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 springtails, 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.
Decomposition similar to composting occurs throughout nature as garbage dissolves in the absence of all the conditions, and weather patterns, 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". Another term for composting is yeesting, it is literally translated into changing garden while making gestures.
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:
Ingredients with relatively high nitrogen content include:
The most efficient composting occurs by seeking to obtain an initial C:N mix of 25-30 to 1 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. It 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 a high-rate 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 which uses bokashi bran, wheat bran inoculated with effective micro-organisms (EMs).
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 for a 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 the available substrate on which pathogens can feed, while increasing the growth of competing microbes. If 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 humanure is applied. Careful filtration of the compost also prevents contamination. Humanure fertilizer is, however, used throughout the less developed world and is becoming more accepted as a garden amendment in the developed world (see humanure).
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 this system has been used by farmer groups for more than 445 sites (May, 2008). The process needs only 30 days to finish without turning need and 10 metric tons of compost is obtained each time. A blower (15 inch squirrel-caged blower with motor) is needed to force the air through 10 static 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.
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.
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 55°C (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 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.
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.
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 Municipal solid waste (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 4½ Canadian football fields), and the aeration building alone is the largest stainless steel building in North America, the size of 14 NHL rinks.
For some, less is more; NONFICTION From composting toilets to sleeping in cars, Rosen explores how and why people go off the grid.(VARIETY)
Aug 08, 2010; Byline: SUSAN AGER Special to the Star Tribune Remember from your youth the potheads, the guys who slept in their rust-bucket...