Compost bins hold organic matter while it is being broken down by microbes or worms. Having the material under controlled conditions while it undergoes decomposition allows you to manipulate the speed at which the items break down.
You control the contents, so you can eliminate odors and add greater air flow, moisture, or green and brown materials as needed. Green materials are nitrogen-rich items like grass clippings, healthy plant waste (not pernicious weeds or diseased plants), and fruit and vegetable scraps.
Brown materials provide carbon, an energy source for the microbes. These are usually dried plant matter like hay or straw, but can also include paper and cardboard, coffee grounds, leaves, branches or twigs. These work together with green materials to assure the health of the bin.
A balance of materials is important in order to create passageways which allow air to circulate, to prevent odors and to prevent green items from clumping together, slowing the process. Finding the right balance may take several tries as you experiment to achieve the best mix for your combination of materials and conditions. To begin, try a 3 to 1 ratio of brown matter to green matter.
Hot composting is aerobic, meaning that the microorganisms that keep the process moving require air. Anaerobic composting relies upon microbes which do not need air. This version can generate unwanted odors and--since it does not achieve the internal temperatures of an aerobic compost bin--parasites or pathogens may survive.
Aerobic compost piles release heat as a byproduct of the decomposition process. Cold composting also works, but the process is far slower. Vermicomposting, where worms are used to help break down the organic materials, operates at lower temperatures. It is an efficient method of creating compost, but requires more maintenance than a bin relying on microbes alone.
For a hot pile to work well, 1 to 3 cubic feet of material is best. If you have multiple bins, you can allow one pile to decompose as you gather new waste material in a second or third bin.
Keep the browns and greens separate until you are ready to build the pile. Layer the materials inside. Add water until everything is damp, but not soaking wet. This is easiest to achieve if you spray each layer as you add it, rather than trying to moisten the full pile all at once. Adding a top to the bin is optional. Closed compost bins help keep insects and animals out.
Compost needs to be stirred to help distribute the materials, to rotate them into and out of the hot core. This brings air to the microorganisms, keeping them alive and active. Bins should be turned about every two weeks. For a more specific time and faster composting, the internal temperature must be monitored.