Pulping is the process by which the bonds within the wood structure are ruptured either mechanically or chemically. Chemical pulps can be produced by either alkaline (i.e., sulphate or kraft) or acidic (i.e., sulphite) processes. The highest proportion of pulp is produced by the sulphate method, followed by mechanical (including semi-chemical, thermomechanical and mechanical) and sulphite methods (figure 1). Pulping processes differ in the yield and quality of the product, and for chemical methods, in the chemicals used and the proportion that can be recovered for reuse.

Figure 1. Worldwide pulp capacities, by pulp type

Mechanical Pulping

Mechanical pulps are produced by grinding wood against a stone or between metal plates, thereby separating the wood into individual fibres. The shearing action breaks cellulose fibres, so that the resulting pulp is weaker than chemically separated pulps. The lignin connecting cellulose to hemicellulose is not dissolved; it merely softens, allowing the fibres to be ground out of the wood matrix. The yield (proportion of original wood in pulp) is usually greater than 85%. Some mechanical pulping methods also use chemicals (i.e., the chemi-mechanical pulps); their yields are lower since they remove more of the non-cellulosic materials.

In stone groundwood pulping (SGW), the oldest and historically most common mechanical method, fibres are removed from short logs by pressing them against a rotating abrasive cylinder. In refiner mechanical pulping (RMP, figure 2), which gained popularity after it became commercially viable in the 1960s, wood chips or sawdust are fed through the centre of a disc refiner, where they are shredded into finer pieces as they are pushed out through progressively narrower bars and grooves. (In figure 2, the refiners are enclosed in the middle of the picture and their large motors are on the left. Chips are supplied though the large diameter pipes, and pulp exits the smaller ones.) A modification of RMP is thermomechanical pulping (TMP), in which the chips are steamed before and during refining, usually under pressure.

Figure 2. Refiner mechanical pulping

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One of the earliest methods of producing chemi-mechanical pulps involved pre-steaming logs before boiling them in chemical pulping liquors, then grinding them in stone grinders to produce “chemi-groundwood” pulps. Modern chemi-mechanical pulping uses disc refiners with chemical treatment (e.g., sodium bisulphite, sodium hydroxide) either prior to, during or after refining. Pulps produced in this manner are referred to either as chemi-mechanical pulps (CMP) or chemi-thermomechanical pulps (CTMP), depending on whether refining was carried out at atmospheric or elevated pressure. Specialized variations of CTMP have been developed and patented by a number of organizations.

Chemical Pulping and Recovery

Chemical pulps are produced by chemically dissolving the lignin between the wood fibres, thereby enabling the fibres to separate relatively undamaged. Because most of the non-fibrous wood components are removed in these processes, yields are usually in the order of 40 to 55%.

In chemical pulping, chips and chemicals in aqueous solution are cooked together in a pressure vessel (digester, figure 3) which can be operated on a batch or continuous basis. In batch cooking, the digester is filled with chips through a top opening, the digestion chemicals are added, and the contents cooked at elevated temperature and pressure. Once the cook is complete, the pressure is released, “blowing” the delignified pulp out of the digester and into a holding tank. The sequence is then repeated. In continuous digesting, pre-steamed chips are fed into the digester at a continuous rate. Chips and chemicals are mixed together in the impregnation zone at the top of the digester and then proceed through the upper cooking zone, the lower cooking zone, and the washing zone before being blown into the blow tank.

Figure 3. Continuous kraft digestor, with chip conveyor under construction

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The digesting chemicals are recovered in most chemical pulping operations today. The principal objectives are to recover and reconstitute digestion chemicals from the spent cooking liquor, and to recover heat energy by burning the dissolved organic material from the wood. The resulting steam and electricity supplies some, if not all, of the mill’s energy needs.

Sulphate Pulping and Recovery

The sulphate process produces a stronger, darker pulp than other methods and requires chemical recovery to compete economically. The method evolved from soda pulping (which uses only sodium hydroxide for digestion) and began to gain prominence in the industry from the 1930s to 1950s with the development of chlorine dioxide bleaching and chemical recovery processes, which also produced steam and power for the mill. The development of corrosion-proof metals, such as stainless steel, to handle the acidic and alkaline pulp mill environments also played a role.

The cooking mixture (white liquor) is sodium hydroxide (NaOH, “caustic”) and sodium sulphide (Na₂S). Modern kraft pulping is usually carried out in continuous digesters often lined with stainless steel (figure 3). The temperature of the digester is raised slowly to approximately 170°C and held at that level for approximately 3 to 4 hours. The pulp (called brown stock because of its colour) is screened to remove uncooked wood, washed to remove the spent cooking mixture (now black liquor), and sent either to the bleach plant or to the pulp machine room. Uncooked wood is either returned to the digester or sent to the power boiler to be burned.

