Introduction
The lumber industry is one of the oldest and most important in our world. wood as a building material has been the primary resource of residential construction for centuries. Over the years, advancements in the industry have created a highly efficient system with very little waste. This insures maximum profits from, and minimum impact on, such a valuable resource. Among these advances lies the advent of pressure treated, or “PT,” lumber. Pressure treated lumber is wood that has been milled, and then soaked in preserving chemicals under pressure to promote longevity. This technology creates wood that lasts decades longer than raw wood under similar conditions, and may even be used where raw wood would rot away in a matter of months. This analysis will attempt to reveal which material, raw wood or PT wood, is most appropriate from an environmental standpoint; including water use and pollution, net carbon release, and lumber resource use as viable factors.
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Figure 1: Incised Treated 4*4 (stock photo) |
Figure 2: Raw Cedar 4*4 (stock photo) |
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| Figure 3: Estimated Service Life of Wood Products (Quarels et. al.) |
Since pressure treated lumber is essentially raw lumber with added preservatives, they both follow the same path up to the end of the milling process. The trees are harvested in a forest. Here they are de-limbed, cut into logs, and moved to a temporary processing area known as a landing. This produces sawdust from the original cut and bucking process, which is left to decompose on the ground, and excess branches from the de-
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limbing process. The branches are either burned in a “slash pile,” collected for an experimental charcoal production process, or mulched to be sold as fertilizer. Next, the logs are loaded onto trucks and sent to a mill. Here they are stripped of bark and graded. The bark is burned, used as garden bedding or fertilizer, or used as playground floor material. Then the logs are sawn into the desired sizes. The lumber that comes out needs to dry before being shipped out for use. This is accomplished either in large, open drying yards, or in a kiln. The kiln is often powered by burning sawdust, scrap wood, and bark from the milling process, and the ash left over makes excellent fertilizer. Sawdust and wood chips from the cutting process can also be used to create entirely different building materials, like particle board, plywood, and composite wood, which is a blend of sawdust and plastic. Water is used throughout the process to cool saw blades and prevent over-drying of the wood before milling. (EPA)
The modern lumber system is incredibly efficient, with almost no wood waste. (EPA) All
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| Figure 5: Small Lumber Mill (Griffith Lumber) |
the byproducts are used, whether it’s for fertilizer, energy, or new material. However, this is the point where PT and raw lumber go separate ways.
Raw Lumber
Raw lumber is taken straight from the mill, once it’s been graded and dried, and distributed to hardware stores and construction suppliers ready for use. This wood will be used mostly in places where it will stay dry and rot free, like in the walls of a house; sandwiched in layers of sheet rock and insulation. Raw lumber simply can’t be used, according to most building codes, for many moisture-heavy installations. The wood is usually cut further by the installer, producing sawdust and scrap wood, which are either land filled, burned, or, recycled. Outside applications include deck joists, deck surfacing, temporary structures, and fencing. (Quarles et. al.) I am mostly assessing outside installation.
There are many end of life scenarios. Assuming the wood was Douglas fir, a common type easily accessible in Arcata; it could last ten to fifteen years if not in contact with the ground. In a ground-contact application it is estimated to last only three to six years. If used in a temporary structure which would be abandoned, for example a child’s fort, it would decompose and return to the soil. In more likely situations it would need replacement after ten to fifteen years, at which point it would be either burned, sent to a landfill, or, depending on how rotten it was, reused for non-structural purposes. (Quarles, et. al.)
