Environmentally Benign Approaches for Pulp Bleaching (Developments in Environmental Management)

Environmentally Benign Approaches for Pulp Bleaching
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Stringent pollution regulations are now in force, however, and, as new CTMP mills are built, they incorporate the necessary pollution control devices to ensure that mill effluents pass toxicity monitoring. In fact, the new goal of the pulp and paper industry is the development of "zero-emission" or "closed-loop" CTMP mills, where all wastes are recovered and process water is recycled.

ISAP 2019 Strengthening the Synergies between Waste, Climate and the SDGs

There are very few sulphite pulp mills still operating in Canada, and the newly built CTMP mills generally meet pollution effluent standards. This leaves Kraft pulping as the primary source of toxic pulp mill effluents. Logs and debarked and then put through a chipper. The evaporated phase, which may contain inorganic sulphur in the form of sulphate, sulphite, or dithionite, is trapped in a condenser producing a concentrated sulphur waste water stream termed evaporator condensate.

Highly volatile fugitive emissions from the condenser, and volatile compounds and gases from the burnt concentrated black liquor are released to the atmosphere. Air contaminants released from pulping include particulate matter, sulphur dioxide, and total reduced sulphur TRS compounds. The TRS compounds consist primarily of hydrogen sulphide, methyl mercaptan, dimethyl sulphide, and dimethyl disulphide. It is these compounds that contribute to the characteristic foul odors associated with pulp mills.

The very low odour threshold of these pollutants means that disagreeable odours can be discerned by smell at concentrations that are seldom hazardous to human health.

Accordingly, the main effect of pulp mill airborne emissions is the reduction of aesthetic air quality, although, the blackening by hydrogen sulphide of homes and buildings in proximity to pulp mills is not uncommon. Subsequent bleaching treatments may vary, but usually proceed through a series of hypochlorite, chlorine dioxide, alkali, and chlorine dioxide processing steps. After each treatment stage the pulp is filtered and the process liquids combined as bleachery effluent. During bleach plant treatment, approximately 1 kg of extractives, 19 kg of polysaccharides, and 50 kg of lignin are dissolved from 1 tonne of softwood pulp.

Chlorine can react with all of these organic wastes to produce structurally diverse organochlorine compounds. However, most reactions take place with the lignin fraction producing simple but toxic monoaromatic compounds such as chlorinated phenols, guaiacols, and catechols, as well as high molecular weight chlorolignins.

The latter compounds are thought to be nontoxic, as their large size precludes their penetration or transport across cell membranes. In spite of this, they are not environmentally benign, as they carry chromophoric structures that discolour receiving waters. Also, there is the potential for the release of toxic compounds as the chlorolignins slowly degrade. Their concentration in stack gases from the burning of condensed black liquor is very low. PCDDs and PCDFs also are produced during pulp bleaching, where they are formed from chlorinated phenols, and particularly from chlorinated 2-phenoxyphenols.

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Accordingly, the Swedish pulp industry is responsible for only a very small amount 1. This has been verified by comparison of isomer patterns.

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Environmentally Benign Approaches for Pulp Bleaching. Volume 1 in Developments in Environmental Management. Book • Authors: Pratima Bajpai. ykoketomel.ml: Environmentally Benign Approaches for Pulp Bleaching ( Developments In Environmental Management) (): Pratima Bajpai.

The pulp industry produces primarily 2, 3, 7, 8-tetrachlorodibenzo-p-dioxin and 2, 3, 7, 8-tetrachloro-dibenzo-p-furan, while the isomer profiles from incinerator operations are distinctly different. Although total environmental contribution is quite low, the pulp and paper industry has spent large sums of money on research and development to eliminate or at least greatly reduce these compounds in effluents and paper products. The industry is threatened by publicity and public fear of dioxins, which peaked with the discovery of dioxin in milk cartons.

It has been possible to reduce the dioxin level in milk carton cardboard to 2 parts per trillion, a level indistinguishable from typical background levels in milk. During the past century many chlorinated organic pesticides, solvents, refrigerants, etc. These man-made chemicals are called xenobiotics. They are not natural chemicals, in that they have never been produced in nature by animal or plant metabolism.

As such, they have no natural enemies; that is, microorganisms have not been in contact with these chemicals long enough to evolve the enzyme systems necessary for their rapid degradation. Similarly, many of the chlorinated organic compounds randomly synthesized during pulp bleaching are toxic xenobiotics that will persist for long periods in the environment.

The situation can often be made worse if pulp mill effluents are released to oxygen-limited or depleted anaerobic waters. Certain species of anaerobic bacteria can methylate chlorinated organic compounds, thereby making the compounds less toxic to the bacteria. Unfortunately, this mechanism generally increases both the toxicity and lipophilicity of the compound to higher animals. Lipophilic compounds are preferentially soluble in fats. When a methylated-chlorinated compound is consumed by fish or fowl, rather than being completely eliminated in the urine it may become sequestered in body fat in a phenomenon termed "bioaccumulation.

The magnitude of this waste problem necessitates the identification of those chemicals that pose a significant threat to human health. Although a large variety of chlorinated compounds are present in pulp mill effluents, they are seldom at concentrations that would cause acute toxicity. For example, the minimum lethal oral doses for human beings of the common bleach effluent compounds trichloromethane and pentachlorophenol are The toxic compounds most often identified in aquatic life below pulp and paper mill effluent discharge points are tri-, and tetrachloroguaiacols, tetrachlorocatechols and di-, tri-, tetra-, and pentachlorophenols.

The most toxic of these is 1, 1, 3-trichloroacetone which has been found in bleachery effluents at concentrations up to 2. Considerable attention has also been directed toward identification of the mutagenic and thereby potentially carcinogenic compounds in pulp mill effluents.