The black liquor collected from the digester and brown stock washers contains dissolved organic material whose exact chemical composition depends on the wood species pulped and the cooking conditions. The liquor is concentrated in evaporators until it contains less than 40% water, then sprayed into the recovery boiler. The organic component is consumed as fuel, generating heat which is recovered in the upper section of the furnace as high-temperature steam. The unburned inorganic component collects at the bottom of the boiler as a molten smelt. The smelt flows out of the furnace and is dissolved in a weak caustic solution, producing “green liquor” containing primarily dissolved Na₂S and sodium carbonate (Na₂CO₃). This liquor is pumped to a recausticizing plant, where it is clarified, then reacted with slaked lime
(Ca(OH)₂), forming NaOH and calcium carbonate (CaCO₃). The white liquor is filtered and stored for subsequent use. CaCO₃ is sent to a lime kiln, where it is heated to regenerate lime (CaO).

Sulphite Pulping and Recovery

Sulphite pulping dominated the industry from the late 1800s to the mid-1900s, but the method used during this era was limited by the types of wood which could be pulped and the pollution created by discharging untreated waste cooking liquor into waterways. Newer methods have overcome many of these problems, but sulphite pulping is now a small segment of the pulp market. Although sulphite pulping usually uses acid digestion, both neutral and basic variations exist.

The cooking liquor of sulphurous acid (H₂SO₃) and bisulphite ion (HSO₃^–) is prepared on-site. Elemental sulphur is burned to produce sulphur dioxide (SO₂), which is passed up through an absorption tower that contains water and one of four alkaline bases (CaCO₃, the original sulphite base, Na₂CO₃, magnesium hydroxide (Mg(OH)₂) or ammonium hydroxide (NH₄OH)) which produce the acid and ion and control their proportions. Sulphite pulping is usually carried out in brick-lined batch digesters. To avoid unwanted reactions, the digester is heated slowly to a maximum temperature of 130 to 140°C and the chips are cooked for a long time (6 to 8 hours). As the digester pressure increases, gaseous sulphur dioxide (SO₂) is bled off and remixed with the raw cooking acid. When approximately 1 to 1.5 hours of cooking time remains, heating is discontinued and the pressure is decreased by bleeding off gas and steam. The pulp is blown into a holding tank, then washed and screened.

The spent digestion mixture, called red liquor, can be used for heat and chemical recovery for all but calcium-bisulphite-base operations. For ammonia-base sulphite pulping, the dilute red liquor is first stripped to remove residual SO₂, then concentrated and burned. The flue gas containing SO₂ is cooled and passed through an absorption tower where fresh ammonia combines with it to regenerate the cooking liquor. Finally, the liquor is filtered, fortified with fresh SO₂ and stored. The ammonia cannot be recovered because it is converted into nitrogen and water in the recovery boiler.

In magnesium-base sulphite pulping, burning the concentrated pulping liquor gives magnesium oxide (MgO) and SO₂, which are easily recovered. No smelt is produced in this process; rather MgO is collected from the flue gas and slaked with water to produce magnesium hydroxide (Mg(OH)₂). SO₂ is cooled and combined with the Mg(OH)₂ in an absorption tower to reconstitute the cooking liquor. The magnesium bisulphite (Mg(HSO₃)₂) is then fortified with fresh SO₂ and stored. Recovery of 80 to 90% of the cooking chemicals is possible.

Recovery of sodium-base sulphite cooking liquor is more complicated. Concentrated spent liquor is incinerated, and approximately 50% of the sulphur is converted into SO₂. The remainder of the sodium and sulphur is collected at the bottom of the recovery boiler as a smelt of Na₂S and Na₂CO₃. The smelt is dissolved to produce green liquor, which is converted to sodium bisulphite (NaHSO₃) in several steps. The NaHSO₃ is fortified and stored. The regeneration process produces reduced sulphur gases, in particular hydrogen sulphide (H₂S).

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Adapted from Soskolne 1997, with permission.

Hazardous wastes include, among other things, radioactive materials and chemicals. The movement of these substances from their source to other locations has been termed “toxic trade”. It was in the late 1980s that concern was raised about toxic trade, in particular with Africa (Vir 1989). This set the stage for the recently recognized issue of environmental justice, in some situations also known as environmental racism (Coughlin 1996).

Vir (1989) pointed out that as environmental safety laws became increasingly stringent in Europe and in the United States, and as the cost of disposal increased, “dumpers” or “waste merchants” began to turn their attention to poorer nations as potential and willing recipients of their waste products, providing a much needed source of revenue to these poorer countries. Some of these countries had been willing to take such waste at a fraction of the cost that developed nations would otherwise have had to pay for their disposal. To “nations that are drowning economically, this is an attractive deal” (Vir 1989).

