57. Timber, sawmills, wood processing and wood products
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2. Environmental impacts and protective measures
2.1 Mechanical woodworking
2.2 Derived wood product manufacture
3. Notes on the analysis and evaluation of environmental impacts
4. Interaction with other sectors
5. Summary assessment of environmental relevance
Wood is man's oldest material and energy source; it is particularly important as it is a renewable resource. Despite the availability of metal, synthetic (plastic/ chemical) and mineral materials it is still important as a raw material. Because of their technological properties, tropical woods have been accepted - particularly in the last thirty years - as valuable functional and decorative materials. In most tropical and subtropical countries, wood still plays a vital role as an energy source.
The major timber sub-sectors are as follows:
- production (timber industry, incl. reforestation), felling and transport
- mechanical woodworking (sawing, shaping, milling, sanding)
- manufacture of wood board materials (plywood, chipboard and fibreboard)
- transformation into other products with extensive chemical modification of timber
This article focuses mainly on the primary, i.e. mechanical processing of wood, the manufacture of wood products and charcoal production, with only a brief look at combustion.
The manufacture of paper and pulps from wood is regarded as a separate, secondary processing sector and is not covered by this brief but by the environmental brief Pulp and Paper.
The environmental impacts of woodworking and wood processing operations, in the form of dust, noise and odours, can be countered to a large extent by appropriate siting, namely downwind from residential areas (see also the environmental brief Planning of Locations for Trade and Industry). The problem of wastewater, on the other hand, calls for closer attention. Direct effects on personnel can be at least reduced by the wearing of suitable hearing protection and breathing equipment.
In terms of the scale of environmental impacts it should be borne in mind that advancing slash and burn (shifting cultivation) can be the most dangerous environmental impact of timber felling, and is frequently the most significant factor in forest destruction.
The major effect on employment to be considered is that the workforce in the timber industry is almost exclusively male.
2. Environmental impacts and protective measures
2.1 Mechanical woodworking
Wood is a raw material which regrows and is obtained mainly from natural woodland, with plantations still playing only a minor role in many countries.
The dual system of national forestry authorities and private timber concessionaries often results in a clash of business management and forestry policy interests - being founded largely on conflicting principles.
Woodworking per se begins in the sawmill with debarking, unless this has already been done in the forest, followed by the cutting into lengths and cutting to size of timber supplied from the forest. The cut timber is either used directly as building lumber or is upgraded by shaping, milling, sanding and painting or impregnation.
Sawmills are workshops in which round wood is processed into sawn lumber (primary processing). Mechanical woodworking goes hand in hand with noise and dust, and is often carried out in the same mills which carry out surface treatments with paint, stains etc., processes which result in the formation of gaseous and substances with strong odours.
The mechanically driven transport, cutting, milling, shaping and dust extractor installations in the timber industry generate noise, a problem which is exacerbated where sawmills are built to an open design in warmer climates.
However, because most sites are selected because they are close to raw material sources, they tend to be far from residential areas. Thus it is only mill personnel who are mainly affected. For this reason, the wearing of hearing protection should be compulsory and, where new plant or new equipment is to be installed, emphasis should be placed on providing tooling which is enclosed and designed to reduce noise.
Further negative effects on machine operators exist in the form of vibrations. The reduction of vibrations is an important factor when laying the foundations and erecting operating and control stations.
· Dust emissions
Alongside noise, dust is emitted from mechanical woodworking processes. In sawmills, wood cutting produces wood shavings, but because the wood is in most cases supplied fresh from the forest or is fibre-saturated, dust emissions do not present a major problem in relative terms, and fabric filters or wet extractors are not generally required. However, where wood shavings are stored in the open air, measures must be taken to protect against airborne dust.
Far more significant is the dust generated by mechanical woodworking in joinery works, cabinet-making and similar businesses, where both dust quantity and qualitative dust composition differ from that in a sawmill. The crucial factor is the fineness of the dust, expressed by its grain size and grain size distribution. Fine dust is naturally more difficult to remove than coarse dust and constitutes a greater health hazard to man, particularly where the particles are small enough to reach the lungs. The fine dust content is particularly high in sanding operations, and not so high in operations which produce shavings.
