Anaerobic composting is a form of composting that requires little to no oxygen. In fact, just as aerobic means with air, anaerobic means without air.
In the process, anaerobic bacteria, i.e. bacteria that do not need air, turn organic material into compost. They also release gases, which include large amounts of methane.
The process occurs naturally in nature in places where there is no or little oxygen – for example in the wetlands that produce peat. It is more likely to take place when high-nitrogen (green) materials are used.
Typically, there are two ways in which anaerobic composting can take place in the garden.
First, there’s anaerobic composting that happens by accident when there is not enough oxygen in the compost.
In fact, it’s likely that some anaerobic composting takes place in most home compost heaps at some stage.
Then there’s anaerobic composting which is done on purpose. This is often done in an airtight container, allowing organic material to break down in the absence of oxygen.
It can be done at home, but it is also done commercially, and sometimes used to produce energy.
What causes accidental anaerobic composting?
Too much moisture
A compost heap can become anaerobic if it is too wet due to excess rain or watering. The water takes up free air space, forcing oxygen out of the pile.
Lack of aeration
Failing to ensure there is air in your compost is a surefire way to get aerobic composting.
There are many ways to aerate your compost, from using bulking materials to turning the heap. Fortunately, not all involve a lot of energy!
Too much nitrogen
Nitrogen is essential to aerobic composting. However, if there is a lot of nitrogen and not much carbon, there will be an imbalance in these essential compost nutrients.
If composted without browns, nitrogen-rich materials such as manure, food scraps, and grass clippings break down too quickly. In the process, they release nitrogen gas into the environment, reducing the oxygen available for aerobic bacteria and other aerobic organisms.
As a result, anaerobic bacteria can begin to form and dominate the heap, leading to limited oxygen and anaerobic conditions.
Aenarobic composting on purpose?
For the home composter
Anaerobic composting can also be done deliberately. This can be as simple as leaving organic material in a plastic bag to decompose, putting it in a sealed container or burying it in a pit.
Popular methods include using a Bokashi Bin or using a digester such as the Green Cone (only available in the UK, unfortunately). (Do note the Bokashi system produces pre-compost rather than compost.)
Leaf mold is also often made using an anaerobic processor.
Anaerobic composting is often said to involve less work. That does depend on the method you use.
For example, if you choose a method of composting that requires a lot of turning, aerobic composting can be labor-intensive. However, other methods of aeration require less work.
On the other hand, some anaerobic systems can be pretty fiddly. I personally lost interest in the bokashi system after using it for a few weeks. The amount of effort in return for a small amount of pre-compost didn’t seem worth it when I had other systems that could produce a large amount of compost.
Can deal with smaller amounts of waste
Systems like bokashi bins and the plastic bag method are ideal for dealing with small amounts of kitchen waste.
Can deal with a wider range of items
Items such as oil, meat and fish can be tricky to compost in a traditional system. On the other hand, some anaerobic systems are ideal for use with materials that would otherwise attract pests.
Can be used to produce energy
Some systems are used to capture methane (which is also called biogas) and use it as an energy source. However, this is usually done by commercial systems.
Typically, anaerobic systems take longer to produce finished compost. The material produced through anaerobic composting can be very acidic and is often left to mature for a full year.
If not kept in a sealed container, the process can produce a gas that smells like rotten eggs.
All composting processes release some greenhouses gasses. However, anaerobic composting produces high levels of methane, which is considered many times more harmful than carbon dioxide.
May not be as effective at killing pathogens
Much is made of how heat kills pathogens in hot compost piles. The aerobic composting process also eliminates pathogens in other ways. For example, some microorganisms release antibiotics that kill harmful bacteria.
However, is also possible that the harsh environment produced by anaerobic composting also kills pathogens.
Can be messy
If you’ve ever tried to compost grass without mixing it, you can see the mess it can cause! Materials can become wet and slimy in the early stages of the process. However, this may not be a problem if the material does not need to be handled until the process is complete.
Note that http://whatcom.wsu.edu/ag/compost/fundamentals/biology_anaerobic.htm has some useful information, but is an insecure link.