Asante-Duah, Saccomanno and Shortreed (1992) show the exponential growth in the United States in the production of hazardous wastes since 1970, with the costs associated with treatment and disposal similarly increasing. They argue in favour of a controlled hazardous waste trade, one that is “regulated and informed”. They note that “countries generating small quantities of hazardous wastes should view the waste trade as an important economic option, as long as the waste recipients do not compromise their environmental sustainability”. Hazardous wastes will continue to be generated and there are countries for which an increase in some of these substances would not increase the risk to health of either present or future generations. It might therefore be economically efficient for such countries to accept waste.

There are others who argue that waste should be disposed of only at the source and not be transported at all (Puckett and Fogel 1994; Cray 1991; Southam News 1994). The latter argue from the position that science is incapable of providing any guarantees about the absence of risk.

One ethical principle that emerges from the foregoing argument is that of respect for autonomy (i.e., respect for persons), which also includes questions of national autonomy. The crucial question is one of the ability of a recipient country to adequately assess the level of risk associated with a shipment of hazardous waste. Assessment presupposes full disclosure of the contents of a shipment from the originating country and a level of local expertise to assess any potential impacts on the recipient country.

Because communities in developing countries are less likely to be informed about the potential risks associated with waste shipments, the NIMBY phenomenon (i.e., not in my backyard) so evident in the more affluent regions of the world is less likely to manifest in poorer regions. Furthermore, workers in developing regions of the world tend not to have the infrastructure related to worker protection, including information concerning the labelling of products with which they come into contact. Hence, workers in poorer nations involved in the management, storage and disposal of hazardous waste would lack the training to know how to protect themselves. Regardless of these ethical considerations, in the final analysis the economic benefits to be derived from accepting such waste shipments would need to be weighed against any potential harms that could arise in the short, medium and longer terms.

A second ethical principle emerging from the preceding argument is that of distributive justice, which involves question regarding who takes risks and who derives benefits. When there is an imbalance between those who take risks and those who derive benefits, the principle of distributive justice is not being honoured. It has often been financially poor labourers who have been exposed to hazards without any ability to enjoy the fruits of their efforts. This has occurred in the context of production of relatively expensive merchandise in the developing world for the benefit of first world markets. Another example related to the testing of new vaccines or drugs on people in developing countries who could never afford access to them in their own countries.

Towards Controlling the Transport of Hazardous Wastes

Because of the recognized need to better control the dumping of hazardous wastes, the Basel Convention was entered into by ministers of 33 countries in March 1989 (Asante-Duah, Saccomanno and Shortreed 1992). The Basel Convention addressed the transboundary movements of hazardous wastes and required the notification and consent of recipient countries before any waste shipments could take place.

Subsequently, the United Nations Environment Programme (UNEP) launched its Cleaner Production Programme, in close cooperation with governments and industry, to advocate low- and non-waste technologies (Rummel-Bulska 1993). In March 1994, a full ban was introduced on all transboundary movements of hazardous wastes from the 24 rich industrialized countries of the Organization for Economic Cooperation and Development (OECD) to other states that are not members of the OECD. The ban was immediate for wastes bound for final disposal and enters into force at the beginning of 1998 for all hazardous wastes that are said to be destined for recycling or recovery operations (Puckett and Fogel 1994). The countries most opposed to the introduction of a total ban were Australia, Canada, Japan and the United States. Despite this opposition from a handful of powerful industrial governments through the penultimate vote, the ban was finally agreed to by consensus (Puckett and Fogel 1994).

Greenpeace has stressed the primary prevention approach to solving the mounting waste crisis by addressing the root cause of the problem, namely minimizing waste generation through clean production technologies (Greenpeace 1994a). In making this point, Greenpeace identified major countries exporting hazardous wastes (Australia, Canada, Germany, the United Kingdom and the United States) and some countries importing them (Bangladesh, China (including Taiwan), India, Indonesia, Malaysia, Pakistan, the Philippines, the Republic of Korea, Sri Lanka and Thailand). In 1993, Canada, for example, had exported some 3.2 million kilograms of ash containing lead and zinc to India, the Republic of Korea and Taiwan, China, and 5.8 million kilograms of plastic waste to Hong Kong (Southam News 1994). Greenpeace (1993, 1994b) also addresses the extent of the problem in terms of specific substances and approaches to disposal.

Risk Assessment

Epidemiology is at the centre of human health risk assessment, which is invoked when concern is raised by a community about the consequences, if any, of exposure to hazardous and potentially toxic substances. The scientific method that epidemiology brings to the study of the environmental determinants of ill health can be fundamental to protecting unempowered communities, both from environmental hazards and from environmental degradation. Risk assessment conducted in advance of a shipment likely would fall into the legal trade arena; when conducted after a shipment has arrived, risk assessment would be undertaken to determine whether or not any health concerns were justified from what likely would have been an illegal shipment.

