Table 1 provides an overview of the types of exposures which may be expected in each area of pulp and paper operations. Although exposures may be listed as specific to certain production processes, exposures to employees from other areas may also occur depending on weather conditions, proximity to sources of exposure, and whether they work in more than one process area (e.g., quality control, general labour pool and maintenance personnel).

Table 1. Potential health and safety hazards in pulp and paper production, by process area

RMP = refining mechanical pulping; CMP = chemi-mechanical pulping; CTMP = chemi-thermomechanical pulping.

Exposure to the potential hazards listed in table 1 is likely to depend on the extent of automation of the plant. Historically, industrial pulp and paper production was a semi-automatic process which required a great deal of manual intervention. In such facilities, operators would sit at open panels adjacent to the processes to view the effects of their actions. The valves at the top and bottom of a batch digester would be manually opened, and during the filling stages, gases in the digester would be displaced by the incoming chips (figure 1). Chemical levels would be adjusted based on experience rather than sampling, and process adjustments would be dependent on the skill and knowledge of the operator, which at times led to upsets. For example, over-chlorination of pulp would expose workers downstream to increased levels of bleaching agents. In most modern mills, progress from manually controlled to electronically controlled pumps and valves allows for remote operation. The demand for process control within narrow tolerances has required computers and sophisticated engineering strategies. Separate control rooms are used to isolate the electronic equipment from the pulp and paper production environment. Consequently, operators usually work in air-conditioned control rooms which offer refuge from the noise, vibration, temperature, humidity and chemical exposures inherent to mill operations. Other controls which have improved the working environment are described below.

Figure 1. Worker opening cap on manually controlled batch digester.

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Safety hazards including nip points, wet walking surfaces, moving equipment and heights are common throughout pulp and paper operations. Guards around moving conveyors and machinery parts, quick clean-up of spills, walking surfaces which allow drainage, and guard-rails on walkways adjacent to production lines or at height are all essential. Lock-out procedures must be followed for maintenance of chip conveyors, paper machine rolls and all other machinery with moving parts. Mobile equipment used in chip storage, dock and shipping areas, warehousing and other operations should have roll-over protection, good visibility and horns; traffic lanes for vehicles and pedestrians should be clearly marked and signed.

Noise and heat are also ubiquitous hazards. The major engineering control is operator enclosures, as described above, usually available in wood preparation, pulping, bleaching and sheet-forming areas. Air-conditioned enclosed cabs for mobile equipment used in chip pile and other yard operations are also available. Outside these enclosures, workers usually require hearing protection. Work in hot process or outdoor areas and in vessel maintenance operations requires workers to be trained to recognize symptoms of heat stress; in such areas, work scheduling should allow acclimatization and rest periods. Cold weather may create frostbite hazards in outdoor jobs, as well as foggy conditions near chip piles, which remain warm.

Wood, its extracts and associated micro-organisms are specific to wood preparation operations and the initial stages of pulping. Control of exposures will depend on the particular operation, and may include operator booths, enclosure and ventilation of saws and conveyors, as well as enclosed chip storage and low chip inventory. Use of compressed air to clear wood dust creates high exposures and should be avoided.

Chemical pulping operations present the opportunity for exposures to digestion chemicals as well as gaseous by-products of the cooking process, including reduced (kraft pulping) and oxidized (sulphite pulping) sulphur compounds and volatile organics. Gas formation may be influenced by a number of operating conditions: the wood species used; the quantity of wood pulped; the amount and concentration of white liquor applied; the amount of time required for pulping; and maximum temperature attained. In addition to automatic digester capping valves and operator control rooms, other controls for these areas include local exhaust ventilation at batch digesters and blow tanks, capable of venting at the rate the vessel’s gases are released; negative pressure in recovery boilers and sulphite-SO₂ acid towers to prevent gas leaks; ventilated full or partial enclosures over post-digestion washers; continuous gas monitors with alarms where leaks may occur; and emergency response planning and training. Operators taking samples and conducting tests should be aware of the potential for acid and caustic exposure in process and waste streams, and the possibility of side reactions such as hydrogen sulphide gas (H₂S) production if black liquor from kraft pulping comes into contact with acids (e.g., in sewers).

In chemical recovery areas, acidic and alkaline process chemicals and their by-products may be present at temperatures in excess of 800°C. Job responsibilities may require workers to come into direct contact with these chemicals, making heavy duty clothing a necessity. For example, workers rake the spattering molten smelt that collects at the base of the boilers, thereby risking chemical and thermal burns. Workers may be exposed to dust when sodium sulphate is added to concentrated black liquor, and any leak or opening will release noxious (and potentially fatal) reduced sulphur gases. The potential for a smelt water explosion always exists around the recovery boiler. Water leaks in the tube walls of the boiler have resulted in several fatal explosions. Recovery boilers should be shut down at any indication of a leak, and special procedures should be implemented for transferring the smelt. Loading of lime and other caustic materials should be done with enclosed and ventilated conveyors, elevators and storage bins.

