Wood preservation facilities, creosote: chapter D-5

5. Description of Preservative Application and Potential Chemical Discharges at Creosote Wood Preservation Facilities

In 2012, there were five active pressure wood preservation creosote facilities in Canada, and only one used it as their sole preservative (1). The impregnation of creosote into wood is carried out in pressure treatment plants and is for industrial and commercial purposes only. Refer to Part 1, General Background Information, Section 2.2.3 Figure 4, for an overview of the process.

A highly individualized approach exists to design and operating practices, which is emphasized by the fact that the technical support services at those facilities depend upon internal resources. Most of the plants have been in operation for 50 years or more. These older plants, although state of the art at the time of construction, did not have the benefit of today’s knowledge of environmental protection. However, the plants have used the recommendations contained in the previous technical recommendations documents to upgrade designs and operating practices (22).

5.1 Description of Process

Creosote is used either in a mixture of 50:50 creosote/petroleum oil or alone--full strength. Creosote and petroleum oil are delivered to wood preservation facilities by bulk truck or rail tanker and are stored in a bulk storage tank.

The Western Wood Preservers Institute, Wood Preservation Canada, the Southern Pressure Treaters’ Association and the Southern Forest Products Association have developed U.S. and Canadian recommended guidelines for the production and use of treated wood products in aquatic and other sensitive environments. The industry associations’ best management practice (BMP) is to place enough preservative into a product to provide the needed level of protection while also minimizing use of the preservative above the required minimum industry standard to reduce the amount potentially available for movement in the environment (23).

After delivery of the creosote and petroleum oil, the following process steps occur (Refer to Part 1 - General Background Information, Section 2.2.3, Figure 4).

Chemical Mixing

In Canada, creosote/petroleum oil mixtures are blended by pumping transfers and by recirculation between bulk tanks. The benefits of blending creosote with oil are lower cost and improved penetration (lower viscosity) in applications such as railway ties, where conditions of use allow for less protection than that usually provided by 100% creosote. The physical properties of wood treated with a mix are quite similar to those of material treated with 100% creosote,: better dimensional stability (compared with untreated or water-borne treated wood), improved mechanical wear, corrosion inhibition, resistance to chemicals, water repellency and improved resistance to electrical conductivity. Full strength creosote is used where maximum biocidal protection is desired, such as for timbers exposed to marine borers.

Wood Conditioning

In order to enhance penetration of the water-immiscible creosote into the wood, the moisture content of the wood is reduced by a conditioning process. Conditioning may be achieved by air seasoning, kiln drying or by processes carried out in the treatment cylinder--for example, application of steam and subsequent vacuum, or boiling under a vacuum in the presence of the treating solution (Boultonizing). For certain wood products, conditioning procedures are stipulated by the Canadian Standards Association (5).

Preservative Application

If Boultonizing or steam-vacuum processes have been used for conditioning, creosote is applied in the following steps, either by the full-cell treatment process or the empty-cell treatment process. It should be noted that in contrast to treatments with water-borne preservatives, creosote solutions are applied at an elevated temperature (70-90°C).

Depending on the species of wood, the wood product and the moisture content of the wood, the operator of the facility determines the appropriate treatment process (full cell or empty cell), and the pressure, temperature and times for various process sequences.

An expansion bath and final vacuum are usually added after the pressure cycle so as to render product surfaces relatively dry and to minimize long-term “bleeding” of preservative and to improve the surface cleanliness of the material. The expansion bath can be applied before removal of the preservative from the cylinder, by quickly re-heating the oil surrounding the material to the maximum temperature permitted by the CSA Standard for a specific species, either at atmospheric pressure or under vacuum. The steam shall be turned off as soon as the maximum temperature is reached. The cylinder shall then be quickly emptied of preservative. A vacuum equal to or stronger than -75 kPa (562.5 mmHg) shall be created promptly and maintained until the material can be removed free of dripping preservative. (5) Condensate from steam should be recuperated to be treated in a proper manner, see section 9.

