Composting wood preservatives?
Can I compost wood that have been treated with wood preservatives?
Many think the answer is no, but in most cases it is YES you can!
We have done our best to distal the facts, read the comments below and note we can only act as an information source not a guarantor of safety. With that said, what we found is: modern preservatives supplied in the EU/Can/US to the general public during the last 10 years are biodegradable and compostable.
The golden rule: follow the manufacturer’s label.
By the time the wood is no longer usable – you will have most likely have got rid of the tin! Look at the problem logically. The purpose of the preservative is to kill bacteria and fungi that rot wood – they contain biocides and fungicides that kill bacteria. Hopefully you are not throwing ‘good’ usable wood on your compost heap – it is more likely your wood is rotting/decaying. If the bacteria are already decomposing the wood -the chemical that was preventing bacterial growth is no longer working! Composting merely accelerates and completes the process.
The question now is where are the leftover chemicals and is the compost safe?
Why do so many composting sites advise not to compost wood treated with preservatives
This relates back to toxicity –10-20 years ago ‘heavy metal’ wood preservatives (see table below) were nasty, dangerous and were banned. This toxicity issue appears to be applied to all wood preservatives!
It is right to be concerned about toxic chemicals many mistakes have been made – but I also advise, whenever you see toxicity cited as a reason not to do something, step back and ask: toxic to what (plants, humans, animals?) at what level (parts per billion or gallons of the stuff? and how is it toxic (when it touches the skin, ingestion, inhalation)?
10-20 years ago ‘heavy metal’ wood preservatives (see table below) were found to be nasty, dangerous to both marine life and workers exposed to heavy doses of fumes and liquids – they were banned.
Heavy metal preservatives & old wood
The arsenic and chromium would remain in your compost/soil – even in small quantities, the risk of them entering the food chain is unacceptable. Do not compost old wood treated with old arsenic or chromium based preservatives.
Newer organic and organic+copper preservatives
The organic (carbon) fraction is used by the bacteria. The copper ions are most likely to be left in the soil/humus matrix. Most soils already contain metal ions – the quantity looks small and non copper is not a problematic metal (our water system run through copper pipes).
If your wood has been treated with a newer version and is rotting – the preservative is already gone/well on way out – composting bacteria will decompose the wood and any copper ions will return to the compost/soil.
Brief History – 2000 years of wood preservative use
||Olive-oil, linseed oils to help protect woods
||Coated their ships hulls with tar
|Creosote – the dark brown stinky stuff
||A natural product from wood or coal. Will fully biodegrade with time. Became common place to treat railway sleepersAll types of creosote are composed of phenol derivatives (as found in wood). For its useful effect, wood-tar creosote relies on the presence of methyl ethers of phenol, and coal-tar creosote on the presence of naphthalenes and anthracenesToday, creosote is manufactured from the distillation of coal tar. Creosote is regulated as a pesticide, and is not usually sold to the general public – but it is still used
|Heavy metals compounds 60-90s
- CCA (Chromated Copper Arsenate) green tinted
- ACQ (Alkaline Copper Quaternary)
|The bad period – nasty chemicals started being used to treat wood without the knowledge of hazards. Typically arsenic, copper, chromium based compounds.These were banned in late 90’s as they were found to be toxic to marine life (small amounts) and caused problems for factory workers via inhalation and ingestion (exposed to large amounts)
- Copper azole & Other copper compounds
- PTI – – Propiconazole-Tebuconazole-Imidicloprid
|They all have incredibly long chemical names like tebuconazole or propiconazole – but I did note these same chemicals as used in food production – so safe? CuHDO used in EuropeCurrently limited to above ground applications such as decks (why – because below ground it decomposes! (Note: all three of the PTI components are also used in food crop applications)
Wiki has a very informative section on wood preservatives http://en.wikipedia.org/wiki/Wood_preservative
We have done our best to distal the facts. As always with H&S – you are the guardian of you own safety. We can only act as an information source not a guarantor of safety.