Anaerobic composting made simple for the gardener

Anaerobic composting is a quite fascinating process, with potential to solve a number of environmental problems. The art of hot (aerobic) composting is made accessible and simple by demonstrations like that given by Marcé, a permaculturist, at the TUFCO gathering. but the 'cold' or anaerobic form I'm finding a little tougher to understand, and had to do a quite a bit of research.

Anaerobic means without air. So a closed container, or restricted air entry is what would group a process in this class. The decomposition processes can be fermentation, or other processes. 

Anaerobic composting can be done on any scale. On a commercial scale it is often called anaerobic digestion. While essentially the same process as that you would do on a small scale, anaerobic digestion is much more controlled and fine tuned by the methods and technology applied, to produce products like biogas, and this is where an understanding of bio-chemistry comes in, and large scale anaerobic composting can be more complicated than large scale commercial aerobic composting.

a bokashi bin we make: one of the best known forms of anaerobic composting in the homebokashi is anaerobic composting

anaerobic composting at home ?

Many anaerobic methods of composting are brought forward on the web, from Bokashi, to simply putting your organic waste in a sealed plastic bag or bucket. Even forms of fermentation like making wine, kefir and vinegar are related to the breakdown processes in anaerobic digestion. These fermentations can all play a role in boosting your soil's effective micro-organism populations. It is generally claimed that the anaerobic form of composting is low maintenance, as the digging over needed to spread oxygen through an aerobic compost heap falls away, and the material can just be left alone, unmonitored, for months. The problem is, if it proceeds far enough, methane is produced, a gas whose ability to cause global warming is more potent than that of carbon dioxide.

Some anaerobic composting methods are closed, absolutely airtight and others are not, involving regular opening of the vessel to add new organic matter, or covering the material to be composted with loose sand. Simply putting your kitchen waste in a plastic bag and adding water, and then closing it tight, will induce anaerobic bacterial activity, and you will know if you open the bag, I assure you, by the smell. Aerobic processes do not make such a smell, and a stench is a sign of the anaerobic process having taken over.

Organic garden fertilizer made easy
and smelling good

I’ve made anaerobic compost by closing material in bags. In my mum’s garden there are lots and lots of big trees and there was always a thick carpet of leaves on my little paved entertainment area which I would sweep up and put in bags. In my experience it took very very long, a year or more. I can think of two reasons why… firstly the consensus is on the web, that anaerobic composting is slow. I will question this in my article on anaerobic digestion. I had closed the leaves in a plastic bag and aerobic composting would quickly use up all the oxygen and they would then go anaerobic, so this MAY have made it so slow. Another cause is that dead leaves are very high C, so the C:N ratio is not conducive even when a large bulk is piled together, and aerated, to anything but slow slow disintegration, and lignin, a substance in dead leaves, is resistant to breakdown by most composting organisms.  After a year, the stuff in the black bags was black mush, sweet not foul smelling, and not fully broken down. Scattered on the ground it made an attractive dark leaf mold compost. The high C would also be responsible for the lack of stench, that is of course in addition to the long curing time. I observed that the masses of leaves brought up from the bottom of the pond smell like horse manure, so they must have ingredients like proteins that go through the smelly anaerobic processes, in addition to the resistant lignin and other large “high Carbon” molecules. This makes them useable to grow organic food, probably not the heavy feeding kinds of food, but those which prefer a lower nutrient level like carrots, which I learned how to plant in a permaculture workshop. 

What's next ? some traditional and more home grown anaerobic composting methods

After this brief introduction I will go on to discuss traditional and home anaerobic composting methods, and the production of biogas, in a similar process, but on a larger scale.

For more information about composting have a look at Compostjunkie an excellent source of information. 

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