Wood preservation facilities, alkaline copper quaternary: chapter G-5

5. Description of Preservative Application and Potential Chemical Discharges

5.1 Description of Process

ACQ preservatives, whether amine or ammonia versions, are water-based products that are applied by pressure treatment in essentially the same manner as chromated copper arsenate (CCA) (refer to Part 1 - General Background Information, Section 2.2.3 Figure 3).

ACQ-A (amine version) is prepared on-site at wood preservation facilities from concentrates and water to form a working strength solution of 0.5-3.4% actives (copper plus quat). The main differences between ACQ-A and the other ACQ types is that the quat is DDA-chloride, and is present in a 1:1 copper:quat ratio within the treatment solution.

ACQ-C (ammonia version) is prepared on-site at wood preservation facilities from concentrates and water to form a working strength solution of 0.5-3.4% actives (copper plus quat).

ACQ-D (amine version) is shipped as two premixed solutions that are then mixed together at the facility. It is prepared by adding a known quantity of amine copper to a measured amount of water in a mix tank to form a working strength solution of 0.5 to 5.0% actives (copper plus quat). The strength of the working solution is determined by the amount of preservative to be retained in the wood. When using recuperated water in the working solution, ethanolamine can be used for makeup to adjust mixture concentration. The working solution is applied to wood that has been loaded into pressure cylinders varying in size depending on the wood products being treated.

The specific treatment times and pressures are dictated by the species of wood, the wood product and the moisture content of the wood. A predetermined range of process parameters is defined by the applicable treatment standards of the CSA-O80 Series of Standards (5). Good practice, after the impregnation cycle, includes a prolonged vacuum to remove ammonia. Ammonia loss causes precipitation of the active ingredients in the wood.

5.2 Potential Chemical Releases

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

Figure 1. Potential Releases from ACQ Pressure Treating Plants

Liquid Discharges

The ACQ process uses water-borne ingredients and can be operated as a “closed system.” Once dripping has cease and a minimum of 48 hours has passed, freshly treated lumber should be wrapped prior to storage in the yard or under sheds. If incidental drippage occurs, the lumber must be returned immediately to the drip pad area until all dripping has been confirmed to have stopped. Immediate cleanup of the drippage should be initiated to prevent any potential for causing stormwater contamination or tracking. Dripped solution or contaminated storm runoff water can be reused in the process. Primary facility design features that should be used for ACQ containment and recycling include the following:

• concrete containment surfaces and diking with a second barrier for major process areas, including the cylinder and tanks
• containment surfaces for chemical drips from treated wood on the cylinder discharging track and in the freshly treated drip pad
• 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.

Under normal operating practices in a properly designed facility there should be few contaminated liquid discharges. The most common potential sources of contaminated liquid discharges in an ACQ facility are stormwater runoff waters from unpaved and unroofed treated product storage areas. The quantities of preservative in these waters depend on many factors, including quantity of precipitation, stabilization method, elapsed stabilization time, 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 where containment surfaces are used for freshly treated wood. These contaminated yard soils, at high concentration, have potential for causing groundwater contamination.

Solid Wastes

During normal operating conditions, solid waste generation at ACQ facilities should be small. See Chapter A, Section 5.2 for more details.

Air Emissions

The use of the ammonia formulation implies a significant potential for ammonia emissions at the ACQ facility if proper controls are not in place, and likewise with ethanolamine emissions when the amine formulation is used. Potential sources of release for either of these components include storage and mix tank vents, vacuum pump exhausts, vapours released when cylinder doors are opened, and freshly removed wood charges. Air emission levels should be monitored and appropriate controls such as exhaust extractors employed where necessary. Air emissions are generally intermittent and restricted to localized areas.

Concentrations of ACQ active components have generally been measured to be below occupational health limits. However, as was the case with ammoniacal copper arsenate (ACA) facilities, emissions of ammonia have the potential to be higher; dictating the use of a combination of process controls and personal protective equipment, specifically on mixing and in the immediate vicinity of cylinder doors during openings and vacuum exhausts (25).

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 or surface waters, the amount and concentration of preservative released, the frequency of releases, and contingency measures in place at the facility.

Improperly designed and/or operated facilities would have the potential to contaminate site soils and groundwater to levels that would prevent the use of such groundwater for drinking.