Recycling and Deinking of Recovered Paper

Strategies for the evaluation of pulp-mill effluents for mutageni potential have generally followed three approaches: assessment of whole effluent streams, of fractionated effluent streams, and of pure compounds known to be effluent components. In the first approach, authentic waste solutions are tested, in which chemical and physical interactions and possible synergistic effects are maintained.

The many studies performed on whole pulp mill waste streams are all in agreement that the first-chlorination-stage effluent shows high mutagenic activity, while other effluents and process streams show no or reduced mutagenicity. Examination of fractionated effluent streams, and the individual testing of approximately known constituent chemicals of pulp mill effluents, showed that 39 compounds exhibited weak to very strong mutagenic activity. Two resin acids neoabietic and 7-oxodehydroabietic acid , two chlorinated spirodiones, tri-, tetra-, penta-, and hexachloroacetone, and a number of chlorinated alipathic hydrocarbons were identified as mutagenic agents.

Chloroacetones, although demonstrating substantial mutagenicity, do not pose a serious human health risk as these compounds are quite unstable and tend to degrade rapidly in receiving waters. Resin acids have also been found to bioaccumulate in fish at sublethal concentrations. The combination of persistence, bioaccumulation, and mutagenic activity makes these compounds a potential health risk.

The most potent bacterial mutagen in pulp mill effluents was identified as 3-chloro dichloromethyl - 5-hydroxy-2 5 H -furanone MX. In the Ames test, MX demonstrated a mutagenic potential nearly two-fold greater than the extremely potent mould metabolite aflatoxin B1. Accordingly, MX should be deactivated and destroyed at pulp mills where the bleach plant effluent goes directly for conventional waste water treatment. However MX could pose a serious risk if mill effluents were discharged directly to receiving waters. This is particularly so in Canada and Northern Europe where lake and river waters may be slightly acidic due to poor buffering capacity, high humic acid content, and acid rain.

Dioxins are routinely described in the news media as extremely dangerous, toxic, cancer-causing agents. In spite of the extreme public fear of dioxins, it has not been established that the polychlorinated dibenzo-p-dioxins and polychlorinated dibenzo-p-furans pose a serious threat to human health. In these incidents many contaminated animals died; however, human suffering was limited to nausea and cases of severe chloracne. Follow-up studies have not shown an increased incidence of cancer or birth defects. Although dioxin and furan cogeners do not appear to be highly toxic to human beings, they are extremely toxic to certain species of animals.

For example, in the guinea pig the LD50 dosage for dioxin is 0.

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Controlling the concentration and build-up of impurities in the liquor cycle is of paramount importance as more bleach plant effluents are collected for evaporation and burning. Berkeley Workshop on Environmental Politics. In: Neidleman, S. Kovacs, T. The pulp suspen- sion is then pumped to one or more medium-consistency high-shear mixers fed with the ozone—oxygen gas mixture, and then to a pressurized reactor.

Many countries are now legislating discharge limits for pulp and paper mill effluents. For the Swedish permissible discharge limit was set at 1.

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Three Canadian provinces have initiated discharge regulations using the more stringent generic classification of Absorbable Organic Halogen AOX. The Ontario discharge limit was set at 2. A similar preliminary limit was set in British Columbia, with mandatory secondary treatment in , and reduction to 1. In Quebec AOX levels of 1. In December , the Canadian government announced new federal regulations to control pulp mill pollution. As of 1 January , any measurable level of dioxins or furans in pulp mill effluent will constitute a violation of the regulations.

In addition, wood chips that might be contaminated with preservatives and certain defoaming agents can no longer be used in the pulping process as they might contribute to the release of toxic pollutants. Of the many pollutants in pulp and paper mill effluents, only dioxins and furans have been evaluated by Health and Welfare Canada as "priority substances.

For this reason, the federal government legislated specifically on dioxins and furans and did not include other known pollutants. Meeting these discharge limits will not be easy. In some cases it will mean the earlier closure of out-dated plants as the pulp and paper industry phases out old sulphite and Kraft mills and replaces them with new, more competitive, chemithermomechanical pulping plants. Compliance ill definitely mean the adoption of alternative methods to reduce chlorine bleaching and the establishment of high-efficiency treatment facilities. Attainment of pollution abatement goals will also be very expensive.

This mill now meets Ontario AOX discharge regulations. Research and the development of new processes and techniques to curtail the release of toxic effluents from pulp and paper mills are following two approaches. The first strategy is the development of new pulping processes with emphasis on improved delignification, and the replacement or partial replacement of chlorine for bleaching.

The second approach is the development of new biological treatment processes, particularly hybrid or dual systems that capitalize on the advantages afforded by both anaerobic and aerobic digestion. A new process called oxygen delignification has been found to be quite effective in removing the lignin and hemicellulose fraction from cellulose, and leaving the fibres relatively undamaged. The major environmental advantage is greater lignin removal, so that less lignin enters the bleach plant.

No toxic chlorinated organic compounds would be released in mill effluents if chlorine could be completely eliminated from the pulp-bleaching process. Partial replacement of chlorine by chlorine dioxide has been found to reduce greatly the emission of organochlorines, while complete substitution with hydrogen peroxide eliminates these toxic compounds. Sign up. Already have an account? Log in.

You have no items in your shopping cart. Environmentally Benign Approaches for Pulp Bleaching. Add to cart Buy now. Sign in to add to Wish List. Pulp and paper production has increased globally and will continue to increase in the near future. Approximately millions tons of wood pulp is produced worldwide and about millions is projected for Seller Inventory n. Pratima Bajpai; P. Pratima Bajpai ; P. Publisher: Elsevier Science , This specific ISBN edition is currently not available. View all copies of this ISBN edition:. Synopsis About this title Pulp and paper production has increased globally and will continue to increase in the near future.

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