Composting
Composting is the aerobic decomposition of biodegradable organic matter, producing compost. (Or in a simpler form: Composting is the decaying of food, mostly vegetables or manure.) The decomposition is performed primarily by facultative and obligate aerobic bacteria, yeasts and fungi, helped in the cooler initial and ending phases by a number of larger organisms, such as ils, and other families representing ants, nematodes and oligochaete worms.
Composting can be divided into home composting and industrial composting. Essentially the same biological processes are involved in both scales of composting, however techniques and different factors must be taken into account.

# Importance
Composting recycles or "downcycles" organic household and yard waste and manures into an extremely useful humus-like, soil end-product called compost. Examples are fruits, vegetables and yard clippings. Ultimately this permits the return of  needed organic matter and nutrients into the foodchain and reduces the amount of "green" waste going into landfills.  Composting is widely believed to speed up the natural process of decomposition appreciably as a result of  the raised temperatures that often accompany it. The elevated heat results from exothermic processes, and the heat in turn reduces the generational time of microorganisms and thereby speeds the energy and nutrient exchanges taking place. It is a popular misconception that composting is a "controlled" process; if the right environmental circumstances are present the process virtually runs itself. Hence a popular expression, "compost happens". It is nonetheless very necessary to provide as optimal circumstances as possible for large amounts of organic waste to break down properly. This is especially so when  it is accompanied by heating, since at elevated temperatures oxygen within the piles is consumed more rapidly, and if not controlled, will lead to malodor.
Decomposition similar to composting occurs throughout nature as garbage dissolves in the absence of all the conditions that modern composters talk about; however, the process can be slow. For example, in the forest bark, wood and leaves  break down into humus over 3-7 years. In restricted environments, for example, vegetables in a plastic trash container,  decomposition with a lack of air encourages growth of anaerobic microbes, which produce disagreeable odors. Another form of degradation practiced deliberately in absence of oxygen is called anaerobic digestion-  an increasingly popular companion to composting as it enables capture of residual energy in the form of biogas, whereas composting releases the majority of bound carbon-energy as excess heat (which helps sanitize the material) as well as copious amounts of biogenic CO2 to the atmosphere.
It is important to distinguish between terms such as "biodegradable", "compostable", and "compost-compatible".
- A biodegradable material is capable of being broken down completely under the action of microorganisms into carbon dioxide, water and biomass. It may take a very long time for a material to biodegrade depending on its environment (e.g. hardwood in an arid area), but it ultimately breaks down completely.
- A compostable material biodegrades substantially under composting conditions, into carbon dioxide, methane, water and compost biomass. Compost biomass refers to the portion of the material that is metabolized by the microorganisms and which is incorporated into the cellular structure of the organisms or converted into humic acids etc.  Compost biomass residues from a compostable material are fully biodegradable. "Compostable" is thus a subset of "biodegradable". The size of the material is a factor in determining compostability because it affects the rate of degradation. Large pieces of hardwood may not be compostable under a specific set of composting conditions, whereas sawdust of the same type of wood may be.
- A compost-compatible material does not have to be compostable or even biodegradable.  It may biodegrade too slowly to be compostable itself, or it may not biodegrade at all.  However, it is not readily distinguishable from the compost on a macroscopic scale and does not have a deleterious effect on the compost (e.g. it is not a biocide).  Compost-compatible materials are generally inert and are present in compost at relatively low levels.  Examples of compost-compatible materials include sand particles and inert particles of plastic.

