2.1 Factors Influencing the Production of Organic Substances in Pulp Mill EffluentOrganic material is present in effluent from the manufacture of all types of pulp. Various sum parameters are commonly used to measure the quantity of material present because the procedures involved in their determination are simple and can be performed routinely without elaborate equipment. Sum parameters are valuable for monitoring purposes within mills and provide a partial indication of the type of material present, but do not provide any information about the individual substances present. The determination of individual substances is much more complicated and is discussed in Section 2.3. Since environmental effects are often associated with specific substances or groups of substances, they should not be attributed to levels of sum parameters. Common sum parameters used to assess effluent characteristics are described below.
Total Organic Carbon (TOC) is a measure of the amount of all organic substances in effluent. Chemical Oxygen Demand (COD) is a measure of the oxygen consumed in the chemical oxidation of organic material in effluent by a strong chemical oxidant. TOC and COD are general sum parameters which can be used on effluent from all stages and types of pulp production. Effluents also contain chlorinated substances which are produced during the bleaching of chemical pulp as shown in Figure 2. Chlorinated organic substances can be measured by the sum parameter, adsorbable organic halogen (AOX). Extractable organic halogen (EOX) is a less commonly used sum parameter which refers to the amount of halogenated organic material which can be extracted from the effluent by a very non-polar solvent.
By the very nature of the test, AOX (or EOX) does not provide any information on the chemical species present and, with the mixture of compounds typically present in pulp mill effluent, AOX does not correlate with persistence or bioaccumulation. The use of this measurement for regulatory purposes also ignores the fact that the toxicity and biological properties of the organochlorine substances vary widely; the lower chlorinated substances generally presenting less risk than those with a higher degree of substitution (Solomon et al., 1993). Several studies have shown that AOX concentration is not related to biological responses (NCASI, 1990; O'Connor et al., 1993; Robinson et al., 1994).
The amount of organic material in pulp mill effluent from bleaching is determined by the lignin content and amount of organic material entrained in the brownstock arriving at the bleach plant. Prebleaching steps, such as extended delignification and oxygen prebleaching decrease the lignin content of the brownstock. These processes, and improved pulp washing, decrease the organic load of the material entering the bleach plant which, in turn, decreases the consumption of bleaching chemicals and the amount of organic byproducts produced during bleaching. Chlorinated organic substances result mainly from reaction of residual lignin in pulp with chlorinating agents. Extended delignification and oxygen prebleaching therefore also decreases the amount of chlorinated substances formed during bleaching. Since the lignin content of hardwood pulps is less than softwoods, effluents from the bleaching of hardwoods contain smaller amounts of chlorinated substances.
It is well established
that concentrations of chlorinated organic substances (as AOX) in effluent
from ECF (elemental chlorine free) bleaching are five to ten times lower
and different in composition from those of chlorine-based effluents (Solomon
et al., 1993). As discussed in the previous assessment, chlorine
dioxide acts primarily as an oxidant during bleaching. The only chlorinated
material formed results from reaction of the byproduct hypochlorous acid
with organic material.
To obtain a perspective of any possible environmental impact of pulp mill organochlorines, and, given the rapid increase of information within this field, the structure, formation, and sources of natural chlorinated material must be considered.
Several recent original papers and reviews have stressed the considerable concentrations of chlorinated organic material occurring as natural background levels in receiving waters (Grimvall, 1995; Grimvall et al., 1994; Gribble, 1994; 1996; Dahlman et al., 1993a; 1993b, Moore, 1995). These background concentrations show considerable area-specific variations, which must be accounted for when assessing environmental risks from the pulp industry. At the time when bleaching with chlorine came into the political and environmental spotlight, natural background concentrations of organochlorine compounds were not considered, leading to assumptions of large-scale contamination of the environment by organochlorines discharged from the pulp and paper industry. These assumptions were based on measures of AOX and EOX from receiving waters. Recently, some 2000 chlorinated and other halogenated compounds had been identified as entering the environment via synthesis in plants, marine organisms, bacteria, insects, fungi and mammals (Gribble, 1994). Abiotic processes such as active volcanoes, forest fires, chemical reactions in the atmosphere etc., further contribute to the formation of organochlorine compounds within the biosphere.
