Is My Worm Bin On Fire?

Here is a question from Heather:

Hi Bentley,
I have notice that the shredded bedding in my worm bin looks like it
has been burning (almost charred black and that brownish tinge). I
tried to wet it more, but then my worms seemed to disappear, so i
don’t know if I overwatered. Is it possible that my worm bin could be
heating up to the point where the paper is catching fire?
Thanks for helping me out. Love the newsletters too. very helpful.

Hi Heather,
That is indeed an intriguing question (and your subject header certainly made me smile). I’ll be honest, my initial reaction was “no way – not possible!”, but I decided that rather than assuming the role of a closed-minded ‘Negative Nelly’ (haha), I would look into it further.

I have certainly heard of spontaneous combustion occurring in straw storage areas, and even in largescale compost heaps, but my assumption with home vermicomposting systems is that they are too small, and too wet! That being said, I decided to read up a little on the process to see what I could learn.

I found a lot of really interesting info on this page: Fire – Compost and Organic Matter.

The temperatures required for the spontaneous combustion of organic matter are apparently as low as 150-200 C (212-392 F). What’s really interesting, is that while the heating is initiated by microbes, once it gets up past 70-80 C (158-176 F) – i.e. once the microbes are killed off – heat-releasing chemical reactions take over, continuing to drive the temperatures upwards.

Large volumes of material seems to be one important requirement (just as it is with hot composting), since a certain ‘critical mass’ is required in order to overcome the various cooling mechanisms at work (air flow, evaporative cooling).

It seems that my notion about the material needing to be quite dry isn’t necessarily true. According to the article, moisture contents as high as 45% still fall within the acceptable range for initiating the process. Given the fact that moisture can quickly evaporate from a pile during hot composting, and that large compost heaps often sit outside in the sun, I can now see how this could easily happen.

Getting back to the idea of this happening in a worm bin…

While I am still not sure that you could ever initiate a full-blown ‘fire’ in a worm composting system (it would have to be a huge system, and most of your worms would likely be killed off long before it happened), the aerobic decomposition process is in a sense akin to a slow biological ‘burning’. Paper that has been partially composted for example, often looks almost burnt. I suspect that the colonization of certain organisms can also make it look like there has been some charring of the material. For example, I’ve written previously about the way in which coffee grounds seem to develop a burnt look, even when decomposing in relatively small quantities – something I suspect is caused by the presence of certain fungi.

As for your worms disappearing once you added water, I’m not really sure what might be going on there! Hopefully they have shown up again since the time of your writing.

Anyway, I’m not sure if this has helped to answer your question at all, Heather – but thanks again for bringing up the interesting topic. It was cool to learn a little more about spontaneous combustion.

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    • Harlene geier
    • September 26, 2009

    Hey bentley,
    just thought I say hi & ask a quick question.
    That article on the bin looking like it caught fire was
    interesting. That makes me wonder if maybe something
    similar might be going on with my small bag of composting
    food that I have saved for my worms. Two weeks ago I put
    a few tomatoes in with others stuff to compost, I have since
    then given it to my worms & started another batch, but
    I have started to notice a burning tire smell coming from
    the bag? Do you think to little oxegen is the cause? Harley.

    • Nathanael
    • September 28, 2009

    Hi Bentley
    I have not experienced burning but I have had problems with the food waste getting hot (100-110 F). It usually happens a day after I have added some food waste that I have been storing in a sealed 5 gallon container. I am guessing that it is because the waste is shifting from aerobic to anaerobic decomposition.
    In addition, it happens inside the bin if I add food waste from my juicer. I think this is because the finely shredded scraps quickly start to decompose. The worms will crawl up the sides of the bin for a few days and sometimes I put ice cubes inside the bin to cool things down. Eventually things settle down and the worms seem happy to have plenty of food to eat!

    • Bentley
    • September 29, 2009

    HARLENE – based on your description of what you did, I would definitely assume that any stinky smell would be the result of food waste going anaerobic. I’ve never encountered ‘stinky tire’ myself (haha), but perhaps it is a result of the particular mix of wastes you have.


    NATHANAEL – thanks for sharing. I would guess that your hunches are right on the money. It’s ALL about aerobic microbial activity – so if you provide the right combination of c-rich and n-rich materials, moisture, oxygen and accessibility (surface area – shredding increases this), you are more than likely going to end up with some heating. Especially if you have a lot of material!

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