Among the concerns to the risk assessor would be hazard assessment, i.e., questions about what hazards, if any, exist and in what quantities and in what form they might be present. In addition, depending on the type of hazard, the risk assessor must make an exposure assessment to establish what possibilities there are for people to be exposed to the hazardous substance(s) through inhalation, skin absorption or ingestion (by contamination of the food chain or directly on foodstuffs).

In terms of trade, autonomy would require the informed consent of the parties in a voluntary and non-coercive milieu. However, it is hardly possible that non-coerciveness could ever pertain in such a circumstance by virtue of the financial need of an importing developing world country. The analogue here is the now accepted ethical guideline which does not permit the coercion of participants in research through payment for anything but direct costs (e.g., lost wages) for the time taken to participate in a study (CIOMS 1993). Other ethical issues involved here would include, on the one hand, truth in the presence of unknowns or in the presence of scientific uncertainty and, on the other hand, the principle of caveat emptor (buyer beware). The ethical principle of non-maleficence requires the doing of more good than harm. Here the short-term economic benefits of any trade agreement to accept toxic wastes must be weighed against the longer term damage to the environment, the public health and possibly also to future generations.

Finally, the principle of distributive justice requires recognition by the parties involved in a trade deal as to who would be deriving the benefits and who would be taking the risks in any trade deal. In the past, general practices for dumping waste and for locating hazardous waste sites in unempowered communities in the United States have led to the recognition of the concern now known as environmental justice or environmental racism (Coughlin 1996). In addition, questions of environmental sustainability and integrity have become central concerns in the public forum.

Acknowledgements: Dr. Margaret-Ann Armour, Department of Chemistry, University of Alberta, provided valuable references on the topic of toxic trade as well as materials from the November 1993 Pacific Basin “Conference on Hazardous Waste” at the University of Hawaii.

The Greenpeace office in Toronto, Ontario, Canada, was most helpful in providing copies of the Greenpeace references cited in this article.

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Workers involved in municipal waste disposal and handling face occupational health and safety hazards which are as diverse as the materials they are handling. Workers’ primary complaints relate to odour and upper respiratory tract irritation usually related to dust. However, actual occupational health and safety concerns vary with the work process and the waste stream characteristics (mixed municipal solid waste (MSW), sanitary and biological waste, recycled wastes, agricultural and food wastes, ash, construction debris and industrial wastes). Biological agents such as bacteria, endotoxins and fungi may present hazards, particularly for immune system-compromised and hypersensitive workers. In addition to safety concerns, health impacts have involved predominantly respiratory health problems among workers, including symptoms of organic dust toxic syndrome (ODTS), irritation of the skin, eyes and upper airways and cases of more severe pulmonary diseases such as asthma, alveolitis and bronchitis.

The World Bank (Beede and Bloom 1995) estimates that 1.3 billion tonnes of MSW were generated in 1990 which represents an average of two-thirds of a kilogram per person per day. In the US alone, an estimated 343,000 workers were involved in MSW collection, transport and disposal according to 1991 US Census Bureau statistics. In industrialized countries waste streams are increasingly distinct and work processes are increasingly complex. Efforts to segregate and better define the compositions of waste streams are often critical for identifying occupational hazards and appropriate controls and for controlling environmental impacts. Most waste disposal workers continue to face unpredictable exposures and risks from mixed wastes in dispersed open dumps, often with open burning.

The economics of waste disposal, reuse and recycling, as well as public health concerns, are driving rapid changes in waste handling globally to maximize recovery of resources and reduce dispersion of refuse into the environment. Depending on local economic factors this results in the adoption of either increasingly labour-intensive or capital-intensive work processes. Labour-intensive practices draw an increasing number of workers into hazardous work environments and commonly involve informal sector scavengers who sort mixed refuse by hand and sell recyclable and reusable materials. Increased capitalization has not automatically led to improvements in working conditions as increased work within confined spaces (e.g., in drum composting operations or incinerators), and increased mechanical processing of wastes can result in increased exposure to both airborne contaminants and mechanical hazards, unless proper controls are implemented.

Waste Disposal Processes

A variety of waste disposal processes are used, and as waste collection, transportation and disposal costs increase to meet increasingly stringent environmental and community standards, an increasing diversity of processes can be cost-justified. These processes break down into four basic approaches which may be used in combination or in parallel for various waste streams. The four basic processes are dispersal (land or water dumping, evaporation), storage/isolation (sanitary and hazardous waste landfills), oxidation (incineration, composting) and reduction (hydrogenation, anaerobic digestion). These processes share some general occupational hazards associated with waste handling, but also involve work-process-specific occupational hazards.

General Occupational Hazards in Waste Handling

Regardless of the specific disposal process being utilized, simply processing MSW and other wastes involves common defined hazards (Colombi 1991; Desbaumes 1968; Malmros and Jonsson 1994; Malmros, Sigsgaard and Bach 1992; Maxey 1978; Mozzon, Brown and Smith 1987; Rahkonen, Ettala and Loikkanen 1987; Robazzi et al. 1994).