Pressure Treated
After the mill, wood destined to become PT lumber is shipped to a treatment plant. Here, the wood is first conditioned to allow maximum absorption of the preservative. This is a major source of wastewater for the system. The wood is also incised, or perforated, to improve deeper absorption. Next the wood is treated. There are several types of treatment, but I will focus on water-borne inorganic pressure treatment, as it is the most common. Once conditioned, the wood is sent to a “retort,” or the cylinder where treatment occurs. A vacuum is created in the retort, then preservative is pumped in until the wood will not absorb any more. The excess preservative is drained and recycled, and the wood is allowed to drip dry. By law, the wood must remain on a special pad or inside the retort until it stops dripping preservative. It is then held for a short time to ensure no preservative is left unabsorbed and dripping before being shipped out to the users. (EPA)
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| Figure 6: Treatment retorts (Lagarde) |
The use of PT lumber is slightly different than raw lumber. PT lumber is meant to be used in high exposure situations, and is even required by code for many outdoor applications. A Douglas fir PT board will last twice as long as a raw board in off ground installations, over thirty years, and doesn’t show a decrease in longevity with ground-contact application. This makes replacement of PT lumber a rare occurrence. However, because of the chemicals involved, disposal of scrap wood and sawdust is much more serious. Burning of PT lumber is strongly discouraged to prevent volatilization of the chemicals. It also cannot be composted or used as fertilizer, because of its resistance to decomposition and the danger of introducing the chemicals to food crops. PT lumber must be disposed of in an approved landfill. (Quarles, et. al.)
PT lumber will continue to leach preservatives for the duration of its life, even after disposal in a landfill. This was especially a problem with the preservative CCA which contained arsenic as an insecticide. Use of CCA was discontinued in 2003, and ill effects from newer preservatives have not yet been witnessed. There is still a danger of soil and water contamination from the production process and leaching over time, which may adversely affect ecosystems. (Quarles, et. al.)
Calculations
For these figures, water use is defined as the total volume of water taken from municipal sources. This does not represent the total water that requires treatment, as much of it is recycled in-process, and runoff from rainfall and other sources also require treatment on-site.
Mill Water Use
The mill uses approximately 4.3 ML per year to convert 50,000 
of uncut wood into useful raw lumber annually.(Goodfellow inc.)
L/ML = 4,300,000 L 4,300,000 L ÷ 50,000
= 
This means that 86L of water is used to produce 1
of raw lumber. Then some of this wood is sent to a treatment plant to be pressure treated.
Treatment Plant Water Use
The pressure treatment process uses 2,700,000 L of water to treat 12,000,000 BF (board feet) of wood in one year. There are approximately 424 board feet in a cubic meter. (Wood Centre)
12,000,000 BF * 
Add the usage from the milling process for the total:
* Note that these figures are only examples from random facilities that I was able to find data for, and should not be treated as best or worst case scenarios, or an industry average.
Flow Diagrams
Untreated
Treated
Conclusion
Based on my research and calculations, it is apparent that raw, untreated lumber is more environmentally friendly than pressure treated lumber, even considering the frequent replacement of raw lumber.The difference in water use between the two is tremendous. Pressure treatment uses chemicals that can cause serious environmental harm if mishandled. Also, the extra transportation associated with pressure treatment adds more air emissions.
Summary
Environmental Impact |
Raw Lumber |
Treated Lumber |
Water Pollution |
Nutrients and Heavy Metals |
Preservatives, Fungicides, and Insecticides |
Water Use |
86 |
1039 |
Emissions |
Transportation |
Extra Transportation to Treatment Facility |
| Wood Use from Replacement | Every 3-15 years depending on conditions (2x use of treated) | 30+ years |
Works Cited
EPA. Profile of the Lumber and wood Products Industry. Washington, DC: Office of Compliance, Office of Enforcement and Compliance Assurance, U.S. Environmental Protection Agency, 1995. Web.
Goodfellow Inc. "NOTICES OF THE SOCIETY." Journal of the Royal Society of Arts 134.5356 (1986): 219-25. Goodfellow Incorporated. Web.
Quarles, Stephen L., John W. Kobzima, and Pamela M. Geisel. "Publication 8144." Selecting Lumber and Lumber Substitutes for Outdoor Exposures (n.d.): n. pag. Web.
The wood Centre Developement. HUON Development Proposal and Environmental Management Plan (n.d.): n. pag. Southwood Resources. Web.