Effluents from Pulp Mills using Bleaching - PSL1

Section 5 Risk Assessment, Conclusions and Future Considerations

5.1 Source Assessment

In Canada, as of July 1990, there were 47 pulp mills (42 kraft and five sulphite) which use chlorine bleaching. Much variety exists among these mills resulting in different effluent qualities, for example: pulping, bleaching and wastewater treatment processes; type of wood pulped; and the quality of the pulp product. Many of these bleached pulp mills are integrated; therefore, in addition to their production of bleached pulp, the mills (53%) also produce such semi-bleached and unbleached pulp and paper products such as newsprint and paperboard.

In 1989, the Canadian bleached pulp industry discharged between 25 000 and 310 000 m³ of wastewater per day per mill. Pulp mills consumed between 10 to 110 kg of chlorine per day per tonne of pulp produced (or 2.5 to 175 tonnes of chlorine per day). This produced approximately 100 to 1 400 tonnes of bleached pulp per day. On this basis, it is estimated that 610 000 tonnes of chlorine were used to produce 10.2 million tonnes of bleached pulp in 1989.

Effluents from pulp mills using chlorine bleaching contain a wide variety of chlorinated and unchlorinated organic compounds. The chlorinated compounds are produced as a result of complex reactions between lignin and its breakdown products, released during the chemical pulping process, and the chlorine bleaching agent. It is estimated that the Canadian pulp and paper sector discharges over one million tonnes of chlorinated organic material to the aquatic environment annually.

Seventy to eighty percent of the dissolved matter in bleached pulp mill effluents consists of high molecular weight chlorinated organic compounds. Although approximately 250 individual compounds have been characterized in bleachery effluents, they have been estimated to represent only 10 to 40% of the total low molecular weight materials present. High molecular weight chlorinated materials are not generally considered to represent a direct threat to aquatic biota, however, they have been observed to be microbiologically transformed or degraded into low molecular weight compounds that add to the total low molecular weight loading.

The predominant classes of low molecular weight compounds reported in Canadian bleached pulp mill effluents are: chlorinated and nonchlorinated resin and fatty acids, phenols, alcohols, aldehydes, ketones, sugars and aliphatic and aromatic hydrocarbons. Numerous volatile sulphur-containing compounds, such as hydrogen sulphide, thiophenes, and methyl mercaptan, are also found in bleached pulp mill effluents. The composition of bleached pulp mill effluents is not constant from mill to mill but depends on, for example, the chemical pulping and bleaching processes and the degree and type of wastewater treatment. For example, tri-and tetra-chloroguaiacols are the major low molecular weight chlorinated phenolics in bleached kraft mill effluents, whereas 2,4,6-trichlorophenol is the predominant chlorinated phenol discharged from bleached sulphite mills.

As the majority of chlorinated organic materials released in bleached pulp mill effluents have not been identified, analytical efforts have focused on developing general indicators or surrogate measures of the total quantity of chlorinated organic compounds in a particular environmental matrix. For water, the most commonly used approach is known as the Adsorbable Organic Halogen (AOX) analytical method. The halogen component ("X") of bleached pulp mill effluents is almost entirely composed of chlorine. The AOX method is only applicable to water and not to sediments and tissues; therefore, another related method, known as Extractable Organic Chlorine (EOC1), may be used to determine the organochlorine content of sediments and biological tissues. The major limitation of such surrogate tests is the fact that equal AOX values (or equal EOC1 values) give no indication of effluent composition nor of its potential toxicity, persistence, or fate.

Adsorbable Organic Halogen values measured in Canadian bleached pulp mill effluents in early 1989 ranged between 0.5 and 14.9 kg per air-dried tonne of pulp (kg/ADt) with an average of 3.8 kg/ADt. The AOX level in bleached pulp mill effluents is directly related, for example, to the degree of chlorination applied during the bleaching process and to wastewater treatment. The greater the degree of chlorine dioxide substitution for chlorine as the bleaching agent, the lower the AOX value, in general. In addition, effluents from bleached pulp mills which employ secondary wastewater treatment have lower AOX values than those mills with either primary or no effluent treatment facilities.

The 47 Canadian bleached pulp mills together annually discharge over one million tonnes of chlorinated organic compounds to the aquatic environment. Approximately 250 compounds have been identified but many more remain unidentified. The chemical composition of bleached pulp mill effluents is complex, variable, and not well characterized. Thus, substantial quantities of chlorinated organic matter, both of known and unknown composition, enter the Canadian aquatic environment from bleached pulp mill discharges.

