Timber offcuts, sawdust and ash cannot be used for compost. Please do not put them in the organics bin or in your home compost as it may contaminate them.

  • Place timber offcuts, sawdust and cold bagged ash only in your red rubbish bin when disposing.
  • Let ash cool for at least five days in a metal bucket or container before disposing in the red rubbish bin, otherwise it may cause a fire.
  • Do not burn treated timber offcuts. When we burn treated timber, it releases large amounts of toxic heavy metals into the air we breathe, and the ash left behind contains 100 times the concentration of toxic heavy metals as the original piece of wood.

How is there a fire risk?

Ash waste needs to be cooled for at least 5 days before disposal, to ensure there is no risk of embers reigniting. We are working collaborative with the Fire Brigade to raise awareness of this issue, and as a result of bin, house and collection truck fires from ash waste.

If a bin is collected containing ash waste that has not been cooled, it causes a risk to public safety. We have had a number of fires in trucks, and the collection truck driver has had to act quickly to find a safe place to empty the collected waste. This causes not only a public safety issue but also causes delays to traffic and the remaining bin collections in that area.

So what happens when we burn treated timber?

When you burn treated timber, not only are considerable amounts of toxic heavy metals released into the air we breathe, but the ash waste contains 100 times the concentration of toxic heavy metals than the original piece of timber/wood.

Background information

Heavy metals are found naturally in our environment, like in our tree clippings, but can also be concentrated by certain processes.

Timber may be treated which chemical solutions to prolong its life, particularly when used for outdoor and building purposes. It is difficult to identify whether timber has been treated or not, and once it’s in with all the other organics waste, we can’t get it out again – which means any compost we make from it is contaminated, and can’t be used in any soil.

In New Zealand, the following chemicals may have been used to preserve timber:

  • Arsenic
  • Copper
  • Chromium.

A kilogram of treated wood can contain over 8,000 times the levels of contamination, and we can’t remove it from the organics waste that goes in the green bin, or the compost that it’s turned into. That’s why we need to think about how we manage timber offcuts, sawdust and ash in our waste, to stop it getting in there in the first place.
As little as a third of an ice cream container of ash can be enough to contaminate the contents of one whole green bin collection truck load of organics waste. Once it’s in there, we can’t get it out, and contaminated compost can’t be added to soil to help grow our food and gardens.