## Reducing Waste
Although composting has historically focussed on creating garden-ready soil, it is becoming more important as a tool is reducing solid waste. More than 60 percent of household waste in the United States is recyclable or compostable. But Americans only compost 8 percent of their waste. Surveys have shown that the #1 reason Americans don't compost their waste is because they feel the process is complicated, time-consuming or requires special equipment. However, especially in rural areas, much of the solid waste could be removed from the waste stream by promoting "extremely passive composting" where consumers simply discard their yard waste and kitchen scraps on their own land, regardless of whether the material is ever re-used as "compost". ()

# Materials
Many different materials are suitable for composting organisms.  Composters often refer to "C:N" requirements; some materials contain high amounts of carbon in the form of cellulose which the bacteria need for their energy. Other materials contain nitrogen in the form of protein, which provide nutrients for the energy exchanges. It would however be an over-simplification to  describe composting as about carbon and nitrogen, as is often portrayed in popular literature. Elemental carbon - such as charcoal - is not compostable nor is a pure form of nitrogen, even in combination with carbon. Not only this, but a great variety of man-made, carbon-containing products, including many textiles and  polyethylene, are not compostable - hence the push for biodegradable plastics.
Suitable ingredients with relatively high carbon content include:
- Dry, straw-type material, such as cereal straws
- Autumn leaves
- Sawdust and wood chips
- paper and cardboard (such as corrugated cardboard or newsprint with soy-based inks)
Ingredients with relatively high nitrogen content include:
- Green plant material (fresh or wilted) such as crop residues, hay, grass clippings, weeds
- Manure of poultry and herbivorous animals such as horses, cows and llamas
- Fruit and vegetable trimmings
The most efficient composting occurs  by seeking to obtain an initial C:N mix of 25/30  by dry chemical weight.  Grass clippings have an average ratio of 10-19 to 1 and dry autumn leaves from 55-100 to 1. Mixing equal parts by volume approximates the ideal range.
Poultry manure provides much nitrogen but with a ratio to carbon that is imbalanced. If composted alone, this results in excessive N-loss in the form of ammonia - and some odor. Horse manure provides a good mix of both, although in modern stables, so much bedding may be used as to make the mix too carbonaceous.
For home-scale composting, mixing the materials as they are added increases the rate of decomposition, but it can be easier to place the materials in alternating layers, approximately 15 cm (6 in) thick, to help estimate the quantities. Keeping carbon and nitrogen sources separated in the pile can slow down the process, but decomposition will still occur.
Some people put special materials and activators into their compost. A light dusting of agricultural lime (not on animal manure layers) can curb excessive acidity, especially with food waste. Seaweed meal provides a ready source of trace elements. Finely pulverized rock (rock flour or rock dust) can also provide minerals, while clay and leached rock dust are poor in trace minerals.
Composting in the form of bioremediation can break down petroleum hydrocarbons, TNT and a variety of toxic compounds.  This is the bacterial and in some cases fungal content of the compost, that possess the enzymatic properties to de-polymerize the complex man-made molecules. In other words, there is nothing about the composting process per se that adds or detracts from this, unless as noted above, by warming, to increase the metabolic rate of the constituent organisms.
Some materials are best left to high-rate, a thermophilic composting system, as they decompose slower, attract vermin and require higher temperatures to kill pathogens than backyard composting provides. These materials include meat, dairy products, eggs, restaurant grease, cooking oil, manure and bedding of non-herbivores, and residuals from the treatment of wastewater and drinking water.  Meat and dairy products can be recycled using bokashi, a fermentation method.
Human waste can be composted by industrial, high-heat methods and also composting toilets, even though most composting toilets do not allow for the thermophilic decomposition believed to be necessary rapid kill of pathogens, such as Salmonella This is not a problem, however, since composting toilets also incorporate  the essential element of time required to reduce available substrate on which pathogens can feed, while increasing the growth of competing microbes.  If these high temperatures are reached, the resulting compost can be safely used as a fertilizer for food crops and even directly edible crops provided it is not illegal in the regions where the sludge is applied. Careful filtration of the compost also prevents contamination.

# Approaches
There are two major approaches to composting: active and passive.  These terms are somewhat of a misnomer since both active and passive composts can attain high heating, which increases the rate of biochemical processes. But the terms active and passive are appropriate descriptions for the nature of human intervention used.