5.2 Distribution and Fate Assessment

A number of chlorinated organic compounds generated by pulp mills using chlorine bleaching have been detected and measured in effluents, water, sediments and biota. Water currents, sediment composition, salinity and temperature gradients, and discharge depth of bleached pulp mill effluents affect the distribution of organochlorine throughout the aquatic environment. Tetrachloroguaiacol, for example, has been detected in all aquatic environmental compartments at distances varying up to 1400 km downstream from the nearest bleached pulp mill outfalls.

The persistence of chlorinated organic compounds over time within various aquatic environmental compartments depends on such variables as the water pH, organic carbon content of suspended particulate matter and sediments, presence of a biofilm layer, partition coefficients, water solubility, temperature, light, and microbial population of the receiving environment, but the two most influential variables are the degree of chlorine substitution in the discharged compound and the time of year. Compounds with low chlorine substitution (e.g., dichlorophenols) biodegrade within hours to days whereas highly chlorinated organic compounds may persist within the aquatic environment anywhere from days to weeks or longer, especially in winter. Substances such as chloroguaiacols, therefore, have been discovered in water hundreds of kilometres downstream of bleached pulp mill effluent outfalls during winter conditions. Findings of long-range transportation of chlorinated phenols under ice are not unexpected; however, Canadian field studies have predominantly been conducted in the summer months rather than on a year round basis. Also, under conditions of low flow, particulate matter may settle out relatively close to the outfall; however, under conditions of high flow, for example, a spring freshet or a coastal storm, resuspension occurs which may distribute the contaminated sediments over greater distances.

Nonchlorinated resin acids, such as dehydroabietic acid, have also been found to persist both spatially and temporally within the various environmental compartments (water, sediments and biota), especially where the mills concerned did not practice adequate secondary wastewater treatment.

Some chlorinated organic compounds can be microbially degraded in water or sediments and metabolized in tissues. Generally, the rate of decomposition depends on such factors as: the degree of chlorine substitution of the organic molecule; the organic carbon content of the substrate; ambient oxygen concentration; temperature; and the previous exposure of bacteria to individual or structurally-related chlorinated organic compounds generated by bleached pulp mills. Dichlorodehydroabietic acid and tetrachloroguaiacol are the most resistant to bacterial degradation of the well-characterized compounds discharged in bleached pulp mill effluents and, as such, are commonly detected in biologically-treated wastewaters and in the aquatic receiving environment.

The degree of wastewater treatment that these effluents receive can greatly influence both the levels and the types of compounds detected in the receiving waters. Bleached pulp mill effluents from well-managed, biologically-treated, aerated lagoons would be expected to contain less chlorophenols and their associated by-products than those mills with minimal or no wastewater treatment facilities. This is a general observation within the Canadian bleached pulp and paper sector.

Chlorinated organic compounds may also be microbially transformed into more persistent and bioaccumulative compounds. Biological transformation of chloroguaiacols and chlorocatechols, for example, occurs under aerobic conditions producing chloroveratroles and chloroanisoles, respectively. Under anoxic conditions, the reverse reaction takes place. The biotransformation products of chloroguaiacols, therefore, would not be expected and have not in fact been detected in the anaerobic sediments in the immediate vicinity of bleached pulp mill effluent outfalls. Chloroveratroles have, however, been detected in aerobic bleached pulp mill effluents, receiving waters and sediments in Canada and in fish tissues from the Baltic Sea.

Most chlorinated organic compounds generated by bleached pulp mills are not appreciably bioaccumulated within the tissues of aquatic organisms, however, trichlorophenols, tetrachlorophenols and chloroveratroles are often accumulated far above those concentrations found in water. Chloroveratroles are capable of accumulating in fish up to 25 000 times the concentration in water. The bioaccumulation of chloroveratroles is of particular interest as they are products of biotransformation and are not monitored within the various environmental compartments.

