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Composting is the controlled aerobic decomposition of biodegradable organic matter, producing compost. The decomposition is performed primarily by aerobic bacteria, helped by larger creatures such as ants, nematodes and oligochaete worms.

Composting can be divided into home composting and industrial composting. Both scales of composting use the same biological processes, however techniques and different factors must be taken into account.

Composting recycles organic household and yard waste into compost, which returns needed organic matter to the soil. While improving the soil for gardens, composting also reduces the amount of waste going into burgeoning landfills. Composting speeds up the process of decompositon by providing an optimal environment for waste to break down and enable it to return to the earth. This requires the correct mix of carbon, nitrogen, oxygen and water.

Decomposition occurs even without some of these ingredients; however, the process can be slow and unpleasant. For example, vegetables in a plastic bag decompose, but the lack of air encourages the growth of anaerobic microbes, which produce disagreeable odors. Degradation with insufficient air is called anaerobic digestion.

Different materials contain various nutrients for the composting bacteria. Some materials contain much carbon in the form of cellulose which the bacteria need to produce sugars and heat. Other materials contain nitrogen in the form of protein, which allow the compost bacteria to thrive.

Ingredients with higher carbon content include:

• Dry, straw-type material, such as cereal straws
• Autumn leaves
• Sawdust and wood chips
• Some paper and cardboard (such as corrugated cardboard or newsprint with soy-based inks)

Ingredients with higher 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
• Seaweeds
• Coffee grounds

The most rapid composting occurs in matter containing 25 to 30 times more carbon than nitrogen by dry chemical weight. Grass clippings have an average ratio of 19 to 1 and dry autumn leaves about 55 to 1. Mixing equal parts by volume approximates the ideal range.

Poultry manure provides much nitrogen but little carbon, while horse manure provides both. Sheep and cattle manure compost at a lower temperature and so slower than poultry or horse manure.

Eggshells are a good source of nutrients for the compost pile and the soil, but typically take more than one year to decompose.

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 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. 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 also breaks down petroleum hydrocarbons and some toxic compounds for recycling and beneficial reuse. Composting for such purposes is called bioremediation.

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, but milk and oil cannot.

Human waste can be composted by composting toilets, but most composting toilets do not allow for the thermophilic decomposition needed to kill pathogens. If these high temperatures are reached, the resulting compost can be safely used as a fertilizer for food crops and even directly edible crops. Careful filtration of the compost also prevents contamination.

There are two major approaches to composting: active and passive.

An active compost heap, steaming on a cold winter morning. The heap is kept warm by the exothermic action of the bacteria as they decompose the organic matter.

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 in a hot compost (such as a compost oven) occurs if the temperature exceeds 55 °C (130 °F) for three or more days. To achieve it, the compost bin must be kept warm, insulated and damp to encourage the cultivation of actinomycete bacteria. Pasteurised soil is valuable for gardening; pasteurisation is otherwise expensive and complicated, and adding chemicals to produce it makes the compost less healthy.

Aerated Composting is a particularly efficient form of hot composting. With aerated composting, fresh air (i.e. oxygen) is introduced throughout the mix of materials using an electric blower. The oxygen stimulates the micro-organisms that are already in the mix, and their by-product is heat. In a properly operated compost system, pile temperatures are sufficient to pasteurize 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 greatly expedites the composting process and a finished product can be rendered in 30 to 60 days, without turning the compost pile. 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.

Passive (cold) composting is composting in which temperatures never reach above 30 °C (86 °F). It is slower and leaves dormant pathogens, 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 has a very high water content and no aeration, and so becomes anaerobic and creates foul smell and emits significant adverse greenhouse gases. To improve drainage and airflow, a gardener can mix in wood chips, small pieces of bark, leaves or twigs.

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.

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 enjoy temperatures of 20 to 40 °C (70 to 110 °F). thermophilic (heat-loving) bacteria prefer around 60 °C (140 °F), and are the fastest decomposers. This is the ideal temperature as it also kills most pathogens and weed seeds. To 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. Adding water at this time helps 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 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 microorganisms 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 systems are increasingly being installed as a waste management alternative to landfills, along with other advanced waste processing systems. It has been done even for landfills from foundry furan resin sand dust. Industrial composting or anaerobic digestion combined with mechanical sorting of mixed waste streams and is called mechanical biological treatment. Treating biodegradable waste before it enters a landfill reduces global warming; untreated waste breaks down anaerobically in a landfill, producing landfill gas that contains methane, a potent greenhouse gas.

Most commercial and industrial composting operations use active composting techniques. This ensures a high-quality product and a short processing time (see compost windrow turner). The greatest control, and so the highest quality, is achieved by composting inside an enclosed vessel and controlling its temperature, air flow, moisture and other parameters. See In-vessel composting (indoor composting) (also called en-vessel).

Large-scale composting systems are used by a few urban centers around the world. Co-composting is a technique which combines solid waste with de-watered biosolids. The world's largest 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 4½ Canadian football fields), and the aeration building alone is the largest stainless steel building in North America, the size of 14 NHL rinks.

• BSI PAS 100
• Compost
• Composting Association
• Container composting
• Ecological sanitation
• Humanure Composting
• In-vessel composting
• List of composting systems
• Compost tea
• Comparison of anaerobic and aerobic digestion

• How-to video on home composting from the nonprofit group Kitchen Gardeners International.
• Cré, the Irish Composting Association Contains information on composting in Ireland.
• Online compost calculator that provides the carbon:nitrogen ratio for many common ingredients and calculates the best mix.