Composting is a system of waste treatment that involves engineered biological decomposition of solid biodegradable residues under controlled conditions. It is essentially controlled decomposition and shares some properties with the decomposition of waste that happens in nature. Natural micoroganisms break down the waste material so that after a period of time - several months to several years - the waste is no longer recognizable as the material it started from. The processed waste is called "compost" and can be used as a soil adjuvant. A variation called vermicomposting uses worms to eat the waste and break it down.
While many households engage in composting on a small scale, large composting operations run all over the country, especially where agriculture is important. Many will take waste from businesses. Some operations are commercial and some municipalities have compost collection systems.
Another benefit of composting is that it can be done near where the waste is generated, and can be done on a small scale. However, there are advantages to very large composting operations as the heat the pile generates and environment it provides for bacteria and worms make the waste degrade more rapidly.
Composting works best when the waste material is not too far removed from nature. Yard waste is ideal, as composting is just a managed way of facilitating what would happen in nature. Most yard waste is cellulose and hemicellulose. Paper, which is also cellulose and derived from wood, can decompose through natural processes, but it is slower. For that reason paper is usually excluded from compost bins, although incidental paper is not a worry.
Composting efficiency is enhanced when the material is porous enough to allow free flow of liquid water, air, and microorganisms. Studies have shown that smaller particle size results in faster and more efficient composting. Operators of composters often add bulking material such as sawdust and straw. When one batch of waste is finished composting, some of it is added to a fresh batch to act as an inoculum.
An operating parameter of interest is the C:N ratio - carbon to nitrogen. This ratio is important in many ecosystems. Compost operators sometimes attempt to control the ratio of “green” organic materials and "brown" organic materials in their piles. “Green” organic material includes grass clippings, food scraps, and manure, which contain large amounts of nitrogen. "Brown" organic materials includes wood and dried leaves, which contain little nitrogen. Although material like paper can be put in composters, the ideal material for composting is waste food and plants.
Another important factor is levels of oxygen and water. Efficient industrial size compost operations take measures to ensure the aerobic microorganisms have enough air and water to function. Without enough of those composition proceeds by alternative pathways - rot. Most commercial composting is aerobic, although anaerobic composting is possible, too.
Moisture level and temperature inside the composter are also important.
The characteristic size of the particles is important, and the smaller the pieces of organic material, the more efficient the composting. Operations often include grinders to break down large pieces and increase the surface area which increases the rate of microbial decomposition. A bed of smaller particles also allows more air into the pile which helps to even out temperature.
Anaerobic composting is similar to "rot" that takes place in swamps. Also called putrefaction, it operates at a low pH compared to aerobic composters. Sometimes anaerobic composters are called "digesters".
Vessel composting - very simple process in which organic materials are placed in a batch inside a container - commercially available containers come in sizes from 50 gallons to 5000 gallons and more, and have varying levels of environmental control. The compost process is finished in a few months.
Static pile composting - outdoor version of vessel composting appropriate for larger waste batches. Note that when the pile is finished, no new waste is added until all material has been composted. At that time the compost can be removed and another pile can be started on the same site. This is a batch process, not a continuous one.
Windrow composting - this is appropriate for large quantities of organic waste such as material collected from entire neighborhoods or communities. The “windrows” are long piles of waste. Per the EPA, "the ideal pile height is between four and eight feet with a width of 14 to 16 feet." While the compost process happens (over the course of several months), operators aerate them by turning the piles. In compost piles of this size, the temperature is elevated. Large compost piles can operate at temperatures over 100 degrees F, and indeed up to 140 F.Do people compost medical waste?
Biological waste is not routinely put into composters. An exception are placenta pits used in some cultures. But nobody puts waste blood, materials that have contacted diseased patients, or pathological waste into composters.
However, healthcare and research facilities produce compostable materials - yard waste from the grounds, waste food from the cafeteria. If there is enough of this stuff, the organization may find it feasible or attractive to have an on-site composter - or at least to send that waste to a composting facility. Not only would this cut waste disposal costs, but the organization may get positive publicity for being environmentally responsible.