Unidentified, highly hazardous materials are often intermixed with normal waste. Pesticides, flammable solvents, paints, industrial chemicals, and biohazardous waste, may all be intermixed with household waste. This hazard can be handled primarily through segregation of the waste stream and in particular separation of industrial and household waste.

Odours and exposure to mixed volatile organic compounds (VOCs) can induce nausea but are typically well below American Conference of Governmental Industrial Hygenists (ACGIH) threshold limit values (TLVs), even within enclosed spaces (ACGIH 1989; Wilkins 1994). Control typically involves isolation of the process, as in sealed anaerobic digesters or drum composters, minimizing worker contact through daily soil cover or transfer station cleanup and controlling biological degradation processes, particularly minimizing anaerobic degradation by controlling moisture content and aeration.

Insect- and rodent-borne pathogens can be controlled through daily cover of waste with soil. Botros et al. (1989) reported that 19% of garbage workers in Cairo had antibodies to Rickettsia Typhi (from fleas) which causes human rickettsial disease.

Injection or blood contact with infectious waste, such as needles and blood soiled waste, is best controlled at the generator by segregation and sterilization of such waste prior to disposal and disposal in puncture resistant containers. Tetanus is also a real concern should skin damage occur. Up-to-date immunization is required.

Ingestion of Giardia sp. and other gastrointestinal pathogens can be controlled by minimizing handling, reducing hand-to-mouth contact (including tobacco use), supplying safe drinking water, providing toilet and clean up facilities for workers and maintaining appropriate temperature in composting operations in order to destroy pathogens prior to dry handling and bagging. Precautions are particularly appropriate for Giardia found in sewage sludge and disposable baby diapers in MSW, as well as for tape and round worms from poultry and slaughterhouse wastes.

Inhalation of airborne bacteria and fungi is of particular concern when mechanical processing increases (Lundholm and Rylander 1980) with compactors (Emery et al. 1992), macerators or shredders, aeration, bagging operations and when moisture content is allowed to drop. This results in increased respiratory disorders (Nersting et al. 1990), bronchial obstruction (Spinaci et al. 1981) and chronic bronchitis (Ducel et al. 1976). Although there are no formal guidelines, the Dutch Occupational Health Association (1989) recommended that total bacteria and fungal counts should be kept below 10,000 colony forming units per cubic metre (cfu/m³) and below 500 cfu/m³ for any single pathogenic organism (outdoor air levels are about 500 cfu/m³ for total bacteria, indoor air is typically less). These levels may be regularly exceeded in composting operations.

Biotoxins are formed by fungi and bacteria including endotoxins formed by gram-negative bacteria. Inhaling or ingesting an endotoxin, even after killing the bacteria which produced it, can cause fever and flu-like symptoms without infection. The Dutch Working Group on Research Methods in Biological Indoor Air Pollution recommends that airborne gram-negative bacteria be kept below 1000 cfu/m³ to avoid endotoxin effects. Bacteria and fungi can produce a variety of other potent toxins which may also present occupational hazards.

Heat exhaustion and heat stroke can be serious concerns particularly where safe drinking water is limited and where PPE is utilized in sites known to contain hazardous wastes. Simple PVC-Tyvek suits show a heat stress equivalent of adding 6 to 11°C (11 to 20°F) to the ambient wet bulb globe temperature (WBGT) index (Paull and Rosenthal 1987). When the WBGT exceeds 27.7°C (82°F) conditions are considered hazardous.

Skin damage or disease are common complaints in waste handling operations (Gellin and Zavon 1970). Direct skin damage from caustic ash and other irritating waste contaminants, combined with high exposures to pathogenic organisms, frequent skin lacerations and punctures and, typically, poor availability of washing facilities result in a high incidence of skin problems.

Wastes contain a variety of materials that can cause lacerations or punctures. These are of particular concern in labour intensive operations such as waste sorting for recycling or manual turning of MSW compost and where mechanical processes such as compacting, crushing or shredding can create projectiles. The most critical control measures are safety glasses and puncture and slash resistant footwear and gloves.

Vehicular-use hazards include both operator hazards such as rollover and engulfment hazards and collision hazards with workers on the ground. Any vehicle that works on unsound or irregular surfaces should be equipped with rollover cages that will support the vehicle and allow the operator to survive. Pedestrian and vehicular traffic should be separated to the extent possible into distinct traffic areas, particularly where visibility is limited such as during open burning, at night and in composting yards where dense ground fogs may develop in cold weather.

Reports of increased atopic bronchopulmonary reactions such as asthma (Sigsgaard, Bach and Malmros 1990) and skin reactions can occur in waste workers, particularly where organic dust exposure levels are high.