The inhalation of wood dust, particularly hardwood dust, can result in the absorption of harmful substances found in wood, which in turn can lead to serious illnesses. Thus, before any wood processing is undertaken, the health risks arising from working with wood must be thoroughly investigated and adequate precautions taken.
To reduce the quantity of dust generated at workplaces machines must be fitted with extractor systems, a measure which is justified as both a health precaution for employees and a fire and explosion prevention measure. Machines must be enclosed whenever possible and the extraction and transportation installations must be designed to handle the quantities of dust produced. If the extractor unit is likely to generate a high partial vacuum in the workroom, a pressure compensation system must be provided, but this must not cause any draughts in the workplace. Even where the industrial building is of an open design, every effort should be made to prevent draughts.
If harmful substances are released during the woodworking operations, the exhaust air cannot be returned to the work areas. Furthermore, where exhaust air is returned, the dust load at the workplace must not exceed permitted levels. The extracted dust must be discharged through non-flammable, fracture and wear-resistant extraction pipes, which must be designed and their rate of extraction dimensioned so that no undesirable deposits are able to form in the system.
Before the exhaust air is discharged into the environment the dust it contains must be separated off, for which purpose centrifugal separators or fabric filters are used. More costly and more effective fabric filters are required where extracted air contains sanding dust. Due to the risks of fire and explosion the extractor installations must be fitted with preventive safety devices, such as pressure relief valves, bursting discs, spark detection installations, smouldering fire alarms and fire extinguishing equipment.
· Gaseous emissions
When wood is dried, volatile constituents of wood in the exhaust air generate odours, and this exhaust air must therefore be released so as to avoid any odour nuisance.
Since wood processing mills are often sited in isolated locations, as already mentioned, the employees are those most subject to gaseous emissions.
This problem can be minimised by an appropriate choice of site (in terms of distance, allowance for the prevailing wind direction).
Otherwise, gaseous emissions are only of minor significance in sawmills.
· Analysis and evaluation of environmental impacts
In Germany timber mills are governed by the Technische Anleitung zur Reinhaltung der Luft TA-Luft [Technical Instructions on Air Quality Control] and the Technische Anleitung zum Schutz gegen Lärm TA-Lärm [Technical Instructions on Noise Abatement]. Accordingly the TA-Luft of 1986 restricts the mass concentration of wood dust in inhalable form to 20 mg/m3 at a mass flow of 0.5 kg/h. Lower limit values apply correspondingly to various dusts from woods treated with certain wood preservatives.
For most of the organic substances involved in wood processing, the upper limit is 150 mg/m3 at 3 kg/h. For airborne dust, which is a health hazard, concentration values of 0.45 mg/m3 and 0.30 mg/m3 are specified.
The acoustic pressure level is taken as the basic unit to describe the noise situation. Where measured and assessed values are indicated, three fundamentally different frequency weighting curves are used: single measured value, effective level and evaluation level (German DIN standards, guidelines of the Association of German Engineers VDI). In Germany the permissible limit values are 35 and 70 dB(A), depending on the preconditions for assessment.
In the case of wood preservatives their composition must be carefully examined (preservatives containing PCB's are banned in Germany). They must be kept sealed and accident-proof during storage. No wood preservatives dripping from treated timber are allowed to seep away in an uncontrolled fashion. Appropriate fire and accident prevention measures must be taken and waste must be disposed of correctly.
Where sawmills or mechanical woodworking mills are to be built or re-equipped, these statutory provisions must be applied as guidelines where there are no national regulations.
· Interaction with other sectors
The sawmill industry generally obtains its raw products from nearby forests. It must be ensured that the timber comes only from a properly managed forest (management strategy, coordination of individual usage plans, yield regulation, forestry and wood crop techniques) operating on the principle of sustainability.
Sawmills supply their products primarily to the wood-processing trade and industry (building, furniture and packaging sectors) and also for export. On the other hand the waste material produced contributes to supplying the derived wood product industry, particularly the chipboard industry, with raw materials.
The burning of waste wood concerns all aspects of timber usage and is therefore considered in a separate section.
Mechanical woodworking is primarily associated with the generation of noise and dust, with gaseous emissions and odours occurring only to a limited extent during artificial drying or treatment, and merely constituting a nuisance. Generally speaking, the sawmill industry does not damage or endanger the environment, except where wood preservatives are used, but even this problem can be avoided by careful siting of mills relative to residential areas.
