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Guidance for Wood Preservation Facilities Reporting to the NPRI

IV-D Ammoniacal Copper Zinc Arsenate (ACZA) Wood Preservation Facilities

 

Overview

The treating process is similar to that of CCA in that a full-cell treating process is used to achieve preservative retentions similar to those required for CCA. The treating process is a closed system, and process water and liquid releases are collected and reused. The active preservative constituents are kept in solution at high pH with ammonia. Copper, zinc, arsenic and ammonia constituents are all NPRI reportable substances-arsenic is a CEPA-toxic substance.

Process Description

(Figure 4.3)

 

ACZA preservative is made on site by mixing arsenic acid, copper oxide, zinc oxide, ammonium hydroxide, ammonium bicarbonate and water. Zinc oxide, copper oxide and ammonium bicarbonate are manually added into a mixing tank containing water. After mixing and slurry formation, ammonium hydroxide is added followed by arsenic acid, and mixing continues until the copper is dissolved, which takes 10 to 72 hours. This initial concentrate of 8 to 13% total oxides with the ratio of the ZnO:CuO:As2O5 of 1:2:1, is then diluted to a working solution concentration of 2 to 7% total oxides.

The preservation process is similar to that of CCA. Wood impregnation is obtained using the full-cell or Bethell process. Wood is loaded on trams and sealed in the pressure retort. The impregnation process starts with applying vacuum for 30 to 60 minutes to remove air from the pressure retort and from the accessible wood cells. Treating solution, at ambient temperature, is drawn into the cylinder with the vacuum. Liquid pressure up to 1,040 kPa is applied.

The pressure is held until the designated amount of preservative enters the wood, which can take up to several hours depending on the wood species used. Pressure is then released and excessive preservative is pumped out to a storage tank. A final vacuum is usually applied to remove excess preservative from the wood surfaces. The treated wood is then allowed to stand in a protected area (drip pad) until no more solution drips from the wood. ACZA fixation is achieved by the loss of ammonia from the treated wood, ideally under ambient temperature conditions.

Please refer to Figure 4.4 for an illustration of potential sources of emissions of ACZA components.

Arsenic, copper, zinc and ammonia are the compounds that have to be monitored during the above processes for NPRI reporting. Processing steps that can result in emissions to air, soil, and groundwater or for off-site disposal are shown in Figure 4.4. Airborne emissions of all components as aerosols are possible at the retort door (especially when opened after a treatment charge), at vacuum pump exhausts and tank vents. In addition, fugitive emissions of ammonia will occur wherever treating solution and freshly treated wood is exposed to the environment. Liquid emissions to the soil, groundwater and surface water can occur wherever uncontained drips, leaks and spills occur. Unprotected, treated wood in storage will leach small amounts of all components when subjected to rain.

Solid contaminated wastes result from used filters, contaminated dirt, dust and sawdust and from process sludges that build up in the solution storage tanks.

Table 4.3: Probable Reportable Compounds with CAS Numbers-ACZA Preservation Facilities

CompoundCAS* Registry Number
Ammonia7664-41-7
Ammonium bicarbonate 
Ammonium hydroxide1336-21-6
Arsenic pentoxide1303-28-2
Copper oxide1317-38-0
Criteria air contaminants (including oxides of nitrogen (expressed as NOx), sulphur dioxide (SO2), carbon monoxide (CO), volatile organic compounds (VOCs), total particulate matter (TPM), particulate matter with a diameter = 2.5 microns (PM2.5), and particulate matter with a diameter = 10 microns (PM10))Refer to the Canada Gazette Part I Notice for the year being reported.
Zinc oxide1314-13-2

* CAS denotes Chemical Abstracts Service.

Figure 4.3: ACZA Preservation Plant LayoutACZA preservation plat layout

Figure 4.4: Potential Sources of Releases from an ACZA Wood Preservation Plant
Potential sources of releases from and ACZA wood preservation plant

Estimation Methodologies to Determine Releases and Transfers-ACZA Wood Preservation Facilities

Estimation methodologies for emissions are similar to those for CCA wood preservation facilities, with two exceptions:

  • Emission factors have not been developed for ACZA airborne emissions.
  • There will be a release of ammonia during the process and from freshly treated wood.