Chlorinated organic compounds from bleached pulp mills have been detected in water, sediments, and biota up to 1400 km from bleached pulp mill outfalls. Compounds with low chlorine substitution degrade within hours to days whereas highly chlorinated compounds may persist from days to weeks or longer. Persistence is longer in winter especially under ice. Some chlorinated organic compounds can be biologically degraded or transformed, and transformation may lead to more persistent and bioaccumulative compounds. Chloroveratroles, for example, transformation products of chloroguaiacols, are capable of accumulating in fish up to 25 000 times the concentration in water. Other compounds detected in fish tissues reflect repeated or long-term exposure rather than high bioaccumulative potentials.

5.3 Exposure and Effects Assessment

Seventy-five percent of Canadian bleached pulp mills discharge effluents that have been shown to be acutely lethal to rainbow trout in mandatory laboratory tests. Canadian effluent concentrations as low as 3.2% have been reported to cause mortalities in fish, and the average concentration required to kill 50% of the test organisms within 96 hours (96-h LC₅₀) is 16% effluent. A strong correlation exists between the results of the acute toxicity tests and the degree of wastewater treatment employed. Effluents from bleached pulp mills which employ well-managed secondary treatment are often nontoxic to rainbow trout, whereas those from mills with only primary or no wastewater treatment are toxic. Accidental spills and in-plant malfunctions can also contribute to acute toxicity. The acute lethality of whole bleached pulp mill effluent to aquatic organisms is also enhanced during periods of depressed, dissolved oxygen levels.

There is an appreciable amount of lethal toxicity information on some of the individual chlorinated organic compounds generated by bleached pulp mills, with 96-h LC₅₀s ranging from 200 to 2 800 ppb. Chlorinated phenolics, particularly guaiacols and catechols, and chlorinated resin acids have been identified as major toxicants in bleached pulp mill effluents. Although few bleached pulp mill effluents have been monitored for individual chlorinated organic compounds, the limited information suggests that the concentration of any single compound in the effluent does not usually approach its 96-h LC₅₀ even if the effluent taken as a whole is acutely toxic. Concentrations of tetrachloroguaiacol, trichlorophenol, and chlorodehydroabietic acid, however, on occasion approach or surpass the 96-h LC₅₀ concentrations. It is important to note that differences are small between acutely toxic and nontoxic concentrations of chlorinated compounds to specific organisms. For instance, in solutions of tetrachloroguaiacol, 100% fish mortality occurred by 96 hours at a concentration of 350 ppb, but there were no deaths after this interval of time at a concentration of 300 ppb.

In addition to the acute lethality of effluents from bleached pulp mills, whole bleached pulp mill effluents have been observed to cause such chronic toxic effects as reproductive anomalies, deformities, biochemical changes and behavioral alterations in aquatic organisms. In general, chronic effects were observed in the Canadian aquatic environment at 0.5 to 5% bleached pulp mill effluent (e.g., at concentrations of only one-tenth of the 96-h LC₅₀). Artificial stream studies conducted by the U.S. National Council for Air and Stream Improvement (NCASI) are compatible with Canadian findings in that they demonstrate chronic toxicity at the same dilution level of the effluents. Laboratory studies of individual chlorinated organic compounds induced such chronic effects as vertebral deformities, and embryo and larval mortality in fish at levels as low as 2.8 ppb.

The effluents from bleached pulp mills are diluted by local receiving waters to a degree which depends upon watercourse flow, tides, time of year, and other factors. Prom the limited data available, predominantly from freshwater mills, the receiving waters even after dilution in the mixing zone usually have concentrations of bleached pulp mill effluents well into the range necessary to exhibit chronic effects. In fact, seventy percent of the Canadian bleached pulp mill effluents discharged to freshwater systems are not diluted below the concentrations which have been associated with chronic effects.

The acute or chronic toxicity of whole bleached pulp mill effluent cannot be explained by the toxicity of individual chlorinated organic compounds which have so far been characterized. The toxicity of those individual chlorinated organic compounds generated by bleached pulp mills which have been studied in the laboratory does not summate to more than a few percent of the observed toxicity of the effluents. The few detailed biological studies conducted downstream of representative Canadian bleached pulp mills all demonstrated continued and repetitive chronic effects on the aquatic biota. The observed chronic effects, such as reproductive anomalies, are significant irreversible factors which jeopardize the continuance of the species and the integrity of the ecosystem.

Seventy-five percent of Canadian bleached pulp mills discharge effluents that are acutely lethal, and even after dilution by the receiving waters seventy percent of the freshwater bleached pulp mill effluents are still within the range of chronic toxicity. The chronic effects observed downstream of Canadian bleached pulp mills include significant irreversible factors which jeopardize the continuance of the species and the integrity of the ecosystem.

