Wood preservation facilities, chromated copper arsenate: chapter B-5
5. Description of Preservative Application and Potential Chemical Discharges at CCA Wood Preservation Facilities
5.1 Description of Process
In 2012, 31 facilities used CCA and only 13 used it as their sole preservative (18). The impregnation of CCA into wood is carried out in pressure treatment plants (refer to Part 1 - General Background Information, Section 2.2.3 Figure 3).
Suppliers of CCA generally offer a high level of support services, including facility design and routine safety and consulting expertise on operations, maintenance and emergency response procedures. This approach provides a relatively high level of control over preservative use at most facilities (19). Design and operational guidance from the suppliers provides a national level of general consistency to CCA facilities.
CCA is normally purchased as a premixed concentrate (50% or 60%) shipped by bulk truck and rail tanker. The concentrate is stored in tanks and diluted with water to a 1.5-5.0% strength working solution. This dilution is accomplished by pumping transfers and by recirculation between bulk tanks. The working solution is then applied to the wood in a pressure cylinder, which may be up to 45 m long and 2 m in diameter.
Treatment conditions must be calibrated to yield the target retention levels described on the pesticide label. The CAN/CSA O80 (4) 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
CCA wood preservation plant designs and operational practices do differ, 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.
The CCA 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. Economics and the toxicity of the process chemicals have led the CCA 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 CCA containment and recycling at well-operated facilities are illustrated in Part 1 - General Background Information, Section 2.2.3 Figure 3.
Under normal operating practices, liquid discharges from a CCA treatment facility are confined to stormwater runoff from unpaved and unroofed areas where treated products are stored. The quantity of copper, chromium or arsenic in such waters depends on many factors, such as quantity of precipitation, the degree of chemical fixation in the treated wood as determined by fixation time and temperature prior to the precipitation event, and soil characteristics of the storage yard. Uncontained liquid releases other than stormwater are generally confined to yard soils.
Refer to Part 1, Chapter A, Section 5.2 for details on potential solid waste discharges.
Several monitoring studies in the vicinity of CCA air emission sources (e.g. cylinder doors, tank storage areas) have been reported in the literature, and concentrations of copper, chromium and arsenic below existing occupational health limits were reported (19, 20, 21).
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 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 released amount, the frequency of releases and contingency measures in place at the facility.
Environmental assessments of CCA facilities (19, 21) do exist. Environmental or worker health effects are commonly not caused by “normal” CCA usage at wood preservation facilities. However, available information indicates that improperly designed and/or operated facilities do have the potential to contaminate site soils and groundwater to levels that would prevent the use of such groundwater for drinking purposes (19). Also, surface runoff waters that exceed various regulatory limits have been reported (9). Nevertheless, accidental events in well-designed and maintained facilities can occur and are of equal concern.
Figure 1: Potential Chemical Releases From CCA Pressure Treating Plants
Figure 1 is schematic representation of the CCA pressure treatment process with indication where there are potential chemical releases.
- Date modified: