Wood preservation

Preservatives are applied to wood to achieve a longer lifetime for wood typically used outdoor. Wood is preserved to protect it against fungal and insect attack and against weathering. Products include telegraph poles, railway sleepers, and construction materials. Around 6 million m³ of timber were preserved annually in the EU before the membership of Finland, Sweden, and Austria. There are around 1,000 wood-impregnation sites in the EU, thus the average size of these businesses is small.

Creosote, organic solvents, and water-borne preservatives exist. VOC emissions from the preservation of wood were 136,000 tons in Europe in the mid-1990s (Klimont et al., 1997). Wood preservation accounted for 0.8% of total VOC emissions in Europe in the mid-1990s.

Two drying methods, air seasoning and kiln drying, are used for applying water-based preservatives after debarking. Steam vacuum drying is primarily used for creosote. In that method, the wood undergoes a pressurized steam treatment, and a vacuum is then used to remove excess moisture from the wood (UN, 1983).

Creosote is oil prepared from coal tar distillation. VOCs make up approximately 10% of the creosote used for wood preservation. Creosote is used for wood destined for outdoor uses, such as for telegraph poles and railway sleepers (Giddings et al., 1991). The lifetime of creosote-impregnated poles is 40–70 years (Metsäliitto, 1999). In most applications, creosote can be replaced with water-borne preservatives.

Water-based preservatives consist of solutions of salts in water (Giddings et al., 1991).

The lifetime of salt-impregnated poles is 30–70 years (Metsäliitto, 1999). Water-based treatments are popular for applications where it is necessary to paint the wood after treatment or where odors from creosote treatment are unacceptable (UN, 1983).

Solvent-based preservatives are used predominantly in the construction industry. These preservatives have the advantage of keeping very precise dimensions of the wood intact.

Introducing water-based preservatives into an existing plant may be costly because of the stainless-steel requirements. Another advantage compared with water-borne preservatives is the faster drying time.

In 1991, water-based solutions accounted for 13% of total preservative consumption in the EU. The share of creosote consumption was 54% and that of solvent-borne solutions was 33% (Giddings et al., 1991). Preservatives can be applied by using vacuum processes, dipping, spraying, or brushing. The efficiency of spraying is only 10%, that of the other applications is around 90%.

In the vacuum process for creosote application, timber enters a chamber that can be pressurized with air. The chamber is flooded with hot creosote for 1–3 hours. After draining, a vacuum is applied to draw off the excess creosote. The timber is then left to dry in the open air. Water-borne preservatives are applied in the same way.

In the vacuum process for organic solvent application, timber is placed in a chamber, which is subsequently evacuated. The chamber is flooded with preservative and pressurized for 5–20 minutes. After draining the chamber, a final vacuum is used to draw off the excess preservative. The timber is left to dry in the open air.

A thermal treatment process is also available but is not widely used. The VOC emissions from this process are considerable. Capturing these vapors and condensing and returning them to the tanks is the only control alternative (UN, 1983).

Pressure treatment leads to VOC emissions, mainly from the drying of impregnated wood and to a smaller extent from preparation and handling. Drying is usually performed in the open air (Klimont et al., 1997). Emissions can be reduced through proper solvent management, by enclosing the process wherever possible so that air can be extracted through abatement equipment, and by using alternative low-solvent coatings where possible (Atmospheric Emission Inventory Guidebook, 1996).

The primary water pollutants are condensate and condensed vapors from the conditioning and treatment processes. The amount of effluents is, however, small. The three primary steps for the treatment of wastewater contaminated by oil-borne preservatives are as follows:

• Primary separation of free oil and solids

• Removal of emulsified oil and suspended solids

• Removal of dissolved organic compounds (UN, 1983)

Gravity separation, dissolved air flotation (as in papermaking), and granular media filtration are the alternatives for the first step. Free creosote can be recovered.

In the second step, emulsion is broken by coagulation or heating. Sedimentation, dissolved air flotation, and granular media filtration are then used.

Biological treatment is usually performed in the last step. Waste stabilization ponds and activated sludge treatment are used. Waste stabilization ponds do not require high capital investments and are inexpensive to operate. However, they do require warm temperatures the whole year. It has been estimated that even by performing only the first step, over 80% of CODs can be removed at relatively low annual costs. Introducing the other steps results in considerably higher costs.

If water-borne preservatives are used, chemical reduction and oxidation reactions followed by precipitation or filtration are used to remove different metals. Lime is added to neutralize acidic wastewaters. Ion exchange and precipitation can be used to remove some metals. The costs of those measures are not high (UN, 1983).

Part of the emissions may be fugitive emissions and hence abatement techniques can capture only a part of the total emissions. (Klimont et al., 1997). Fugitive emissions occur throughout the handling, application, and drying stages of the processes. Timber impregnation using the closed double vacuum process minimizes fugitive loss. It has been estimated that the closed double vacuum process can decrease VOCs by 40%.

Activated carbon adsorption and incineration can be used as an abatement technology.

Those techniques are discussed in Section 9. Because of fugitive emissions, the efficiency estimation for those techniques is only 60% (Klimont et al., 1997). The cost of double vaccum impregnation is about half that of activated carbon adsorption. The cost is still relatively high for existing plants (Klimont et al., 1997).

Emissions from burning creosote-impregnated wood do not differ much from those from non-impregnated wood, provided a high incineration temperature is used (Metsäliitto, 1999).

9. VOC abatement technologies of in the mechanical forest