In bleach plants, field operators may be exposed to the bleaching agents as well as chlorinated organics and other by-products. Process variables such as bleaching chemical strength, lignin content, temperature and pulp consistency are constantly monitored, with operators collecting samples and performing laboratory tests. Because of the hazards of many of the bleaching agents used, continuous alarm monitors should be in place, escape respirators should be issued to all employees, and operators should be trained in emergency response procedures. Canopy enclosures with dedicated exhaust ventilation are standard engineering controls found at the top of each bleaching tower and washing stage.

Chemical exposures in the machine room of a pulp or paper mill include chemical carry-over from the bleach plant, the papermaking additives and the chemical mixture in the waste water. Dusts (cellulose, fillers, coatings) and exhaust fumes from mobile equipment are present in the dry-end and the finishing operations. Cleaning between product runs may be done with solvents, acids and alkalis. Controls in this area may include complete enclosure over the sheet drier; ventilated enclosure of the areas where additives are unloaded, weighed and mixed; use of additives in liquid rather than powder form; use of water-based rather than solvent-based inks and dyes; and eliminating the use of compressed air to clean up trimmed and waste paper.

Paper production in recycled paper plants is generally dustier than conventional paper production using newly produced pulp. Exposure to micro-organisms can occur from the beginning (paper collection and separation) to the end (paper production) of the production chain, but exposure to chemicals is less important than in conventional paper production.

Pulp and paper mills employ an extensive maintenance group to service their process equipment, including carpenters, electricians, instrument mechanics, insulators, machinists, masons, mechanics, millwrights, painters, pipefitters, refrigeration mechanics, tinsmiths and welders. Along with their trade-specific exposures (see the Metal processing and metal working and Occupations chapters), these tradespeople may be exposed to any of the process-related hazards. As mill operations have become more automated and enclosed, the maintenance, cleaning and quality assurance operations have become the most highly exposed. Plant shutdowns to clean vessels and machines are of special concern. Depending on mill organization, these operations may be carried out by in-house maintenance or production personnel, although subcontracting to non-mill personnel, who may have less occupational health and safety support services, is common.

In addition to process exposures, pulp and paper mill operations entail some noteworthy exposures for maintenance personnel. Because pulping, recovery and boiler operations involve high heat, asbestos was used extensively to insulate pipes and vessels. Stainless steel is often used in vessels and pipes throughout pulping, recovery and bleaching operations, and to some extent in papermaking. Welding this metal is known to generate chromium and nickel fumes. During maintenance shut-downs, chromium-based sprays may be applied to protect the floor and walls of recovery boilers from corrosion during start-up operations. Process quality measurements in the production line are often made using infrared and radio-isotope gauges. Although the gauges are usually well shielded, instrument mechanics who service them may be exposed to radiation.

Some special exposures may also occur among employees in other mill-support operations. Power boiler workers handle bark, waste wood and sludge from the effluent treatment system. In older mills, workers remove ash from the bottom of the boilers and then reseal the boilers by applying a mixture of asbestos and cement around the boiler grate. In modern power boilers, this process is automated. When material is fed into the boiler at too high a moisture level, workers may be exposed to blow-backs of incomplete combustion products. Workers responsible for water treatment may be exposed to chemicals such as chlorine, hydrazine and various resins. Because of the reactivity of ClO₂, the ClO₂ generator is usually located in a restricted area and the operator is stationed in a remote control room with excursions to collect samples and service the saltcake filter. Sodium chlorate (a strong oxidizer) used to generate ClO₂ can become dangerously flammable if it is allowed to spill on any organic or combustible material and then dry. All spills should be wetted down before any maintenance work may proceed, and all equipment should be thoroughly cleaned afterward. Wet clothing should be kept wet and separate from street clothing, until washed.

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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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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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The basic structure of pulp and paper sheets is a felted mat of cellulose fibres held together by hydrogen bonds. Cellulose is a polysaccharide with 600 to 1,500 repeated sugar units. The fibres have high tensile strength, will absorb the additives used to modify pulp into paper and board products, and are supple, chemically stable and white. The purpose of pulping is to separate cellulose fibres from the other components of the fibre source. In the case of wood, these include hemicelluloses (with 15 to 90 repeated sugar units), lignins (highly polymerized and complex, mainly phenyl propane units; they act as the “glue” that cements the fibres together), extractives (fats, waxes, alcohols, phenols, aromatic acids, essential oils, oleoresins, stearols, alkaloids and pigments), and minerals and other inorganics. As shown in table 1, the relative proportions of these components vary according to the fibre source.

Table 1. Chemical constituents of pulp and paper fibre sources (%)

nd = no data available.

Coniferous and deciduous trees are the major fibre sources for pulp and paper. Secondary sources include straws from wheat, rye and rice; canes, such as bagasse; woody stalks from bamboo, flax and hemp; and seed, leaf or bast fibres, such as cotton, abaca and sisal. The majority of pulp is made from virgin fibre, but recycled paper accounts for an increasing proportion of production, up from 20% in 1970 to 33% in 1991. Wood-based production accounted for 88% of worldwide pulp capacity in 1994 (176 million tonnes, figure 1); therefore, the description of pulp and paper processes in the following article focuses on wood-based production. The basic principles apply to other fibres as well.

