Wood preservation facilities, pentachlorophenol thermal: chapter F-5

5. Description of Preservative Application and Potential Chemical Discharges

In 2012, there were three thermal impregnation facilities applying PCP/oil in Canada. Design and operating practices are highly individualized at these plants (4).

5.1 Description of Process

PCP dissolved in oil is the only wood preservation chemical used at thermal treatment facilities in Canada. Discussions and recommendations in this document are therefore limited to the use of PCP in thermal operations (PCPT), although the objectives would be applicable to any alternative wood preservative chemical that may be used in the thermal process.

Thermal treatment is used to achieve penetration and subsequent stabilization of the chemical agent within wood. The process can be used to treat entire lengths of timbers (e.g. poles) or only the ends of poles (butt treatment). The latter type of treatment would be used for the protection of only the portion of the utility pole that normally is placed below ground level.

PCP is generally purchased in solid blocks (usually weighing 907 kg/2000 lbs.). Petroleum oils used as carriers for PCP are delivered by bulk truck or rail tanker and stored in tanks. Following delivery of PCP and the carrier oils, the chemicals are mixed and the wood is treated as outlined below. (Refer also to Part 1 - General Background Information, Section 2.2.3 Figure 5).

Chemical Mixing

PCP solid blocks

The PCP blocks are lowered into a thermal treatment tank. Hot petroleum oil is circulated between the tank and a PCP/oil storage tank until a dissolved solution is attained. The prepared solution is then transferred to an insulated storage tank. Solution concentrations may range from 5% to 9% PCP.

PCP liquid solution (minor use in Canada)

The concentrate is then diluted to working concentration (5-8% PCP (8)) by recirculation between the cylinder or mix tank and the bulk storage tank.

Wood Conditioning

Air seasoning is used to reduce moisture in wood in preparation for thermal treatments. Dry kilns or steaming is sometimes used if air seasoning has not been sufficient prior to treatment. Prior to treatment, the wood products may be incised and shaped to end-user specifications. Poles are the most common thermally treated product, although cross-arms may also be treated by this process (19). The poles are loaded into horizontal, rectangular tanks for full-length treatment, or into tanks for upright butt treatment of pole ends, using cranes or custom forklift equipment. Poles placed in the horizontal full-length tanks are held in place by steel cross-beams. Full-length tanks are normally covered with plywood or steel lids prior to application of the preservative. Typical dimensions of such tanks are 4 m x 4 m x 30.5 m.

Preservative Application

Thermal treatment is analogous to the full-cell pressure treatment process. The treatment cycle basically consists of a hot and cold bath as described in Section 2.2.2 (Preservation Processes) of Part I - General Background Information. After the impregnation cycle is completed, the poles are left in the treatment tanks to cool and to allow any excess preservative to drip off. They are then inspected and sampled for quality control purposes.

For butt treatments, the wood is held vertically with only a portion of the pole submerged in oil. The impregnation cycles are similar to the full-length thermal cycles. Butt dip tanks are difficult to cover during treatment. However, the exposed oil surface area is limited. Hollow polypropylene balls are sometimes used to float on the oil surface to help reduce steam and heat loss while also reducing potential foaming. The tanks should be covered when not in use to prevent precipitation entering the tank.

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 considered as such in the event of discrepancy between the standards.

Storage of Treated Product

The cooled treated wood is moved from the treatment tank to a drip pad and then to a storage area. It is common for poles to remain in the treating tank until essentially drip free. The time on the drip pad may vary depending on the facility design, ambient conditions, wood species and application process. The charge must only be removed from the drip pad once drippage has stopped. The treated wood is removed from the drip pad by forklift or crane and stored in a designated area until transport to the customer.

5.2 Potential Chemical Discharges

Thermal wood preservation facility design and operational practices vary (20), and each facility has potential sources of emissions that may affect worker health and/or the environment. The potential sources and releases are illustrated in Figure 1.

Liquid Discharges

Although no liquid process wastes are produced during thermal treatment, the following situations could create liquid releases:

• spills or overflows of liquid from open treatment tanks
• infiltration of groundwater into tank containment systems
• leaks from treatment tanks that have no containment provisions
• surface runoff from the treated wood storage areas
• washwaters

The method for controlling runoff waters depends on analytical and/or bioassay evaluations and on regulatory requirements. Section 10 - Emission and Site Monitoring provides information on this matter.

Solid Wastes

Solid wastes from thermal treatment facilities that use oil-borne PCP may include the following:

• sludges from treatment and storage tanks, in particular the “cold” PCP/oil storage tank
• sludges from wastewater treatment processes (e.g. flocculated material)
• pallets and wrappings from bulk PCP

Air Emissions

Air emissions from thermal treatment facilities are localized and intermittent, and may include the following:

• vapours that escape from the treatment tanks during the treatment cycle
• vapours from tank vents
• vapours from PCP block storage
• vapours from freshly treated charges

Emissions that could disperse beyond facility boundaries include the following:

• vapours from uncovered treatment tanks during the treatment cycle

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

5.3 Potential Effects of Chemical Discharges

The actual impact on the environment of any liquid discharge, solid waste or air emission depends upon 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 thermal facilities could potentially affect the environment, as could any other industrial facility that uses chemicals, if proper control measures are not in place. Documented PCP releases from wood preservation facilities have been due to either poor design or poor operating practices. The effects of these releases appear to be either localized within the plant site (i.e. soil and groundwater contamination) or in the environment immediately adjacent to the plant site.

Fires that have occurred at Canadian PCP thermal facilities illustrate the need for proper contingency planning for fire control and for containment of oil solutions and fire runoff waters.

Human health could be affected if proper precautions are not taken during handling of PCP, exposure to minor spills and residues in working areas, exposure to vapours, or handling of treated products.