Process-specific Hazards

Dispersion

Dispersion includes dumping waste into bodies of water, evaporation into the air or dumping with no effort at containment. Ocean dumping of MSW and hazardous wastes is rapidly declining. However, an estimated 30 to 50% of MSW is not collected in the cities of developing countries (Cointreau-Levine 1994) and is commonly burned or dumped in canals and streets, where it presents a significant public health threat.

Evaporation, sometimes with active heating at low temperatures, is used as a cost-saving alternative to incinerators or kilns, especially for volatile liquid organic contaminants such as solvents or fuel which are mixed with non-combustible wastes such as soil. Workers may face confined-space entry hazards and explosive atmospheres, especially in maintenance operations. Such operations should incorporate appropriate air emissions controls.

Storage/isolation

Isolation involves a combination of remote locations and physical containment in increasingly secure landfills. Typical sanitary landfills involve excavation with earth moving equipment, dumping of waste, compaction and daily cover with soil or compost to reduce pest infestations, odours and dispersion. Clay or impervious plastic caps and/or liners may be installed to limit water infiltration and leachate into groundwater. Test wells may be used to evaluate off-site leachate migration and to allow monitoring of leachate within the landfill. Workers include heavy equipment operators, truck drivers, spotters who may be responsible for rejecting hazardous waste and directing vehicle traffic flows and informal sector scavengers who may sort the waste and remove recyclables.

In areas dependent on coal or wood for fuel, ash can constitute a significant portion of the waste. Quenching prior to dumping, or segregation into ash monofills, may be necessary to avoid fires. Ash can cause skin irritation and caustic burns. Fly ash presents a variety of health hazards including respiratory and mucosal irritation as well as acute respiratory distress (Shrivastava et al. 1994). Low density fly ash can also constitute an engulfment hazard and can be unstable under heavy equipment and in excavations.

In many nations waste disposal continues to consist of simple dumping with open burning, which may be combined with informal scavenging of reusable or recyclable components with value. These informal sector workers face serious safety and health hazards. It is estimated that in Manila, Philippines, 7,000 scavengers work at the MSW dump, 8,000 in Jakarta and 10,000 in Mexico City (Cointreau-Levine 1994). Because of the difficulties in controlling work practices in informal work, an important step in controlling these hazards is to move separation of recyclables and reusables into the formal waste collection process. This may be performed by the waste generators, including consumers or household workers, by collection/sorting workers (e.g., in Mexico City collection workers officially spend 10% of their time sorting waste for sale of recyclables, and in Bangkok 40% (Beede and Bloom 1995)) or in pre-disposal waste separation operations (e.g., magnetic separation of metallic waste).

Open burning exposes workers to a potentially toxic mix of degradation products as discussed below. Because open burning can be used by informal scavengers to assist in separating metal and glass from combustible waste, it may be necessary to recover materials with salvage value prior to dumping in order to eliminate such open burning.

As hazardous wastes are successfully segregated from the waste stream, risks of MSW workers are reduced while quantities handled by hazardous waste site workers increase. Highly secure hazardous waste treatment and disposal sites depend on detailed manifesting of waste composition, high levels of worker PPE, and extensive worker training to control hazards. Secure landfills have unique hazards including slip and fall hazards where excavations are lined with plastic or polymer gels to reduce migration of leachate, potentially serious dermatological problems, heat stress related to work for extended periods in impermeable suits and supplied air quality control. Heavy equipment operators, labourers and technicians depend largely on PPE to minimize their exposures.

Oxidation (incineration and composting)

Open burning, incineration and waste-derived fuel are the most obvious examples of oxidation. Where the moisture content is low enough and the combustible content is high enough, increasing effort is made to utilize the fuel value in MSW either through the generation of waste-derived fuel as compressed briquettes or by incorporating electrical cogeneration or steam plants into municipal waste incinerators. Such operations can involve high levels of dry dusts due to efforts to produce a fuel with consistent heat value. Residual ash must still be disposed of, usually in landfills.

MSW incinerators involve a variety of safety hazards (Knop 1975). Swedish MSW incinerator workers showed increased ischemic heart disease (Gustavsson 1989), while a study of US incinerator workers in Philadelphia, Pennsylvania, failed to show a correlation between health outcomes and exposure groups (Bresnitz et al. 1992). Somewhat elevated blood lead levels have been identified in incinerator workers, primarily related to exposures to electrostatic precipitator ash (Malkin et al. 1992).

Ash exposures (e.g., crystalline silica, radioisotopes, heavy metals) can be significant not only in incinerator operations, but also at landfills and lightweight concrete plants where ash is used as aggregate. Although crystalline silica and heavy metal content vary with the fuel, this may present serious silicosis risk. Schilling (1988) observed lung function and respiratory symptom effects in ash exposed workers, but no changes observable by x ray.