Figure 1. Worldwide pulp capacities, by pulp type

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Use and Disposal of Wood Waste

By-products of the lumber industry which can cause environmental problems may include air emissions, liquid effluent and solid wastes. Most of these problems arise from waste wood, which may include wood chips or sawdust from milling operations, bark from debarking operations and log debris in waterways where logs are stored.

Sawdust and other process dust presents a fire and explosion hazard in mills. To minimize this hazard, dust may be removed by manual means or, preferably, gathered by local exhaust ventilation systems and collected in bag houses or cyclones. Larger wood waste is chipped. Most of the sawdust and chips produced in the lumber industry can be used in other wood products (e.g., particleboard, pulp and paper). Efficient use of this type of wood waste is becoming more common as the expense of waste disposal rises, and as forest companies become more vertically integrated. Some types of wood waste, especially fine dust and bark, are not as easily used in other wood products, so other means of disposal must be sought.

Bark can represent a high proportion of tree volume, especially in regions where the logs harvested are of small diameter. Bark and fine sawdust, and, in some operations, all wood waste including chips, may be burned (see figure 1). Older style operations have used inefficient burning techniques (e.g., beehive burners, teepee burners) which produce a range of incomplete organic combustion products. Particulate air pollution, which can produce “fog”, is a common complaint in the vicinity of these burners. In sawmills where chlorophenols are used, there is also concern about dioxin and furan production in these burners. Some modern sawmills use enclosed temperature-controlled power boilers to produce steam for kilns or power for the mill or other electricity users. Others sell their wood waste to pulp and paper mills, where it is burned to meet their high power requirements (see the chapter Paper and pulp industry). Boilers and other burners usually must meet particulate emission control standards using systems such as electrostatic precipitators and wet scrubbers. To minimize burning of wood waste, other uses can be found for bark and fine sawdust, including as compost or mulch in landscaping, agriculture, surface mine revegetation and forest renewal, or as extenders in commercial products. In addition, use of thin-kerf saws in the mill can result in dramatic reductions in sawdust production.

Figure 1. Conveyor belts transport waste to a beehive burner

Leanne Van Zwieten

Bark, logs and other wood debris may sink in water-based log storage areas, blanketing the bottom and killing benthic organisms. To minimize this problem, logs in booms can be bundled together and the bundles broken apart on land, where the debris can be easily collected. Even with this modification, sunken debris needs to be dredged from time to time. Recovered logs are available for lumber, but other waste requires disposal. Land-based disposal and deep-water dumping have both been used in the industry. Hydraulic debarking effluent can cause similar problems - thus the trend to mechanical systems.

Chip piles can create storm-water run-off problems since the leachate from wood includes resin and fatty acids and phenolics which are acutely toxic to fish. Landfill disposal of wood waste also produces leachate, requiring mitigation measures to protect ground and surface waters.

Antisapstain and Wood Preservation Fungicides

Wood treatment with fungicides to prevent the growth of sapstain organisms has led to contamination of nearby waterways (sometimes with large fish kills), as well as contamination of the soil on site. Treatment systems which involve driving bundled lumber through large, uncovered dip tanks and drainage in the sawmill yard allow rainfall overflows and widespread travel of runoff. Covered dip tanks with automated dipping elevators, spray booths in the production line, and containment berms around both the treatment system and the lumber drying area greatly reduce the potential for and impact of spills. However, although antisapstain spray booths minimize environmental exposure potential, they may entail more downstream worker exposure than dip tanks that treat finished bundled lumber.

Environmental impacts appear to have been reduced by the new generation of fungicides that have replaced chlorophenols. Although toxicity to aquatic organisms may be the same, certain substitute fungicides bind more strongly to wood, making them less bioavailable, and they are more easily degraded in the environment. In addition, the greater expense of many of the substitutes and the cost of disposal has encouraged recycling of liquid waste and other waste minimization procedures.

Thermal and pressure treatment of wood for long-term resistance to fungi and insects has traditionally been done in more enclosed facilities than antisapstain treatment, and therefore tends not to produce the same liquid waste problems. Disposal of solid wastes including sludge from treatment and storage tanks presents similar problems for both processes. Options may include contained storage in leak-proof containers in a bermed impermeable area, burial in a secure, hydrogeologically isolated hazardous-waste landfill or incineration at high temperatures (e.g., 1,000°C) with specified residence times (e.g., 2 seconds).

Special Issues in Plywood and Particleboard Operations

Veneer dryers in plywood mills can produce a characteristic blue haze made up of volatile wood extractives such as terpenes and resin acids. This tends to be more of a problem inside plants, but can also be present in the dryer water-vapour plumes. Particleboard and plywood mills often burn wood waste to produce heat for the presses. Vapour and particulate control methods, respectively, can be used for these airborne emissions.

Wash water and other liquid effluents from plywood and particleboard mills can contain the formaldehyde resins used as glues; however, it is now common practice for waste water to be recycled for making up the glue mixtures.

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