A compost digester is a system for enhancing decay of organic material in an almost completely enclosed container. Aside from the lid at the top, where feedstock is dropped, a digester has no openings save at the bottom where liquid is drained. The great advantage of compost digesters over traditional open-air piles is that one can compost almost any kitchen waste in a digester — meat, bones, some dairy products, even pet feces. (Be aware, that all instruction manuals say that one should never put compost made from pet feces where vegetables will be grown.)
However, some digesters cannot handle any yard waste and none can handle the quantities that a pile can absorb. Nor can they easily break down high-carbon or woody items. The wood chips and pine cones that may slow an outdoor pile will stop a digester completely.
Anaerobic versions can be as simple as a plastic bag full of garbage left in the sun or as complex as a barrel on bricks. (For an indoor version, see Bokashi Composting.) Interestingly, the only back yard anaerobic digesters on the market are designed for and sold as ways to dispose of pet waste. However, any of them can be used to digest food scraps and small amounts of yard waste.
If you are only composting food scraps and not pet feces, a home-made digester is perfectly adequate and making one is extremely easy. A garbage can with a tight-fitting lid works extremely well. Commercial anaerobic digesters are also available. The Greater Victoria Compost Education Center has a number of downloadable fact sheets available, one of them on digesters.
Anaerobic digesters have drawbacks. They are not quick. They produce methane rather than the less serious greenhouse gas carbon dioxide which aerobic composting produces. Their finished product often needs to aerate for several weeks to reduce its acids to levels that will not damage plants. Smells can be an issue. And the beneficial microbes that make compost so valuable an addition to soil do not grow in anaerobic conditions.
However, a well-maintained system can solve all these problems.
The aerobic digester avoids the environmental problems, both local and global, that can plague anaerobic systems. It won’t smell, since it doesn’t produce hydrogen sulfide. It won’t pollute (much) since it produces carbon dioxide rather than methane. This method is unusual because it is designed not to produce compost, but to dispose of kitchen scraps without allowing solids (compost) to build up.