Wood preservation facilities, ammoniacal copper zinc arsenate: chapter C-5

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

5.1 Description of Process

The impregnation of ACZA into wood is carried out in pressure treatment plants using a process similar to that used for chromate copper arsenate (CCA) (refer to Part 1- General Background Information, Section 2.2.3 Figure 3).

The specific treatment times and pressures are dictated by the species of wood, the wood product and the moisture content of the wood. Treatment conditions must be calibrated to yield the target retention levels described on the pesticide label. The CAN/CSA O80 (3) 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.

Good practice, after the impregnation cycle, includes applying heat and a prolonged vacuum to remove ammonia. Ammonia loss causes precipitation of the active ingredients in the wood. Quality control tests are carried out to ensure that treated products meet a minimum quality standard.

5.2 Potential Chemical Discharges

Based on the plant design and operational practices, various potential emission sources exist that may affect worker health and/or the environment. The potential sources and releases are illustrated in Figure 1.

Liquid Discharges

The ACZA process uses water-borne ingredients and is operated as a “closed” system. Dripped solution or contaminated storm runoff water can be reused in the process. Primary facility design features that should be used for ACZA containment and recycling include the following:

• sealed containment surfaces and dyking of major process components including the cylinder and ACZA tanks
• containment surfaces for chemical drips from treated wood on the cylinder discharging track and in the storage area for freshly treated wood
• a collection sump to receive residual preservative from the cylinder (following the treatment cycle) and the accumulated contaminated runoff from other containment surfaces. This material can then be reused in the treatment process following filtration to remove dust and debris.

The greatest potential sources of contaminated liquid discharges from ACZA facilities are stormwater runoff waters from unsealed and unroofed areas where treated products are stored or transferred. The quantities of ammonia, copper, zinc or arsenic in these waters depend on many factors, including quantity of precipitation, elapsed stabilisation time and temperature prior to a precipitation event, and soil characteristics of the storage yard. Uncontained liquid releases other than stormwater are generally confined to yard soils, particularly those locations near drip pads in charging/unloading areas and when freshly treated wood is not stored on containment surfaces. These contaminated yard soils have the potential to cause groundwater contamination.

Solid Wastes

Refer to Part 1, Chapter A, Section 5.2 for details on potential solid waste discharges.

Air Emissions

Due to its volatility, the use of aqueous ammonia implies a significant potential for ammonia emissions at the ACZA facility, if proper controls are not in place. Potential sources of ammonia releases include storage tank vents, ACZA mixing tank hatches and vents, vacuum pump exhausts, vapours released when retort cylinder doors are opened, and from freshly removed wood charges. Scrubbers should be used to control ammonia releases from plant equipment (vent tank). Potential sources of copper, zinc and arsenic emissions include mists from vacuum pump exhaust, cylinder doors and tank vents. Air emissions are generally intermittent and restricted to localized areas.

Air monitoring at one ACZA plant revealed that levels of ammonia were below 10ppm, 8 hour TWA and all levels of other component were below occupational health limits; however, ammonia emissions in the vicinity of retort cylinder doors during openings and near freshly treated wood could still be uncomfortable for the operator. Nevertheless, it is possible that the concentration of emissions can exceed existing occupational health limits depending on the plant design and its operational procedures.

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.

There have been no documented environmental or worker health effects as a result of “normal” ACZA use at wood preservation facilities. It can be assumed 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. Nevertheless, accidental events in well-designed and maintained facilities can occur and are of equal concern.

Figure 1: Potential Chemical Releases From ACZA Pressure Treating Plants