The range of naturally produced compounds includes both simple alkanes such as chloromethane, as well as numerous complex halogenated alcohols, ketones, carboxylic acids, carboxylic amides, aldehydes, epoxides and alkenes (Gribble, 1994 and references therein). For example, it has been estimated that the global emission rate of chloromethane from terrestrial biomass is 5 million tons per year, whereas the anthropogenic input is only 26,000 tons (Gribble, 1994). Other identified organochlorine compounds include a large number of chlorinated terpenes and related compounds that have been isolated from terrestrial and marine organisms, chlorinated fatty acids, chlorinated prostaglandins and chlorinated lipids (Gribble, 1994). Gribble (1996) reports that there are now 2400 natural halogenated substances identified.
When considering the products of ECF bleaching of pulp, it is important to recognize that many of the chlorinated substances produced in this process are also formed in nature. Several chlorophenolic isomers with biogenic origin have been isolated. Studies in Sweden (Grimvall et al., 1994) have demonstrated that 2,4,6-trichlorophenol and its methylated analogue 2,4,6-trichloroanisole are ubiquitous in humus-rich waters and are formed by the action of microorganisms. Some degradation products from natural chlorinated lignin that are similar to those occurring in bleached kraft mill effluents are shown in Figure 3 (Dahlman et al., 1993a).
The formation of chlorinated substances by organisms is catalyzed by commonly occurring enzymes. The occurrence of haloperoxidase enzymes seems to be the requirement for marine organisms, terrestrial plants, fungi, bacteria and mammals to synthesize halogenated compounds in the presence of chloride, bromide or iodide ions. Interestingly, human white blood cells contain myeloperoxidase, which, in the presence of chloride and other halogen ions and hydrogen peroxide, rapidly forms reactive halogens to destroy invading microorganisms. These reactions can extend to the formation of more persistent and bioaccumulative substances from simpler substances. An illustration of this is the enzymatic conversion of chlorophenols into PCDDs and PCDFs by horse-radish peroxidase enzyme (HRP) (Gribble, 1994).
A natural consequence of the formation of chlorinated organic compounds is that there also must be pathways for their biodegradation and that these have been selected for and evolved over many eons. Evidence for these degradative processes includes the direct measurement of the degradation of specific substances as well as inferred degradation through indirect evidence based on measurements of organically bound chlorine in watersheds (Gribble, 1994; Landner et al., 1994). Simple organochlorines such as di- and tri-chloromethane are degraded to carbon dioxide and water in soil. The chloride ion will dissolve in water and leach into ground or surface waters or be incorporated into new microbiologically synthesized natural chlorinated organic products. Also, chlorophenols are degraded by fungi. A white-rot fungus is known to degrade penta-chlorophenol, DDT, and PCBs (Gribble, 1994). Studies on fluxes of organically bound chlorine in Lake Vättern in Sweden showed lower net export than import despite the fact that a pulp mill producing bleached pulp has been in operation on the lake since the 1950s (Landner et al., 1994). As shown by Archibald et al., (1997), both naturally occurring AOX, as well as pulp mill related ECF AOX, is photomineralized in lake water. Lake Vättern is a clear water lake with a relatively deep photic layer and thus AOX will be subjected to extensive photooxidation in the summer time.