Thermal degradation on pyrolysis products resulting from incomplete oxidation of many waste products can pose significant health risks. These products can include hydrogen chloride, phosgene, dioxins and dibenzofurans from chlorinated wastes, such as polyvinyl chloride (PVC) plastics and solvents. Non-halogenated wastes also can produce hazardous degradation products, including polyaromatic hydrocarbons, acrolein, cyanide from wools and silk, isocyanates from polyurethane and organotin compounds from a variety of plastics. These complex mixtures of degradation products can vary tremendously with waste composition, feed rates, temperature and available oxygen during combustion. While these degradation products are a significant concern in open burning, exposures in MSW incinerator workers appear to be relatively low (Angerer et al. 1992).

In MSW and hazardous waste incinerators and rotary kilns, control of combustion parameters and the residence time for waste vapours and solids at high temperatures is critical in destruction of wastes while minimizing the generation of more hazardous degradation products. Workers are involved in incinerator operation, loading and waste transfer into the incinerator, waste delivery and unloading from trucks, equipment maintenance, housekeeping and ash and slag removal. While incinerator design can limit necessary manual labour and worker exposures, with less capital-intensive designs there may be significant worker exposures and a need for regular confined space entry (e.g., chipping for removal of slag from glass waste from incinerator grates).

Composting

In aerobic biological processes the temperature and speed of oxidation are lower than incineration, but it is nevertheless oxidation. Composting of agricultural and yard wastes, sewage sludge, MSW and food wastes is increasingly common in city-scale operations. Rapidly developing technologies for biological remediation of hazardous and industrial wastes often involve a sequence of aerobic and anaerobic digestion processes.

Composting usually occurs either in wind rows (long piles) or in large vessels which provide aeration and mixing. The objective of composting operations is to create a mix of waste with optimum ratios of carbon and nitrogen (30:1) and then maintain moisture at 40 to 60% by weight, greater than 5% oxygen and temperature levels 32 to 60^oC so that aerobic bacteria and other organisms can grow (Cobb and Rosenfield 1991). Following separation of recyclables and hazardous wastes (which typically involves hand sorting), MSW is shredded to create more surface area for biological action. Shredding can produce high noise and dust levels and significant mechanical guarding concerns. Some operations use ganged hammer-mills to allow reduced front-end sorting.

In-vessel or drum composting operations are capital intensive but allow more effective odour and process control. Confined space entry is a significant hazard for maintenance workers as high levels of CO₂ may be released causing oxygen deficiency. Lockout of equipment prior to maintenance is also critical as mechanisms include internal screw-drives and conveyors.

In less capital intensive wind row composting operations, waste is shredded and placed in long piles which are mechanically aerated through perforated pipes or simply by turning, either with front-end loaders or manually. Wind rows may be covered or roofed to facilitate maintenance of constant moisture content. Where specialized wind row turning equipment is used, chain mixing-flails rotate at high speed through the compost and should be well guarded from human contact. As these flails rotate through the wind row, they eject objects which can become dangerous projectiles. Operators must assure safe clearance distances around and behind the equipment.

Regular temperature measurements with probes allow monitoring the progress of composting and assure high enough temperatures to kill pathogens while allowing adequate survival of beneficial organisms. At moisture contents of 20 to 45% when the temperature exceeds 93^oC there can also be a spontaneous combustion fire hazard (much like a silo fire). This is most likely to occur when piles exceed 4 m in height. Fires can be avoided by keeping pile heights below 3 m, and turning when the temperature exceeds 60°C. Facilities should provide water hydrants and adequate access between wind rows for control of fires.

Hazards in composting operations include vehicle and mechanical hazards resulting from tractors and trucks involved in turning wind-rows of waste to maintain aeration and moisture content. In cooler climates the elevated temperatures of compost can produce dense ground fogs in a work area occupied by heavy equipment operators and pedestrian workers. Compost workers report more nausea, headache and diarrhoea than their counterparts in a drinking water plant (Lundholm and Rylander 1980). Odour problems can occur as a result of poor control of the moisture and air required for the composting to progress. If anaerobic conditions are allowed to occur, hydrogen sulphide, amines and other odorous materials are generated. In addition to typical disposal worker concerns, composting involving actively growing organisms can raise MSW temperatures high enough to kill pathogens, but can also produce exposures to moulds and fungi and their spores and toxins, especially in compost bagging operations and where compost is allowed to dry. Several studies have evaluated airborne fungi, bacteria, endotoxins and other contaminants (Belin 1985; Clark, Rylander and Larsson 1983; Heida, Bartman and van der Zee 1975; Lacey et al. 1990; Millner et al. 1994; van der Werf 1996; Weber et al. 1993) in composting operations. There is some indication of increased respiratory disorders and hypersensitivity reactions in compost workers (Brown et al. 1995; Sigsgaard et al. 1994). Certainly bacterial and fungal respiratory infections (Kramer, Kurup and Fink 1989) are a concern for immune-suppressed workers such as those with AIDS and those receiving cancer chemotherapy.