Facilities that meet the NPRI reporting criteria for CACs should refer to the Supplementary Guide for Reporting Criteria Air Contaminants (CACs) to the National Pollutant Release Inventory (NPRI) and other reference documents to estimate emissions of CACs to air.

For the determination of releases to water, soil and off-site transfers of solid wastes, refer to the section on estimating releases for CCA wood preservation facilities.

Airborne Emissions of Ammonia

Ammonia releases can be estimated by direct measurements for occupational exposure monitoring for those process steps where information on flow rates or air-exchange rates are known. Treaters can also develop their own emission factors. It may also be feasible to use a mass-balance approach.

Direct Measurement: Vented or Stack Releases

Example 1

The ammonia concentration is measured at different times during the treating process at the outlet of the wet scrubber installed in the exhaust vent (see table below). Aerosols and vapours from the retort door opening, vacuum pumps and storage tanks are all vented through the one exhaust. The vent is determined to have a flow rate (F) of 1,000 m3 per hour. Air concentrations of ammonia are measured (Ci in mg/m3) at the vent outlet on several occasions when the retort door is opened and the charge removed, when the initial and final vacuum pumps are operating, during the rest of the impregnation process (fill retort, pressure treat, empty solution) and when the treating system is not in operation. The number of days that wood was treated per year (Dt) and the number of days it was inactive (Dnt) are known, as well as the number of hours each day that the different process steps are active (ti).

The estimated emission during a given process step (such as retort opening and unloading) is given by:

Emission (kg) = F (m3/h) x ti (h/day) x Dt (days) x Ci (mg/m3) x 10-6 kg/mg

Examples of estimated releases based on direct measurements (emission factors not developed):

ProcessHours per dayOperating daysHours per yearExhaust vent rate (m3/h)NH3 Conc. in exhaust (mg/m3)Annual NH3 emission (kg)
Door opening, unload33009001000150135
Vacuum230060010008752.2
Fill, pressure, empty5300150010005684.0
Retort idle14300420010002188.2
 2465156010002132.8
     Total392.2
(0.39 tonnes)



Example 2:
Mass Balance


It is known that much of the ammonia used in the process will be released to the environment as a result of process losses and evaporation of ammonia from the treated wood to fix the preservative chemicals. However, it is also known that some ammonia is retained in the final treated product as a reaction product between the wood components and the other preservative components, and process ammonia is recovered by vacuum and other post-treatment processes in the retort. If it is assumed that 50% of the ammonia is retained in the treated wood and the remainder is released, mainly as vapour.

  • A treater uses 120,000 kg dry mass basis of ACZA containing 39.9% copper (47,880 kg). They formulate the solution with ammonia at 1.8 x the weight of copper (1.8 x 47,880 = 86,184 kg ammonia).
  • Assuming 50% is released during and after treatment, the total release is 86,184/2 or 43,092 kg (43.1 tonnes).

Note that this example ignores ammonia recovered by post-treatment conditioning and through scrubbers in vents and will be an overestimate.

Summary of NPRI Reporting Steps-ACZA Wood Preservation Facilities

  1. Gather information on sources of releases to air, soil, groundwater, storm water, and off-site releases.
  2. Determine reporting thresholds for NPRI reportable substances from the Canada Gazette Part I Notice for the year being reported.
  3. Estimate quantities released on site or transferred off site, based on monitoring data, engineering calculations or emission factors.

Sample Release Summary Form

SubstanceType of releaseReleaseKg Released
AmmoniaProcessFugitive aerosol and vapour 
 StorageLeaching losses to soil 
 StorageFugitive vapour loss 
 Remedial actionContaminated groundwater, etc. 
 Storm waterRelease to sewer 
 Catastrophic releasesRelease to air and groundwater 
ArsenicProcessFugitive aerosol and vapour 
 StorageLeaching losses to soil 
 Remedial actionContaminated soil, etc. 
 Storm waterRelease to sewer 
 Catastrophic releasesRelease to soil/groundwater 
 Non-hazardous wastesSolid wood, etc. to landfill 
 Hazardous wasteSolid waste transferred to hazardous-waste site 
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