As also previously noted, terrestrial plants by themselves also are capable of synthesizing chlorinated compounds and may contribute to the occurrence of chlorinated substances in soils. Swedish studies (Asplund, 1992 and references therein) have shown that Norway spruce needles as well as the leaves from common beech contained 20-60 and 10-20 mg/kg (dry weight) organohalogens respectively. These results indirectly imply either that halogenation of organic compounds is occurring in trees used for pulp production or that these are present as a result of long-range transport of these substances from other, more distant, sources. The precise nature of the compounds included in the analysis of total organic halogens (TOX) remains unclear. A recent study by Johansson et al., 1996 (in manuscript) showed that fresh wood parts of spruce contained 10 µg/g d.w. organochlorine compounds. As to the origin of chlorinated organic substances in wood, it is unlikely that these types of high molecular weight compounds would originate from long-range transport of chlorinated substances as they are not characteristic of those found in precipitation as identified by Dahlman (pers. comm.) and Grimvall et al. (1994). Wood attacked by fungi contains higher concentrations of chlorinated substances than fresh healthy wood. Thus, storage conditions as well as the age of the trees may cause differences in the background concentrations of chlorinated substances entering the pulping process. The contribution of such material to the chlorinated organochlorine content in bleached kraft mill effluents (BKMEs) is unclear but would be less than 0.1 kg/tonne of pulp. In general terms, zero AOX concentrations will not occur, even in effluents from TCF production, since chloride ions from the wood furnish will always be present enabling some microbial formation of AOX.
It was shown by Johansson et al. (in manuscript) that the TOX-value increased along with increasing levels of chlorinated methyl esters in decaying spruce and in the humic layer in a spruce forest. This suggests that decomposors were synthesizing low molecular chlorinated compounds, possibly by using lignin as the carbon source for the chlorination. The TOX:organic matter ratio has been found to be more than ten times higher in soil than in fresh plant material. This halogenated material will at least partly be introduced via land run-off water to watercourses. Asplund (1992) showed that the concentration of chlorinated organic substances in surface waters was correlated to humic acid content. High AOX values were primarily found in humus-rich and oligotrophic lakes. Ongoing research (Lehtinen et al. unpublished) has confirmed the presence of considerable concentrations of AOX in peat bog water (130 µg/L). In comparison, an ECF effluent with an AOX discharge of 0.5 kg/adt and a 100 m3/t water consumption with a receiving water dilution of 100:1 would reach an AOX concentration of 50 µg/L.
The great similarity between the chlorolignin of ECF pulp mills and natural chlorolignin, raises the question of which process is producing more AOX, production of pulp or natural processes. For Lake Vättern in Sweden, it was found that 25% of the input of AOX was of natural origin and 60% of the total AOX was degraded. Only 10% of the total input was retained in the sediment despite a 50-year contribution from two pulp mills. The recent work by Archibald et al. (1997) provides evidence that biotreated pulp mill AOX was, to a large extent, photomineralized over a relatively short, 4 month period, supporting the conclusions made for Lake Vättern that AOX from pulp mills is not recalcitrant in nature.
The chemical characterization of pulp mill effluent involves both the identification and quantification of individual compounds present. The process is much more difficult and time-consuming than that of a more general characterization using sum parameters because effluents contain very complicated mixtures of hundreds of different compounds. In addition, changes in bleaching and other pulp processing technologies result in the formation of different types and amounts of chemical byproducts. Fortunately, standard analytical methods developed for important groups of compounds, e.g., PCDDs, PCDFs, chlorophenols, resin and fatty acids, and chloroform, can be applied to any type of effluent. Other, more general analytical procedures have also been used recently to analyze effluents from mills employing ECF bleaching, with the result that a significant amount of information has been obtained since the 1993 review (Solomon et al., 1993) concerning the nature of the compounds present, particularly in bleaching effluent. Less is known about compounds present in effluents from mills employing TCF bleaching because the use of chlorine-free bleaching agents (and therefore the absence of chlorinated byproducts) has been equated with greater environmental benefit. However, it is clear from recent literature discussed in later sections of this document that environmental effects observed in fish are not related to the type of bleaching used. Some information is available on certain groups of compounds, such as chlorinated hydrocarbons (Section 2.3.2) and sterols (Cook et al., 1997) in final, biotreated effluent from mills employing ECF bleaching.