The treated wood is withdrawn from the treating cylinder and stored on a drip pad until drippage has essentially stopped. From there the wood is either taken for storage in the yard or shipped by truck or rail car. Best management practices (BMPs) are promoted by the industry associations to minimize preservative drippage and bleeding during storage and service (23).

Treatment conditions must be calibrated to yield the target retention levels described on the pesticide label. The CAN/CSA O80 (5) also has retention and process standards to ensure effective treatments for specific uses without damage to the wood. The pesticide label is the legal document and should be consider as such in the event of discrepancy between the standards.

5.2 Potential Chemical Discharges

Creosote wood preservation facility design and operational practices vary (22, 24), and each facility has potential sources of emissions that could affect worker health and/or the environment. The potential sources and releases are illustrated in Figure 1.

Liquid Discharges

Leaks and drips of oil solutions can be contained and reused in the oil-borne treatment process. Liquids that cannot be recycled and reused may include the following:

• condensates removed from wood during the conditioning and during the initial application of the vacuum process
• water released by the wood during the treating cycle and subsequently separated from the unabsorbed treatment oil prior to recycling of the oil
• washwaters

Washwaters commonly consist of high pressure steam cleaning since low pressure washing systems are generally not sufficient for the cleaning of creosote stained surfaces or equipment.

These liquids can contain creosote and must be treated before discharge as a waste stream.

Other liquids that may be released from oil-borne creosote facilities include the following:

• steam condensates from indirect heat transfer in cooling and heating coils, which are generally checked for contamination prior to discharge
• condenser cooling water, which is not normally contaminated and is discharged without treatment
• surface runoff from treated wood storage areas contaminated by preservative

The creosote content in runoff waters depends on many factors, including vacuum and drip time in the last step of the pressure process; viscosity of the wood preservative; wood species; moisture content of the wood prior to application of preservative; specific treatment process (i.e. full cell versus empty cell); and exposure to the weather. The need for control of runoff waters would depend upon analytical and/or bioassay evaluations and regulatory requirements.

Solid Wastes

Solid wastes from creosote treatment facilities include the following:

• sludge from tanks, sumps and pressure cylinders
• sludges from wastewater treatment processes (e.g. flocculated material)
• contaminated soils

Air Emissions

Air emissions from creosote treatment facilities are generally localized and may include the following:

• emissions during application of vacuum for wood conditioning, for the full-cell process or during the final vacuum step
• vapours from tank vents
• vapours from treating cylinders
• vapours from opening of cylinder doors;
• vapours from freshly treated charges

Refer to Part 1, Chapter A, Section 5.2 for more details on potential chemical discharges.

Figure 1 Potential Chemical Releases From Creosote Pressure Treating Plants

5.3 Potential Effects of Chemical Discharges

The actual impact to the environment of any liquid discharge, solid waste or air emission depends on many factors, including the location of the wood preservation facility relative to ground or surface waters, the species of aquatic biota in adjacent surface waters, and the amount of preservative released. Variables that can influence effects on worker health include ambient concentrations, frequency of exposure and protective measures during the time(s) of exposure.

All facilities that use creosote could affect the environment, as could any industrial facility that uses chemical, if proper control measures are not in place. Historically, documented releases of creosote from wood preservation facilities have been due either to poor design or to poor operating practices. The effects of these releases appear to be localized at the plant site (i.e. soil and groundwater contamination) or in the immediate environment adjacent to the plant site (25).

Human health could be affected if appropriate precautions are not taken during spills of creosote and residues, operation of the treatment system (e.g. cylinder door opening) and handling of treated products. Skin burns and allergic reactions were reported in a survey of the accident history of 50 pressure treatment plants (26). A review of the literature by Todd and Timbie (27) concluded that “there is neither positive nor negative human experience data in the wood treating industry to indicate that additional or less restrictive exposure control is appropriate.”