Wood preservation facilities, inorganic boron (borate): chapter I-5

5. Description of Preservative Application and Potential Chemical Discharges

5.1 Description of Process

The impregnation of borates into wood is carried out in pressure-treatment plants with generally a standard full-cell impregnation process in a conventional mild steel vessel (pressure cylinder). The standard full-cell impregnation process is described in Part 1 - General Background Information, Section 2.2.2 and the pressure treatment plant design is described in Part 1 - General Background Information, Section 2.2.3, Figure 3. Borate plants are similar to chromated copper arsenate (CCA) plants with the exception of having dried chemical storage.

Process conditions and treatment results are described in the CSA-O80 Series of Standards (1).

The treating chemical is purchased as a powder in 5-25 kg bags. Indoor dry storage is recommended to prevent caking. Usually, a treatment solution is prepared by direct mixing with water in a suitable mild steel tank. Once the treated wood has been removed from the impregnation vessel, it has to be kept under a roof or a wrap until it is used in an interior location. Exposure to precipitation or running water can deplete the treatment chemical from the wood. Diffusion of the chemical into the wood carries on in the presence of adequate wood moisture after the pressure process has been terminated.

5.2 Potential Chemical Discharges

Borate wood preservation plant designs and operational practices vary, and within each plant there are various potential emission sources that may affect the adjacent environment and/or worker health. The potential sources and types of releases are illustrated in Figure 1.

Liquid Discharges

The borate process uses water as a solvent. Therefore, drippage collected on the pad or rainwater collected in the process areas can be reused within the process. The process technology and economics have led the borate wood preservation industry to use closed treatment systems that contain, collect and reuse the chemical mixture to the greatest possible extent. Primary elements that may be used for borate containment and recycling are essentially the same those used in pressure treatment facilities applying other water-borne preservatives.

Under normal operating practices, liquid discharges from a borate treatment facility are confined to liquids that are not contained and reused within the process. For example, stormwater runoff from unpaved and unroofed treated-product storage areas might be the most common liquid from a borate treatment facility. The quantity of borate in such waters depends on many factors, such as quantity of precipitation, the degree of wrapping prior to the precipitation event, and soil characteristics of the storage yard. Uncontained liquid releases other than stormwater are generally confined to yard soils. Potential for groundwater contamination exists in locations where drip pads are not used in discharging areas or where the pad areas are inadequate to hold the treated wood until wrapping is accomplished.

Solid Wastes

Solid waste generation at borate facilities should be minimal. During normal operating conditions, solid waste is limited to cartridge filters that are used for dust and debris removal from recycled water and to the debris and sludge that are periodically removed from the sump, cylinder and tanks. Treated wood, such as stickers, cut-offs or broken products are another source of solid wastes.

Air Emissions

Potential sources of air emissions include mists from vacuum pump exhausts, cylinder doors and tank vents. At this time, no air emissions monitoring data are available from borate treatment plants.

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 chemical release depends on many factors, including the location of the wood preservation facility relative to ground and surface waters, the amount associated with the releases, and contingency measures in place at the facility.

Borates have only been recently introduced into Canada and no environmental assessments from plants are available as of January 2004. It is not expected that there would be environmental and worker health effects in facilities that respect normal operating practices. Although borates have relatively low fish toxicity, contamination of water bodies must be prevented. Boron is an essential micronutrient for plants, but it can be harmful in large quantities via absorption through plant roots.

Borates also have a low vapour pressure. Treatments under ambient temperature would not represent substantial air quality issues. With elevated process conditions, vapour generation would be increased. However, any air releases would likely be in the form of localized mists, and the effect of a facility on the surrounding environment is expected to be of little consequence.