Pulping is the separation and conversion of wood fibers to a "pulp" by physical and/or chemical processes. The kraft process, which involves cooking the wood fibers with a solution of sodium hydroxide and sodium sulfide, is the main process for producing chemical pulp. The resultant pulping (black) liquor contains degraded cellulose, hemicellulose and lignin, and extractives, the specific components of which are not normally investigated since they are combusted during chemical recovery. As a result, no significant new information has been obtained since the 1993 review concerning products produced during pulping. Extended delignification and improved pulp washing increase the amount of organic material retained in pulping liquors. Oxygen delignification further decreases the lignin content of the unbleached pulp. Since effluent from oxygen delignification is also recycled with the pulping liquors, the chemical components present also are not normally investigated. However, based on model compounds studied (Gierer, 1986) and the analysis of functional groups present in lignin after treatment with oxygen (Gellerstedt and Lindfors, 1987; Lachenal et al., 1995), polar, water-soluble, oxygenated compounds such as carboxylic acids are likely present.
In the previous review, fundamental differences in the chemistry of chlorine and chlorine dioxide bleaching were discussed. Chlorine dioxide is a more powerful oxidant than chlorine, and is capable of oxidizing the aromatic phenolic rings of lignin beyond the quinone stage, characteristic of chlorine bleaching, to muconic acids (Figure 4). The only chlorinated products which are formed result from reaction of organic material with the byproduct hypochlorous acid, and not chlorine dioxide.
Approximately one hundred compounds have now been identified in effluents from ECF bleaching. Appendix 1 lists a number of these along with their structures. The total quantity of chlorinated material in ECF bleaching effluent is, however, only 10-20% of that found in chlorine-based bleaching effluent (Solomon et al., 1993). Most (nearly 60%) of the chlorinated compounds fall into two chemical classes -- chlorophenols and chlorinated hydrocarbons. The remainder fall into a variety of classes including acids, furanones, aldehydes and ketones. A special effort has been made to monitor formation of PCDDs, PCDFs and chlorinated phenols. In the previous assessment, it was reported that levels of some compounds produced during bleaching, particularly PCDDs, PCDFs, polychlorophenols and chloroform, decreased dramatically when chlorine dioxide was substituted for chlorine in the first stage of bleaching. Numerous reports have now firmly established that ECF bleaching reduces levels of PCDDs, PCDFs and polychlorophenols in bleaching effluent to below current detection and/or proposed EPA regulatory limits (Federal Register, 1996; Luthe and Berry, 1997; Shariff et al., 1996; NCASI, 1995). These results have commonly been achieved using pulp produced by conventional pulping, without extended delignification or prebleaching.
As reported in the 1993 assessment, the monochlorophenol, 6-chlorovanillin, is the main chlorophenol present in effluent from softwood bleaching. Traces of mono- and dichlorosyringaldehydes are present in hardwood bleaching effluent, particularly from eucalypt pulp (Smith et al., 1995). The chlorinated hydrocarbons were reported in recent work from Finland. Koistinen et al. (1994) reported that final biotreated effluents from the ECF bleaching of birch pulp contained ng/L (parts per trillion) concentrations of alkylchlorophenanthrenes (including chlororetenes -- see Figure 5) and alkylchloronaphthalenes. The authors suggested that aromatic precursors, such as phenanthrene, may be present in oil-based defoamers as well as the unbleached pulp. Rantio (1995; 1996) reported that final biotreated effluent from both softwood and hardwood bleaching contained traces of several isomeric chlorocymenes. Concentrations decreased dramatically compared to those present when chlorine was used for bleaching. Other substances recently identified in ECF bleaching effluent include muconic acid esters derived from guaiacol (Vilen et al., 1996), dichloromethylenefuranones (McKague and Grey, 1996), 4-chloro-3-hydroxy-2H-pyran-2-one (Smith et al., 1994), dichlorocyclohexenediones (McKague and Grey, 1996) and straight chain aldehydes and ketones (Juuti et al., 1996). These other new compounds are non-aromatic (Figure 5) and contain functional groups which render them susceptible to degradation in the environment (Schwarzenbach et al., 1993).
