Worm Composting & Methane Production

Worm Composting...

“Am putting some of our domestic waste into worm farm. Have been told recently that the material put into conventional compost bins contributes to methane production, is it the same for worm farms.” ~ Dan Milliken

Hi Dan,

Hopefully by “domestic waste” you are simply referring to normal household organic wastes, such as food residuals, yard waste, cardboard etc (not human sewage, or chemical waste of any sort).

Methane (CH4) production is not so much dependent on the particular materials used, but on the conditions present during the composting process.

This powerful greenhouse gas is produced under highly anaerobic (without oxygen) conditions. By definition, composting is the bio-oxidative degradation of organic wastes under controlled conditions (i.e. for the process to technically be considered composting, there needs to be ample oxygen present).

That being said, it’s important to mention that anaerobic pockets CAN form quite easily during composting – especially on a large scale, and especially when the process is poorly managed.

As such, there has been considerable research interest focused on measuring the amounts of methane (and other greenhouse gases) produced during composting.

Just to give you some idea of the difference proper oxygenation can make in terms of reducing methane emissions – in a study by Thompson et al. (2004) it was found that unaerated composting produced as much as 24 times the amount of methane produced in aerated piles!

In fact, composting is considered by many researchers to be an ideal strategy to help livestock farmers reduce their methane emissions, since traditional livestock manure storage methods can lead to considerable methane production.

I mentioned above that ‘conditions’ are much more important than ‘materials’ in determining the amount of methane produced. Well, certain kinds of materials are definitely more prone to going anaerobic than others, namely those with a very dense consistency and/or with very high moisture contents. When composting these types of materials it is very important to make sure you are adding bulking agents (straw, fall leaves, shredded cardboard etc) to help increase air flow.

As far as worm farms go, by their very nature these systems tend to be quite aerobic, especially those that allow water drainage and contain ample amounts of ‘bedding’ materials (aka bulking agents). The constant movement of the worms and the materials they are feeding upon serves to greatly aid the oygenation of the composting mass.

Another important consideration is temperature. Methane production increases as temperatures rise (Kaharabata et al. 1998), so the high temperatures of thermophilic (hot) composting are more likely to result in methane production than the ‘mesophilic'(moderate) temps typically associated with worm composting (and small-scale backyard composting in general).

So, the long and the short of it is that worm farming, while having the potential to release a little methane, should by no means be a cause for concern as far as CH4 emissions go.

Worm farming HAS on the other hand been recently implicated as a potentially significant emitter of N20 (nitrous oxide – another potent greenhouse gas), but discussion of that topic will need to wait until another time, I’m afraid!

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