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.