Forty to fifty percent of the material in ECF bleaching effluent has a molecular weight > 1000 (McKague and Carlberg, 1996). Studies performed on this material have shown it is highly polar and characterized by a low (< 2%) chlorine content. The aromatic content is lower in material from hardwood bleaching (Dahlman et al., 1995) and oxidative degradation gives similar products to those obtained from naturally occurring humic substances (Dahlman et al., 1993a). The latter finding suggests humic substances and the high molecular weight material from ECF bleaching have macromolecular structural characteristics in common and that this material from bleaching is harmless in the environment. Carbohydrates, mainly xylan structures, may account for nearly 30% of the high molecular weight material (Dahlman et al., 1995; Wallis and Wearne, 1994).
Pulp mill process streams are usually combined and treated in aerated lagoons or activated sludge systems prior to final discharge. Low molecular weight compounds are degraded by biological and chemical processes while abiotic processes are responsible for the degradation of high molecular weight material. The more highly oxidized products produced during treatment are more water soluble and less likely to bioaccumulate in aquatic organisms. Occasionally, because of local anaerobic or anoxic conditions within biotreatment systems, compounds which are normally biodegradable may be poorly degraded or undergo biotransformation into other products which are more lipophilic and have a greater potential for bioaccumulation. The previous assessment discussed the biological methylation/demethylation of chlorinated catechols, guaiacols, and veratroles. Resin acids have been reported to undergo biological hydrogenation/dehydrogenation and/or decarboxylation to neutral diterpenoids such as dehydroabietin, retene and fichtelite as well as the expected oxidation products (Wilkins and Panadam, 1987; Zender et al., 1994; Stuthridge and Tavendale, 1996). The resultant products may be more resistant to biodegradation, particularly if they are aromatic. In a study of removal efficiencies of sterols, the concentration of stigmasterol was found to increase during biotreatment (Cook et al., 1997). Resin acids and sterols are constituents of pulping effluent which may be carried over to the bleach plant with the pulp because of their lipophilic properties so are not related to ECF bleaching.
Final mill effluent from mills with ECF bleaching may contain traces of halogenated compounds. For example, chlorinated acetic acids, chloroform and chlorinated resin acids have been reported in ECF effluent after biological treatment (Dahlman et al., 1993b). 1,1-Dichlorodimethyl sulfone is known to be highly resistant to biodegradation and appears in final effluent. A study using muconic model compounds showed biodegradation of these types of compounds was dependent on the chemical structure of the substrate (Zheng and Allen, 1996). The high molecular weight material formed during bleaching undergoes little biodegradation, however, as mentioned in section 2.3.2, it has been shown to resemble natural aquatic humic material (Dahlman et al., 1993a) and undergo gradual abiotic degradation in the environment (Roy-Arcand and Archibald, 1993).
Low concentrations of halogenated compounds may also continue to be released from lagoon sediment after conversion to ECF bleaching. In one study, it was estimated that depletion of historic deposits of 2,3,7,8-TCDF from an aerated lagoon sediment at a bleached softwood pulp mill would require one to four years (Pagoria and Kerfoot, 1997). A variety of studies have shown sediments continue to provide a source of chlorophenols (Kvernheim et al., 1993; Palm and Lammi, 1995; Judd et al., 1995). The trace levels of chlorinated hydrocarbons reported in final biotreated effluent after conversion to ECF bleaching (Section 2.3.2) may also originate from historical deposits in lagoon sediment. As mentioned in the 1993 review, chlorate is removed during biotreatment.