2.2 Derived wood product manufacture
The term "derived wood products" covers chipboard, fibreboard and plywood. In addition to wood, these products - with the exception of a few types of fibreboard - contain an organic or inorganic bonding agent and in some cases additives.
The bonding agents used are mainly amino and phenolic resins, condensation products from an amino compound (urea, melamine) or a phenolic substance (phenol, resorcin, cresol or formaldehyde). Chipboard bonding agents on a diisocyanate adhesive base are a relatively new development. Polyvinyl acetate adhesives are used for wood core plywood.
· Chipboard manufacture
Practically all varieties of wood, wood waste and in some cases fibrous plant substances, bark and biomasses can be used as the feedstock for board. The first stage in the process is the machining of the raw material. Long cut or round wood is either cut into chips with drum chopping machines or processed directly into shavings with cutters. The next stage in shaving processing is drying, following by sizing, intermediate storage, bonding and hot pressing of the cut material. The initial production stages take place in enclosed installations without any significant emissions, these occurring only at the stage of hot bonding in the chipboard press at temperatures of 160 to 220°C. The final production stages comprise trimming, sanding and formatting of the board.
The main bonding agents used are amino and phenolic resins, condensation products from an amino compound (urea, melamine) or a phenolic substance (phenol, resorcin, cresol or formaldehyde). Chipboard bonding agents on a diisocyanate adhesive base are a relatively new development. Polyvinyl acetate adhesives are used for wood core plywood.
· Plywood manufacture
The term "plywood" covers veneer plywood and wood core plywood, the latter containing a central layer of rods, while veneer plywood is made by bonding together a number of individual sheets of veneer.
Suitable untreated wood is cut into veneer by sawing, cutting or scraping, dried and then bonded and pressed. The final production stage comprises trimming, sanding and formatting.
· Fibreboard production
A distinction is made between soft fibreboard, medium-density fibreboard (MDF) and hardboard.
Soft fibreboard contains no bonding agents. Hardboard likewise contains no adhesive, or at the most very small quantities of a phenol-formaldehyde bonding resin. MDF, like chipboard, contains 7 to 9% bonding agent.
The first stage of fibreboard production involves the production of fibres from wood, a process carried out by heat or chemical treatment.
It is then pressed by a number of different processes.
· Manufacture of mineral-bonded derived wood products
These products are made from wood chips, wood shavings or wood fibres and a mineral bonding agent such as cement, gypsum or magnesite. Wood is the main component, accounting for at least 85% of the dry weight. Manufacture is similar to that for chipboard, except that drying and hot pressing are not required.
3. Notes on the analysis and evaluation of environmental impacts
Noise emissions are produced in wood transport, cutting and preparation in all four manufacturing processes. Dust emissions may arise in stores. As in sawmills the resultant nuisance can be reduced by a sensible choice of site and by providing suitable precautions for employees (enclosed workstations, personal hearing protection).
Extremely fine dust is produced in the final stages of chipboard, plywood and fibreboard manufacture and this must be removed by means of centrifugal separators or fabric filters, as it is a health hazard to employees.
Gaseous emissions are produced only in the drying of wood shavings and the pressing of shavings and veneers.
In chipboard and plywood pressing, where amino bonding resins are used, formaldehyde is the main substance yielded in terms of the mol ratio of the bonding resin. Where phenol-formaldehyde bonding resins are used, only traces of phenol are found, and smaller quantities of formaldehyde are produced than is the case with amino bonding resins. Phenol and formaldehyde are both potential health hazards. In Germany formaldehyde emissions at the workplace must not exceed 0.6 mg/m3 and the finished board formaldehyde content must not exceed 10mg/100g board weight, according to EC Directives. After fitting the boards formaldehyde concentration must not exceed 0.1 ppm in the ambient room atmosphere.
The Gefahrstoffverordnung [Ordinance on Hazardous Substances] of 1986 specifies the formaldehyde emission values for all derived wood products in Germany. These gaseous immissions do not occur during the manufacture of mineral- bonded derived wood products.
