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A Decade of Research on the Environmental Impacts of Pulp and Paper Mill Effluents in Canada (1992-2002)

5.6 Effluent and Receiving Environment Chemistry

In response to regulatory changes promulgated in the late 1980s and early 1990s, the greatest emphasis within the pulp and paper industry was to reduce the production and release of organochlorine compounds. Process changes, including oxygen delignification, chlorine dioxide bleaching, peroxide bleaching and improved brownstock washing have resulted in reduced organochlorine discharges, including total adsorbable organic halide (AOX), and also alterations in the composition of effluents (Servos, 1996). Oxygen delignification and improved brownstock washing have resulted in less residual lignin associated with the pulpstock prior to bleaching. This in turn means less bleaching is required to achieve the desired stock brightness and lower organochlorine content in bleaching effluents. The increased usage of chlorine dioxide in bleaching has further resulted in lower multiple chlorination of compounds such as chlorophenols, while having a lesser effect on the overall quantity of AOX discharges (Stromberg et al., 1996). In 1994, the use of chlorine dioxide in place of molecular chlorine, or elemental-chlorine-free (ECF) bleaching had become more common than the use of molecular chlorine. The significant differences in the chemistry of chlorine and chlorine dioxide provide a mechanistic basis for understanding the formation and character of chlorinated leaching by-products. Chlorine not only oxidizes lignin to form quinone or ring-opened structures, it also reacts through an electrophilic substitution mechanism to chlorinate aromatic rings and some aliphatic side chain groups. Chlorine dioxide reacts with the residual lignin only as an oxidant, forming ring-opened muconic acid (ester) structures (LaFleur, 1996).

The majority of AOX released from pulp mills is in the form of high molecular weight material (>1000Da), the composition of which varies according to pulping conditions. Questions about the significance of high molecular weight material formed in the bleaching process drove research into characterizing the structures of these materials in the early 1990s (LaFleur, 1996). Increasing the level of ClO2 for Cl2 increases the carbon to chlorine ratio, indicating a lower degree of chlorination. Numerous other studies determined that high molecular weight material after ClO2 bleaching has a higher carboxylic acid content, and a higher phenolic content than residual lignin. A small proportion of the aromatic rings possess chlorine substitution, which should not lead to the formation of highly chlorinated persistent breakdown products from this material under environmental degradation processes. The breakdown of high molecular weight was considered by Millar et al. (1996) using fungal strains and sunlight. Both naturally occurring fungi and white-rot fungi were found to be capable of degrading high molecular weight fractions of effluent, which would reduce effluent colour and AOX. It was also observed that irradiation could result in the release of chlorinated guaiacols and vanillins.

In the early 1990s the majority of pulp mills across Canada had installed secondary treatment to comply with federal BOD and acute toxicity regulations. The effects on effluent compositions are complex and at that time were not well understood. It was recognized then that removal of compounds from effluent in biological systems is possible through microbial degradation, physio-chemical process such as adsorption, or air stripping. In some cases, microbial degradation may not be complete and it may also result in the formation and discharge of new compounds. It was further recognized that much of the work accomplished to date had been completed on the identification of compounds in process sewers, with comparatively less work on biologically treated effluents. This is partially due to the complexity of biologically treated effluents relative to process sewers and the fact that previous studies on process sewer compositions were the result of chlorine-based bleaching (LaFleur, 1996).

A review of the chemical properties of organics in final effluents in the mid 1990s reveals that the bioaccumulation of the majority of low molecular weight chemicals can be predicted with established relationships between bioconcentration factor and octanol-water partition coefficients (Muir & Servos, 1996). However, it was pointed out that the accumulation of many phenolics and neutrals will be overestimated because of metabolism. Many compounds attributed to pulp mill production were found to accumulate in wildlife in surveys conducted in British Columbia (Elliot et al., 1996). High concentrations of PCDD/DFs were found in eggs of various fish-eating birds species from the Strait of Georgia , with the highest concentrations near pulp mills. These results contrasted with concentrations found in the same species collected in the vicinity of pulp mills on the Atlantic Coast . Poor breeding success in blue herons (Ardea herodias) near a bleached kraft pulp mill in the Strait of Georgia was associated with a variety of effects, including EROD induction (Elliot et al., 1996). Principal component analysis of PCDD/DF concentrations in crab hepatopancreas had been used to distinguish individual mill sources and classify congener patterns according to chlorine bleaching and pentachlorophenol wood preservative sources (Yunker & Cretney, 1996). Since the late 1980s, the proportion of 2,3,7,8-chlorinated congeners and the overall PCDD/DF concentrations had begun to decrease near mill outfalls.

In the mid 1990s the concept of a minimal impact mill, based on a workshop conducted on the elimination of ECF and TCF-based bleachery effluents, was conceived. The workshop concluded that process development toward the minimum impact mill should begin by concentrating on minimizing releases from the pulping and recovery processes. The mimimum impact mill does not mean zero bleach plant effluent but one which maximizes pulp yield and produces quality products that are recyclable, maximizes the energy potential of bioamass, minimizes water consumption and wastes (Axegard et al., 1997).

Also in the mid 1990s, an ecological risk assessment of ECF effluents concluded that the risk of chlorinated compounds of concern had decreased dramatically following implementation of ClO2 bleaching, which was supported by the re-opening of fisheries in the vicinities of pulp mills (Bright et al., 1997). Higher chlorinated phenols are not present in ECF effluents and the risk posed by lower chlorinated compounds is minimal. It was concluded that the compounds of concern are those that had received little attention thus far, the non-chlorinated compounds originating from pulping, rather than bleaching. The issue of the production of biologically active compounds of concern during secondary treatment, such as retene, was also raised.

The first work dealing with the ability of semi-permeable-membrane-devices (SPMDs) to concentrate biologically active compounds was conducted in the mid 1990s (Parrott et al., 1997a). Dialysates of SPMDs deployed in effluents from mills employing a variety of process types were able to elevate EROD activity in a fish hepatoma cell line. SPMDs also proved useful in the recovery of AhR ligands from in-mill process streams. In an extension of this work using fish caged in bleached kraft mill effluent, it was first reported that accumulation of AhR ligands could be measured (Figure 5; Parrott et al., 1997b). Male and female fish accumulated ligands for the AhR and these could be attributed to non-dioxin compounds following analysis of liver extract fractions generated by HPLC.

As part of the 1992 amendments to the Pulp and Paper Effluent Regulations under the Fisheries Act, every mill was required to conduct an Environmental Effects Monitoring (EEM) program on a three-year cycle basis. During the first cycle of the program in the mid 1990s, many mills experienced difficulties meeting the requirement to analyze for effluent tracers in fish. The objective behind tracer analysis is to provide collaborative evidence of exposure in the receiver, particularly where fish can move freely between sampling locations. An industry-government working group was established to examine the use and effectiveness of chemical tracers in exposure assessments and to develop recommendations for future cycles of EEM (Ali et al., 1997). Tracers commonly measured in the first EEM cycle were resin and fatty acids in fish bile and liver with additional analyses of chlorophenolics. It was determined that numerous factors affected the presence and concentrations of tracers in fish and that the concentrations of those tracers measured in the first cycle were highly variable. It was concluded that the inclusion of tracers in EEM is accepted in principle but additional round robin studies, quality control studies and field validation must be conducted in conjunction with additional EEM cycles to assess the effectiveness of the tracer program (Ali et al., 1997).