## Active
Active (hot) composting is composting at close to ideal conditions, allowing aerobic bacteria to thrive.  Aerobic bacteria break down material faster and produce less odor and fewer pathogens and destructive greenhouse gases than anaerobic bacteria.  Commercial-grade composting operations actively control the composting conditions such as the carbon-to-nitrogen ratio.  For backyard composters, the charts of carbon and nitrogen ratios in various ingredients and the calculations required to get the ideal mixture can be intimidating, so many rules of thumb exist for approximating it.
Pasteurisation is a misnomer in composting, as no compost will become truly sterilized by high temperatures alone. Rather, in a very hot compost where the temperature exceeds 55 C (130 F) for several days, the ability of organisms to survive is greatly compromised. Nevertheless, there are many organisms in nature that can survive extreme temperatures, including the group of pathogenic Clostridium, and so no compost is completely safe. To achieve the elevated temperatures, the compost bin must be kept warm, insulated and damp.
Aerated Composting is an  efficient form of composting from the chemical point of view as it produces ultimately only energy in the form of waste heat and CO2 and H2O .  With aerated composting, fresh air (i.e. oxygen) is introduced throughout the mix of materials using any appropriate mechanism. The air stimulates the microorganisms that are already in the mix, and their by-product is heat. In a properly operated compost system, pile temperatures are sufficient to stabilize the raw material, and the oxygen-rich conditions within the core of the pile eliminate offensive odors. High temperatures also destroy fly larvae and weed seeds, yielding a safe, high-quality finished product.
Finally, aeration expedites the composting process through the mechanism of heating insofar as the elevated heat will drive biochemical processes faster, so that a finished product can be rendered in 60 to 120 days.  Aerated compost is an excellent source of macro- and micro-nutrients as well as stable organic matter, all of which support healthy plant growth. In addition, the micro-organisms in compost aid in the suppression of plant pathogens. Finally, compost retains water extremely well resulting in improved drought resistance, a longer growing season, and reduced soil erosion.
In Thailand, May 2008, this system has been used by farmer groups for more than 445 sites. The process needs only 30 days to be finished and 10 metric tons of compost is obtained each time. A blower (15 inch squirrel-caged blower with 3 hp motor) is needed to force the air through 10 piles of compost twice a day and 15 minutes each. The raw materials consist of agricultural wastes and animal manure in the ratio of 3 : 1 by volume. For more information please visit www.compost.mju.ac.th/eng

# Passive
Passive composting is composting in which the level of physical intervention is kept to a minimum, and often as a result the temperatures never reach much above 30C (86 F). It is slower but is the more common type of composting in most domestic garden compost bins. Such composting systems may be either enclosed (home container composting, industrial in-vessel composting) or in exposed piles (industrial windrow composting).  Kitchen scraps are put in the garden compost bin and left untended. This scrap bin can have a very high water content which reduces aeration, and so becomes odorous. To improve drainage and airflow, a gardener can mix in wood chips, small pieces of bark, leaves or twigs, or make physical holes through the pile.

## Natural
An unusual form of natural composting in nature is seen in the case of the mound-builders (megapodes) of eastern Indonesia, New Guinea, and Australia as well as in the case of bowerbirds of New Guinea and Australia. These Megapodes are fowl-sized birds famous for building  nests in the form of huge compost heaps containing leaf litter, in which they incubate their eggs. The birds work constantly to maintain the correct, almost exact, incubation temperatures, by adding and removing leaves from the compost pile.  In effect, this teaches us that thermophilic high-temperature composting is not man-made.

# Home composting
Home composters use a range of techniques, varying from extremely passive (throw everything in a pile and leave it for a year or two) to extremely active (monitor the temperature, turn the pile regularly, and adjust the ingredients over time). Some composters use mineral powders to absorb smells, although a well-maintained pile seldom has bad odors. It is usually located in the back garden.

