use of ECF bleaching of pulp. Most North American mills have already achieved virtual elimination of 2,3,7,8-TCDD and similar reductions in 2,3,7,8-TCDF emissions. Corresponding declines in tissue concentrations of these substances in marine fish and shellfish populations have also been documented. Regional differences in the rates of reduction are likely due to differences in the dynamics of existing sediment contamination and are not a reflection of continuing input.
The only other substance which potentially could be introduced into the effluent stream specifically due to use of ClO2 is chlorate; however, this is readily removed during the treatment of final effluent and therefore is not judged to be an environmental concern at any North American pulp mill.
The potential environmental responses to chlorine-containing compounds produced or released during the manufacture of pulp bleached by technologies using 100% chlorine dioxide substitution must be judged in relation to the production of natural background levels of chlorinated organic compounds. This production varies, depending on the geographical location, but it should be recognized that organisms have evolved in environments with background concentrations of natural chlorinated organic compounds. Many compounds, identical, or similar to those produced as a direct result of bleaching using ClO2, are also formed as a result of natural processes. Therefore, it is not surprising that there is evidence that organisms possess mechanisms for the effective breakdown of these chlorinated substances.
As a consequence of this, a fundamental change in thinking is required -- that is -- to dispel the notion that only chlorinated compounds are of primary importance. Nonchlorinated compounds should be regarded as being at least equally important.
The 1993 review pointed out that a more complete characterization of all the substances produced in ClO2 bleaching and their releases to the environment was needed. Since this time, there has been considerable progress in the area. We also have a more extensive knowledge-base of the physical, chemical and biological behavior of the classes of substances produced by this bleaching process. There are, however, many unidentified substances present in pulp mill effluent, both from ECF and TCF bleaching, such as, in the latter case, those using oxygen based chemical bleaching by hydrogen peroxide and ozone. Identification and chemical characterization of these substances should be an ongoing process.
Overall, it is not possible at present to confidently conduct a quantitative risk assessment of the larger suite of non-chlorinated substances released in pulp mill final effluent. Information on the identity and ranges of concentrations in effluent of many of the organic constituents, especially retene and abietic acid, various resin acids, and plant sterols, or knowledge of their toxicological significance is either lacking or incomplete.
The previous review noted that significant changes in the treatment of effluents from pulp mills had occurred. The presently available studies on degradation and removal processes for constituents of pulp mill effluent reemphasize the importance of effective secondary treatment in the reduction or elimination of acute toxicity in final effluent before it is released to the receiving environment. Subsequent to past improvements in kraft mill effluent quality realized through ClO2 substitution, future improvements might be achieved through improvements in secondary treatment, since; i) recent studies suggest that many biologically active compounds in final effluent are nonchlorinated compounds contributed by pulp mill processes other than bleaching, and; ii) most of the known deleterious substances in pulp mill effluent are amenable to degradation and/or removal through adsorption to particulates. Increased recycle and spill recovery within pulp mills may also be a viable means for the continued improvement of effluent quality.
The larger question regarding risk due to the discharge of pulp mill final effluent, i.e., the known or plausible impacts associated with all aspects of mill operations including ClO2 bleaching, pulping and all other processes, is more complicated. It is clear that recent research on the biological effects of pulp mill effluents has undergone a switch in emphasis with regard to contaminants of concern -- to substances more directly associated with the pulping as opposed to bleaching process. Studies completed since 1993 suggest that some of the compounds found in treated pulp mill effluent are similar to resin acids, plant sterols, or naturally-occurring PAHs (e.g., retene) produced or released during the decomposition of plant materials. The ecological risks associated with release of such compounds in final effluent remain unresolved. While it is recognized that many of these substances occur naturally in the environment, a major difference between those released naturally and many of the constituents of effluent released from some pulp mills may be in the rates of input and concentrations produced, at least close to the point of effluent discharge.
There have been a large number of field and laboratory studies since 1993 on contaminant loadings and biological effects from pulp mills, but, the study designs have limited the extent to which cause and effect relationships can be discerned. The results are nonetheless instructive in understanding the potential for environmental impacts from pulp mill final effluent discharges, such as, for example, the lack of correlation between bleaching process and response in fish. Two of the most consistent responses of some fish species exposed to pulp mill effluents continue to be changes in MFO activity (induction and inhibition), and alterations in serum concentration of reproductive steroid hormones. These appear to be separate and unrelated responses of fish to pulp mill effluents. While the hormone response may be the mechanism by which pulp mill effluents affect reproductive processes of some fish at some mills, the specific details are not completely understood and the population-level consequences are unclear. The toxicological significance of increased MFO activity is related to prolonged elevation of MFO activity. The increase in enzyme activity may result in greater degradation of physiologically important endogenous substances, such as hormones. In addition, prolonged gene activation may result in the diversion of energy and substrates away from normal growth and developmental processes.
Change in MFO activity in fish is a common response to exposure to many pulp mill effluents, and not a characteristic unique to bleaching. This response is most likely caused by more than one substance. The MFO-inducing substances in biologically-treated effluent appear to be natural extractives of wood released by pulping, their metabolic by-products, and/or their chlorinated derivatives. These inducing substances appear to be nonpersistent and easily metabolized and excreted by fish. The number and relative amount of these substances produced as a result of ClO2 (ECF) or TCF bleaching is unknown.
Apart from complete ClO2 substitution and secondary treatment, none of the studies reviewed established a clear link between toxicity reduction and installation of specific technologies. Secondary treatment alleviated acute toxicity to fish and invertebrates in laboratory tests, but adverse responses in some fish in receiving waters were still evident, although reduced. Improvements were associated with multiple changes to many aspects of a mill's operation, and could not be ascribed to any single improvement in technology. Field studies have been useful in assessing the aggregate response of organisms to effluent exposure. However, the nature of field studies, in which different observations can only be correlated, and in which numerous factors act simultaneously and independently, prevents the identification of clear cause-effect relationships. Future advances in this area may benefit from mass balance approaches that account for volumes of both liquid and solid wastes and more thorough characterization of the substances present in the effluents.
In general, responses of aquatic communities and populations to improvements in waste treatment and pulping processes have been positive, however, changes have not been sufficient to alleviate reproductive responses in some fish species in all cases. It is clear that the largest environmental gains have come from 100% ClO2 substitution and more widespread use of effective secondary treatment. Still needed, is some attention to the nonchlorinated substances released during pulping and bleaching. These substances will probably be responsive to source control measures. Any risk management strategies employed should be evaluated on a holistic basis, for example, using risk assessment, life cycle analysis, and environmental cost/environmental benefit procedures.
The effects of long-term exposure to pulp and paper mill effluents are deserving of continued evaluation because of their introduction into receiving systems. This recommendation is made since our evaluation "tools" and "strategies" are constantly evolving and improving. However, any decisions based upon this monitoring must be made with a clear understanding of the ecological relevance of the data. Long-term and continuous exposure to effluents may cause changes in reproductive cycles, reduced or enhanced growth, changes in the age-structure of populations, or impairment of an organism's ability to cope with stress. However, all of these effects should be evaluated in the context of their larger environmental significance.