[USCC] Pressure-treated lumber compost bins
Thu Mar 7 10:43:32 2002
Coincidentally. we (BioCycle) were asked a similar question about pressure treated wood and I subsequently developed it into the Q&A column for the March 2002 issue of BioCycle. The near-final version of the Q&A is pasted below. The second part of the question (about cedar) will not be included in the issue due to space limitations.
Although it is too late to change the version that will appear in the magazine, I would be interested in hearing about any corrections or additions or explanations that should have been in the Answer. -- Bob Rynk
Q. If a composter wants to build a compost bin for yard waste and/or manure, and eventually wants to put that compost on a vegetable garden, should they avoid using pressure treated lumber?
A. Prudence suggests that you should not use pressure-treated lumber for constructing a composting bin. It is now well demonstrated that chemical components of the pesticide do leach from treated lumber. The compost is likely to retain a good share of those chemicals (although some would also be carried with water into the soil or drains below). Potentially, this could affect the compostís quality, if not its safety and performance.
The predominant type of pressure treatment chemical currently used is chromated copper arsenate or CCA. Copper (Cu), chromium (Cr) and Arsenic (As) are the chemical elements that can leach from the wood. Generally, the greatest concern is the leaching of As, which is a carcinogen and an acute poison, in certain forms and in sufficiently high amounts.
Recent research has demonstrated that CCA lumber does leach Cu, Cr and As over time. Its been found that soil beneath CCA-treated wood decks has higher than background levels of these chemicals. The research demonstrates that the chemicals move with rain from the wood into the soil. The chemical concentrations are highest near the drip line and decrease with soil depth. Similarly, soil adjacent to raised garden beds made from CCA wood contains higher than normal levels of As, and those levels decrease
with distance from the wood (to background levels at roughly 6 to 15 inches away) The situation is possibly worse for CCA wood used for a composting bin. At least one study found greater Cu losses from CCA-treated blocks of wood (jack pine) exposed to composting vegetable matter, compared to the Cu losses from similar blocks that were soaked in water or buried in soil (Cooper and Ung, 1992; Forest Products Journal, 57(42v.9)57-59). The authors suggested that the organic acids that developed during
composting enhance leaching of Cu. However, it also was reported that the losses were not large enough to affect the "efficacy" of the compost. If you already have a composting bin or storage that has CCA lumber, and you are concerned about it, you can paint the wood or apply a sealant or line it with plastic to increase the retention of the chemicals in the wood.
Cu, Cr, and As are all regulated elements within the U.S. EPA Part 503 regulations on biosolids, which are often used as a standard for other organic residues. Therefore, adding more Cu, Cr, or As via leaching from bin lumber could be a problem if the compost is on the borderline of meeting the regulatory limits. However, yard trimmings and manure composts are rarely in this situation (although livestock manure can have high copper levels).
Unlike the soil next to a deck, the elements leached out of wood will not accumulate for a long period in a given batch of compost. The different batches of compost that move through the bin will disperse the elements into the environment. However, if the compost is consistently used on the same ground, like a vegetable garden, the leached chemicals will accumulate in the soil.
Research conducted in Florida indicates that compost can influence arsenic leaching for good or bad, depending on the characteristics of the compost. Aziz Shiralipour, the principal investigator, reports that " the municipal solid waste compost (MSW) reduced the leachability of As in soil, especially in the native soil, whereas the biosolids compost greatly increased the leachability of native As" (as opposed to the chemical As added for the experiment). The difference appears to be due to the
electrical conductivity (EC) and dissolved organic carbon (DOC) of the composts with more leaching occurring with the biosolids compost that had higher EC and DOC. According to Shiralipour, "these findings indicate that composts with high EC and/or high DOC are not suitable for reducing arsenic leachability. On the other hand, composts with low EC and/or low DOC are effective in reducing arsenic leachability in contaminated soils, therefore, reducing the chance of contaminating the ground water."
Although much depends on the specifics, in general, yard waste compost would tend to act more like the MSW and manure compost more like the biosolids product. The Florida research is expected to be reported in Compost Science and Utilization this year.
An answer to the question of using pressure-treated lumber for composting bins would not be complete or balanced without some mention of risk. Pressure treated wood proponents and some scientists argue that As is naturally ubiquitous in the environment and that the amount lost from the wood does not increase the health risks. In fact the most critical pathway of As exposure from CCA wood appears to be hand to mouth contact for children playing on CCA wood treated playgrounds (neglecting worker
exposure at construction sites). Many proponents would advise you to use the pressure treated wood for the bins because the risks are small.
In any case, the debate may soon be irrelevant. The U.S. EPA and the wood preservative industry recently agreed to phase out the use of arsenic-based wood preservatives in two years. The announcement is careful to state that the agreement does not imply that CCA lumber poses unreasonable health risks. In fact, EPA will continue with the risk assessment for CCA lumber that is currently in progress.
Q. Is cedar a good substitute for pressure treated wood, or are there compounds in the cedar that may also cause problems in the long run?
A. "Cedar" can refer one of several different tree species, depending on where you live. More than likely, you are referring to a common lumber yielding species of cedar like Western Red Cedar. Most species of cedar are resistant to decay. Therefore, lumber from some species of cedar is used for wood applications that require durability, like roof shingles or compost bins. Cedars owe their decay resistance to resins and oils that act as preservatives, resisting attacks by insects and microorganisms
(e.g. fungi). Conceivably, those same qualities could render the wood, or compounds from the wood, toxic to plants or composting microorganisms.
However, cedar does not appear to be a problem for composting or compost. Although it may compost more slowly than other species of wood, there have been no reported problems in composting bark, chips or sawdust from cedar species (e.g. Western Red or White Cedar). With sufficient time, any bio-hindering compounds will likely decompose during composting. In fact, compost bins sold specifically for backyard composting are often made from cedar (most likely Western red Cedar). In addition, the use of
mulch made from cedar woods does not appear to effect plants so compounds lost from cedar should not be a problem for composts either (see the University of Missouri newsletter: News for Missouriís Gardens, Yards and Resources, May 4, 2001; Volume 7, No.3).
It seems that the cedar trees that are currently harvested for lumber are less decay resistant than the cedars of old. This is because the timber from the former trees contained more heartwood, which in turn accumulated more of the compounds that resist biological attack.
Rufus Chaney wrote:
> Dear Aziz:
> Your note is interesting regarding As and composts.
> Have you considered the addition of Fe to composting biomass to serve as sorbent for arsenate and phosphate? Because high phosphate solubility is becoming an adverse property of compost, addition of Fe may become more common. And that should cause a strong reduction in solubility of arsenate as well as phosphate. I should note that if one is adding Fe to improve adsorption of the product, one should be sure that adequate Mn is present either from the feedstocks, or from amendments along the the Fe.
> Rufus Chaney
> Beltsville, MD
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