Wastewater problems occur during the cleaning of the bonding machines and presses. In fibreboard manufacture, wastewater is produced during the wet process and contains wood particles, wood substances, bonding agents and other treatment agents which can be cleaned using physical processes (sedimentation, flotation or filtration) and/or biological processes. Semi-dry and dry processes do not produce any wastewater.
Residues in the form of wood particles can be returned to the production process in chipboard manufacture, but are otherwise burnt.
In addition to the specific statements in the text, the information relating to mechanical woodworking is also applicable to the analysis and evaluation of environmental impacts.
4. Interaction with other sectors
The wood product industry is reliant upon the forest as its raw material supplier, except where wood waste can be used, as is the case in the chipboard and fibreboard industry. The basic tenet here is that of the principle of sustained yield. The environmental brief Forestry contains detailed information on this subject.
Round wood can be fully utilised by linking sawmills wherever possible to fibreboard and chipboard manufacture.
Derived wood product factories are major power consumers which today generate their power with wood very rarely nowadays. The environmental briefs Overall Energy Planning, Thermal Power Stations and Renewable Sources of Energy should be consulted in this regard.
Wastewater management issues are also addressed in a separate environmental brief.
· Charcoal production
Charcoal is produced by the thermal decomposition of wood carried out without air (wood pyrolysis). This process also yields gaseous and liquid reaction products such as wood gas, wood vinegar, wood spirit and wood tar.
Charcoal is produced at temperatures of between 400 and 600°C, and is used as a fuel, a reducing agent in metallurgy and as a raw material for the chemical and pharmaceutical industries. Wood tar and the other liquid organic substances can be processed or alternatively burnt as an energy source.
Charcoal production is the only process still used today on an industrial scale in which wood is chemically modified to a substantial degree. Charcoal production is not therefore classified as part of the timber industry, but constitutes a separate sector of the chemical industry.
In many countries charcoal is an important source of energy for cooking and heating. The favourable ratio of weight to calorific value means that it can also be transported considerable distances from the place of production to its market. This fuel is in particular demand in cities because it produces heat without a great deal of smoke.
Charcoal is often produced in small businesses (with the exception of the East Amazon, Caracas), which either fell the raw material themselves or have charcoal production plants for wood waste in the immediate vicinity of sawmills. The latter arrangement is particularly useful where there is no derived wood product plant downline from the sawmill.
Gaseous emissions from charcoal production, in the form of smoke and strong odours do not merely constitute a nuisance: where the process is inefficiently managed pyrolysis derivatives, such as benzpyrene may constitute a health hazard to employees, and at high concentrations also to the general population (cancer risk). The comments made on the siting of sawmills apply here too.
The charcoal production process yields considerable quantities of pyrolysis water - up to 15% of the feedstock; this wastewater contains, inter alia, pyrolysis tar and water-soluble organic substances. Whereas liquid pyrolysis products must be conditioned according to the regulations for chemical industry installations in charcoal production on an industrial scale, no such solution yet exists for the small business.
If large-scale charcoal production from wood waste is sited close to wood processing mills appropriate measures must be taken to prevent pollutants from penetrating the water or soil.
· Burning of wood waste
The quantity of residual material (sawdust, splinters, bark, pieces) varies from process to process and product to product; tropical hardwood cutting produces extremely large quantities of waste (up to 60%). Waste disposal may take the form of combustion for energy production, as it is not possible to market the wood waste elsewhere because of the siting of the mill close to the saw of supply of the raw material. The presence of a downstream pulp or paper mill is a rare exception.
Complete incineration produces carbon monoxide, organic hydrocarbons, tar and soot. It is almost impossible to influence the nitric oxide emissions from wood furnaces.
5. Summary assessment of environmental relevance
While plywood mills process high-quality round timber, chipboard and fibreboard are the result of the value-adding utilisation of different wood varieties, some of which are low grade.
Gaseous emissions represent further harmful environmental impacts of chipboard and fibreboard plants, the main principal hazardous substance being formaldehyde. By contract, bonding with phenolic resins and diisocyanates help reduce emission values. One exception, in terms of emissions, is the manufacture of adhesive-free fibreboard.
Gaseous emissions from wood shaving driers have few environmentally harmful properties, especially in the case of hardwood, although the intensity of the odours produced constitutes a nuisance. The same siting criteria apply as for sawmills.
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