5.4 Risk Assessment

Canadian bleached pulp mills discharge over one million tonnes of chlorinated organic materials to the aquatic environment annually. Some constituents of this material are of high spatial and temporal persistence; some demonstrate a large potential for bioaccumulation. Some individual chlorinated organic compounds have been isolated from bleached pulp mill effluents at levels which often approach or surpass concentrations that cause acute toxicity. The summated toxicity of all these chlorinated organic compounds represents a high level of risk.

Seventy-five percent of the effluents from Canadian bleached pulp mills are acutely toxic to rainbow trout. Even after dilution by the receiving waters, seventy percent of the effluents from freshwater bleached pulp mill produce significant chronic effects which jeopardize the continuance of the species and the integrity of the ecosystem. Thus, the levels of whole effluent discharged from Canadian bleached pulp mills to the aquatic environment and the acute and chronic toxic effects observed both in the field and in the laboratory combine to represent a significant risk to the aquatic ecosystem.

5.5 Conclusion

The conclusion based on this risk assessment, is that the substance "effluents from pulp mills using bleach" is entering the environment in a quantity or concentration or under conditions having immediate and long-term harmful effects on the environment. Effluents from pulp mills employing bleaching are therefore considered to be "toxic" as defined under Paragraph 11(a) of the Canadian Environmental Protection Act .

5.6 Future Considerations

Numerous and extensive information gaps were observed during the evaluation of data on bleached pulp mill effluents. Information gaps that have been identified and should be filled include acute toxicities and chronic effects of individual chlorinated organic compounds, especially chlorinated phenolics, which constitute a significant portion of the toxic loading of bleached pulp mill effluents.

The evaluation of bleached pulp mill effluent studies revealed many errors in experimental design. Future field studies should endeavor to select enough suitable control sites so that, for example, appropriate statistical analyses can be applied.

Due to the complexity of bleached pulp mill effluents, analytical methods should be focused on developing general indicators or surrogate measures of the total quantity of chlorinated organic compounds in particular matrices. These surrogate measures include AOX for water and EOC1 for sediments and biological tissues. It should be emphasized, however, that the AOX and EOCI parameters have many severe limitations. Of primary importance is their inability to provide an accurate estimate of the potential toxicity, persistence or bioaccumulation of specific chlorinated organic compounds. Efforts must be made to determine and report all limits of detection and of quantitation for all analytical methods employed.

The amount of chlorinated organic compounds formed during the pulping and bleaching processes must be reduced. Such in-plant process changes as oxygen delignification, improvements in pulp washing and substituting other bleaching agents for chlorine gas will reduce the amount of chlorinated organic compounds produced by a bleached pulp mill. In addition, the amount of chlorinated organic compounds released will be reduced through the installation of secondary wastewater treatment facilities where these do not already exist. It should be noted, however, that wastewater treatment leads to the generation of sludges which contain large amounts of chlorinated organic compounds for which as yet no monitoring or disposal strategy has been developed.

Many of the reported acute and chronic toxic effects have been associated with chlorinated phenolic compounds. Accordingly, future considerations should include the monitoring of a range of these compounds in bleached pulp mill effluents, water, sediments and animal tissues, together with their environmental effects. A strategy should also be devised to compel the bleached pulp industry to undertake biomonitoring of the local aquatic ecosystem. The data generated from this enforceable monitoring program would allow the organochlorine concentrations and aquatic effects at Canadian bleached pulp mills to be compared and contrasted. Only then will it be possible to determine if the proposed Fisheries Act national effluent regulation and the CEPA dioxin and furan effluent regulations are adequately protecting the aquatic environment or if a need for further regulations exists.

The members of the Task Group on "Effluents from Pulp Mills using Bleaching" believe that mandatory reporting by mills of their output of organically bound chlorine and of a range of chlorophenolic compounds in effluents, water, sediments or biological tissues should be established. Environmental effects monitoring should also be required. Given the difficulty of dealing with these contaminants, and their transformation products, future regulations if necessary should place maximum emphasis on preventing the formation of chlorinated organic compounds through the use of alternative technologies and better process control. Properly designed and managed secondary effluent treatment will also reduce the organic chlorine loading to the aquatic environment.