Wood preservation facilities, pentachlorophenol pressure: chapter E-5

5. Description of Preservative Application and Potential Chemical Discharges

In 2012, there were eight pressure impregnation facilities applying Pentachlorophenol (PCP)/oil in Canada (19).

5.1 Description of Process

PCP is generally purchased as solid blocks, usually weighing 907 kg (2000 lbs). Petroleum oils used as carriers for PCP are delivered by bulk truck or rail tanker and are stored in exterior tanks. After 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 4).

Chemical Mixing

PCP solid blocks

The PCP blocks are dissolved by placing them in the treatment cylinder or into a mix tank and recirculating heated oil between the cylinder or mix tank and the bulk storage tanks. A concentrated solution may first be prepared. The concentrate is then diluted to working concentration (5-9% PCP (5)) by recirculation between the cylinder or mix tank and the bulk storage tank.

PCP liquid solution (minor use in Canada)

The concentrate is then diluted to working concentration (5-9% PCP (5)) by recirculation between the cylinder or mix tank and the bulk storage tank.

Wood Conditioning

Prior to application of the PCP/carrier oil mixture, the moisture content of the wood is reduced by one of several conditioning processes. Conditioning of the wood may be achieved by air seasoning, kiln drying or by processes carried out in the treatment cylinder--for example, the application of steam and subsequent vacuum, or boiling under a vacuum in the presence of the treating solution (Boultonizing). In Canada, air seasoning is the most common means of conditioning poles, which make up 90% of the total PCP-treated material (19). Dry kilns or steaming is sometimes used if air seasoning has not been sufficient prior to treatment.

Preservative Application

The preservative is applied in a pressure cylinder, which may be up to 45 m long and 2 m in diameter. Specific treatment parameters (e.g. temperature, pressure and duration) are dictated by the species of wood, the wood product and the initial moisture content of the wood. Many of the operating parameters, preservative standards and product quality characteristics (e.g. penetration and preservative retention) are defined by the Canadian Standards Association (5).

After conditioning, an empty-cell treatment process is generally used to apply the oil-borne PCP preservative. Following the drain cycle at the end of the impregnation process, a vacuum is applied to encourage the removal of excess preservative and pressurized air from the wood cells. This process minimizes preservative “bleeding” from the treated product.

Alternatively, an expansion bath or final steam cycle, followed by a vacuum, may be used to minimize surface exudations and long-term bleeding and to improve the surface cleanliness of the material. This expansion bath can be applied before removal of the preservative from the cylinder, by quickly reheating the oil surrounding the material to the maximum temperature permitted by the CSAStandard for a specific species, either at atmospheric pressure or under vacuum. The steam is turned off as soon as the maximum temperature is reached. The cylinder is then quickly emptied of preservative. A vacuum equal to, or stronger than -75 kPa (-22 in. Hg) is created promptly and maintained until the material can be removed free of dripping preservative (5).

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 (20).

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 treated wood is withdrawn from the treating cylinder and put on a drip pad. 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 and stored in a designated area until it is transported to the customer.

5.2 Potential Chemical Discharges

PCP wood preservation facility design and operational practices can vary (21), 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 include the following:

• condensates removed from the wood during 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

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

Other liquids that may be released from facilities that use oil-borne PCP may 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 waters, which are not normally contaminated and are discharged without treatment
• surface runoff from treated wood storage areas, which can contain preservative
• washwaters

The PCP content in runoff waters depends on many factors, including drip and vacuum 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 (i.e. adequacy of conditioning step); specific treatment process (i.e. Rueping or Lowry); effectiveness of the post-pressure-cycle processes applied (expansion bath, final steaming, final vacuum); and exposure to the weather. The method for controlling runoff waters depends on analytical and/or bioassay evaluations and regulatory requirements. Section 10 - Emission and Site Monitoring provides information on this matter.

Solid Wastes

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

• sludges from tanks, sumps and pressure cylinders
• sludges from wastewater treatment processes (e.g. flocculated material)
• containers or wrappings and pallets from bulk PCP
• contaminated soils

Air Emissions

Air emissions from pressure treatment facilities that use oil-borne PCP are generally localized and may include the following:

• vapours from block storage
• emissions during wood conditioning and the final vacuum step
• vapours from tank vents
• vapours from venting cylinders
• vapours from the 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 PCP Pressure Treating Plants

5.3 Potential Effects of Chemical Discharges

The actual impact on 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 PCP pressure treatment 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 appeared 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 at Canadian wood preservation facilities have also been documented (22, 23, 24). These incidents 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 appropriate precautions are not taken during manual preparation of PCP solutions, minor spills in working areas and handling of treated products.