## Moisture and heat
An effective compost pile is about as damp as a well wrung-out sponge. This provides the moisture that all life requires. Microorganisms vary by their ideal temperature and the heat they generate as they digest. Mesophilic bacteria survive best at temperatures of 20 to 44 C (70 to 120 F).  thermophilic (heat-surviving) bacteria grow optimally at around 55C (130 F), and can attain the fastest decomposition, since metabolic processes proceed more rapidly under higher temperatures.  Elevated  temperature is also preferred since it causes the most rapid pathogen reduction, and is more destructive of weed seeds.  To minimally achieve it, the heap should be about 1 m (3 ft) wide, 1 m (3 ft) tall, and as long as is practicable. This provides enough insulating mass to build up heat but also allows aeration.  The center of the pile heats up the most.
If the pile does not heat up, common reasons include that:
- The heap is too wet, limiting the oxygen which bacteria require
- The heap is too dry for the bacteria to survive and reproduce
- There is insufficient protein (nitrogen-rich material)
The necessary material should be added, or the pile should be turned to aerate it and bring the outer layers inside and vice versa.  You should add water at this time to help keep the pile damp. One guideline is to turn the pile when the high temperature has begun to drop, indicating that the food source for the fastest-acting bacteria (in the center of the pile) has been largely consumed. When turning the pile does not cause a temperature rise, it brings no further advantage. When all the material has turned into dark brown crumbly matter, it is ready to use.

## Worm composting
Worm composting or vermicomposting is a method of composting using Red Wiggler worms in a container. Food waste and moistened bedding are added, and the worms and micro-organisms eventually convert them to rich compost. The worms excrete a soil-nutrient material called worm castings.  Worm composting can be done indoors, allowing year-round composting, and providing apartment dwellers with a means of composting.
Worms are low in the food chain, and so are critical to healthy soil. This is why farmers have historically wanted healthy worm populations to live in their fields.
The nutrients and micro-organisms can be concentrated in liquid form called worm tea, made by running distilled water through worm castings.  When it is poured into the soil, the microorganisms multiply, creating a healthy growing environment for plants.

# Industrial composting
Industrial composting systems are increasingly being installed as a waste management alternative to landfills, along with other advanced waste processing systems. Industrial composting or anaerobic digestion combined with mechanical sorting of mixed waste streams is called mechanical biological treatment increasingly used in Europe due to stringent new regulations controlling the amount of organic matter allowed in landfills. Treating biodegradable waste before it enters a landfill reduces global warming from fugitive methane; untreated waste breaks down anaerobically in a landfill, producing landfill gas that contains methane, a greenhouse gas even more potent than carbon dioxide.
Most commercial and industrial composting operations use active composting techniques. These ensure that the process does not get out of control especially with the high through-put demand imposed by contracted,  incoming waste. This means that as short as possible a processing time must be maintained to keep the facility properly functioning (see compost windrow turner).  Partly for this reason composters have declined to support compost maturity standards if it would increase the required holding time. The greatest amount of technological control of composting is seen in systems using an enclosed vessel and controlling its temperature, air flow, moisture and other parameters.  See In-vessel composting (indoor composting).
Large-scale composting systems are used by many urban centers around the world. Co-composting is a technique which combines solid waste with de-watered biosolids, which originated in the 1960s and has fallen somewhat out of favor due to difficulties controlling inert and plastic contamination from MSW.  In Europe, mixed waste composting is virtually illegal. The world's largest MSW co-composter is the Edmonton Composting Facility in Edmonton in Alberta, Canada, which turns 220,000 tonnes of residential solid waste and 22,500 dry tonnes of biosolids per year into 80,000 tonnes of compost.  The facility is 38,690 square metres (416,500 ft2) large (equivalent to 412 Canadian football fields), and the aeration building alone is the largest stainless steel building in North America, the size of 14 NHL rinks.