Reduction (hydrogenation and anaerobic digestion)

Anaerobic digestion for sewage and agricultural waste involves closed tanks, often with rotating brush contacts if nutrients are dilute, which can pose serious confined space entry concerns for maintenance workers. Anaerobic digesters are also commonly used in many countries as methane generators which may be fuelled with agricultural, sanitary or food wastes. Methane collection from MSW landfills and burning or compression for use is now required in many countries when methane generation exceeds specified thresholds, but most landfills have inadequate moisture for anaerobic digestion to proceed efficiently. Hydrogen sulphide generation is also a common result of anaerobic digestion and can cause eye irritation and olfactory fatigue at low levels.

More recently, high temperature reduction/hydrogenation has become a treatment option for organic chemical wastes. This can involve smaller, and therefore potentially mobile, installations with less energy input than a high temperature incinerator because metallic catalysts allow hydrogenation to proceed at lower temperatures. Organic wastes can be converted into methane and used as fuel to continue the process. Critical worker safety concerns include explosive atmospheres and confined space entry for cleaning, sludge removal and maintenance, hazards of transporting and loading the liquid feed wastes and spill response.

Summary

As wastes are viewed as resources for recycling and reuse, waste processing increases, resulting in rapid change in the waste disposal industry globally. Occupational health and safety risks of waste disposal operations often go beyond obvious safety hazards to a variety of chronic and acute health concerns. These hazards are often faced with minimal PPE and inadequate sanitary and wash-up facilities. Industrial waste reduction and pollution prevention efforts are increasingly shifting recycling and reuse processes away from contracted or external waste disposal operations and into production work areas.

Top priorities in controlling occupational safety and health hazards in this rapidly changing industry sector should include:

• integrating informal sector work into the formal work process

• providing adequate toilet and wash-up facilities and safe drinking water

• eliminating open burning and waste dispersion into the environment

• segregating waste streams to facilitate characterization of wastes and identification of appropriate control measures and work practices

• minimizing mixed vehicular and pedestrian traffic in work areas

• following appropriate excavation practices for soil and waste characteristics

• anticipating and controlling hazards prior to entry into confined spaces

• minimizing respirable dust exposures in high dust operations

• using safety glasses and slash and puncture resistant shoes and gloves

• integrating occupational safety and health concerns when introducing process change plans, particularly during transitions from open dumping and landfills to more complex and potentially more hazardous enclosed operations such as composting, mechanical or manual separation for recycling, waste to energy operations or incinerators.

In this period of rapid change in the industry, significant improvements in worker health and safety can be made at low cost.

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Wood may arrive at a pulp mill woodyard in the form of raw logs or as chips from a lumber mill. Some pulp mill operations have on-site sawmills (often called “woodrooms”) which produce both marketable lumber and stock for the pulp mill. Sawmilling is discussed in detail in the chapter Lumber. This article discusses those elements of wood preparation which are specific to pulp mill operations.

The wood preparation area of a pulp mill has several basic functions: to receive and meter the wood supply to the pulping process at the rate demanded by the mill; to prepare the wood so that it meets the mill’s feed specifications for species, cleanliness and dimensions; and to collect any material rejected by the previous operations and send it to final disposal. Wood is converted into chips or logs suitable for pulping in a series of steps which may include debarking, sawing, chipping and screening.

Logs are debarked because bark contains little fibre, has a high extractives content, is dark, and often carries large quantities of grit. Debarking can be done hydraulically with high-pressure water jets, or mechanically by rubbing logs against each other or with metal cutting tools. Hydraulic debarkers may be used in coastal areas; however, the effluent generated is difficult to treat and contributes to water pollution.

Debarked logs may be sawn into short lengths (1 to 6 metres) for stone groundwood pulping or chipped for refiner mechanical or chemical pulping methods. Chippers tend to produce chips with a considerable size range, but pulping requires chips of very specific dimensions to ensure constant flow through refiners and uniform cooking in digesters. Chips are therefore passed over a series of screens whose function is to separate chips on the basis of length or thickness. Oversized chips are rechipped, while undersized chips are either used as waste fuel or are metered back into the chip flow.

The requirements of the particular pulping process and chip conditions will dictate the duration of chip storage (figure 1; note the different types of chips available for pulping). Depending on fibre supply and mill demand, a mill will maintain a 2 to 6 week unscreened chip inventory, usually in large outdoor chip piles. Chips may degrade through auto-oxidation and hydrolysis reactions or fungal attack of the wood components. In order to avoid contamination, short-term inventories (hours to days) of screened chips are stored in chip silos or bins. Chips for sulphite pulping may be stored outside for several months to allow volatilization of extractives which may cause problems in subsequent operations. Chips used in kraft mills where turpentine and tall oil are recovered as commercial products typically proceed directly to pulping.

Figure 1. Chip storage area with front end loaders

George Astrakianakis

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