If you pile up organic matter, moisten it, and throw a tarp over it to deprive it of oxygen, anaerobic composting will result. Any bin with a tight lid and drainage holes can be used. Even a garbage bag will work. A pile with a tarp over it works well but it will smell
Anaerobic composting requires an entirely different set of organisms and conditions than does aerobic composting. The anaerobic process, which is essentially putrefaction (sorry, but there it is), produces a very acidic environment similar to that in the stomach. Hence the term “digester” used to describe anaerobic processes and to distinguish them from aerobic composting. Actually, the stomach still holds the prize for acidic environments. The pH in an anaerobic digester might dip as low as four but that in the stomach is between one and two.
Both anaerobic and aerobic decay produce heat as a by-product. The temperatures in an aerobic system can become hot enough to kill pathogens or weed seeds. Those in a digester cannot. However, the digester’s acidic environment itself eventually does the trick. Eventually is the key term here. The inhospitable environment takes six months to a year to kill off dangerous micro-organisms.
There’s a second advantage to waiting a full year before using the compost from a digester. Fresh anaerobic material — feedstock that has gone through the initial phase of digestion but hasn’t sat for months afterwards — is so acidic that it cannot be directly applied to plants. Nor can it be dug into dirt where it might come into contact with plant roots. Before it is safe to use, it must go through an aerobic phase that lasts about a month to neutralize its pH. Material that has digested for a full year should be safe for the garden — and it will smell much better.
For this reason, it’s best to have two bins going. When one is full, you can close the lid on it for a year while you deposit waste in the other. By the time you open the first bin, all pathogens will be dead and the composted material will be ready to use.
The simplest anaerobic composter is a plastic bag filled and left in the sun. Most consist of a container with just a single compartment, either placed on cement blocks with a vessel underneath to collect the leachate, or partially buried in the ground. The leachate from a buried vessel will fertilize the nearby soil.
Sinking the bottom of the digester into the ground slows the rate at which leachate drains out of the feedstock and ensures that the contents do not dry out. It also gives worms and other soil organisms access to the contents of the digester, though many will only feel at home after the bottom layer of material has passed through its most acidic phase and the pH starts to level off. Finally, burial helps control odors and makes it almost impossible for pests of any kind to gain access to the bin’s contents.
As with aerobic composting, different methods yield different composting times. The throw-everything-in-a-plastic-bag method mentioned above is roughly equivalent to building a hot aerobic pile. The basic composting is accomplished within a few weeks if the container is in hot sun. A barrel or digester to which you continually add material is more like a slow pile. The stuff at the bottom forms mature compost while the stuff at the top isn’t compost at all.
Material is added to a continuous pile and a barrel digester in the same way. But the compost is harvested differently. With a continuous aerobic pile you throw yard waste and food scraps on top and pull compost out the bottom. There’s no easy way to remove the finished compost from the bottom of a digester. It’s just too wet and sloppy. This is why you eventually need to put the lid on an aerobic system and walk away. This also explains why it’s best to have two bins: one to add to while the other digests.
Despite these disadvantages, anaerobic composting is the best way to go in some situations:
You're looking to dispose of a one-time load of wet, potentially smelly, or pest-attracting kitchen waste, such as you'd accumulate after a day spent canning fruits or vegetables, cleaning freshly caught fish, or organizing a big social gathering that generates food scraps.
Pulling spent garden plants at the end of fall leaves you with an enormous pile of organic matter that you don't have the space or time to manage over winter.
Aboveground composting of kitchen scraps without a sealed container isn't allowed where you live.
You aren't keen on the appearance of a compost area in your landscape, but you prefer not to send your organic waste to a landfill.
You want to improve soil structure and fertility in a future garden bed.
You don't have time to monitor the air or moisture requirements of an aboveground compost pile.