According to Wikipedia:
Composting is the process of producing compost through aerobic decomposition of biodegradable organic matter. The decomposition is performed primarily by aerobes, although larger creatures such as ants, nematodes, and oligochaete worms also contribute. This decomposition occurs naturally in all but the most hostile environments, such as within landfills, extremely arid deserts or cold weather such as boreal winters or polar regions, which prevent the microbes and other decomposers from thriving.
Composting can be divided into the two areas of 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 is the controlled decomposition of organic matter. Rather than allowing nature to take its slow course, a composter provides an optimal environment in which decomposers can thrive. To encourage the most active microbes, a compost pile needs the correct mix of the following ingredients:
Carbon
Nitrogen
Oxygen (in the case of aerobic composting)
Water
Decomposition happens even in the absence of some of these ingredients, but not as quickly or as pleasantly. (For example, vegetables in a plastic bag will decompose, but the absence of air encourages the growth of anaerobic microbes, which produce disagreeable odors. Degradation under anaerobic conditions is called anaerobic digestion.)
The goal of a composting systemThe goal in a composting system is to provide a healthy environment and nutrition for the rapid decomposers, the bacteria. The most rapid composting occurs with the ideal carbon to nitrogen ratio of between 25 and 30 to 1 by dry chemical weight. In other words, the ingredients placed in the pile should contain 25 to 30 times as much carbon as nitrogen. For example, grass clippings average about 19 to 1 and dry autumn leaves average about 55 to 1. Mixing equal parts by volume approximates the ideal range. Commercial-grade composting operations pay strict attention to this ratio. For backyard composters, however, 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 to guide composters in approximating this mixture.
Materials for compostingGiven enough time, all biodegradable material will compost. However, not all compost feedstocks are appropriate for backyard composting. Most backyard systems will not reach high enough temperatures to kill pathogens and deter vermin, so pet droppings, non-vegetarian animal manure, meat scraps, and dairy products are best left to operators of high-rate, thermophilic composting systems.
Certain substances should not be composted by the average homeowner, as they require more sophisticated systems.
These substances include non-vegetarian animal manures and bedding, by-products of food production and processing, restaurant grease and cooking oils, and residuals from the treatment of wastewater and drinking water. Composting will also break down petroleum hydrocarbons and some toxic compounds for recycling and beneficial reuse. The use of composting for such purposes is most commonly referred to as a form of bioremediation.
High-carbon sources provide the cellulose needed by the composting bacteria for conversion to sugars and heat, while high-nitrogen sources provide the most concentrated protein, which allow the compost bacteria to thrive.
Some ingredients with higher carbon content: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)
Some ingredients with higher nitrogen content:Green plant material (fresh or wilted) such as crop residues, hay, grass clippings, weeds
Animal manures (choose vegetarian horse manure, cow manure, llama manure, etc.)
Fruit and vegetable trimmings
Seaweeds
Coffee grounds
Poultry manure provides lots of nitrogen but little carbon. Horse manure provides both. Sheep and cattle manure don't drive the compost heap to as high a temperature as poultry or horse manure, so the heap takes longer to produce the finished product.
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 in any event.
Greasy food waste and wastes from meat, dairy products, and eggs should not be used in household compost because they tend to attract unwanted vermin and they do not appropriately decompose in the time required. However, eggshells are a good source of nutrients for the compost pile and the soil although they typically take more than one year to decompose. If recycling of meat and dairy products is desirable, Bokashi is a suitable alternative, which uses fermentation. However, even in Bokashi, liquids like milk and oil should not be used. Manure from non-vegetarian animals should never be used, and neither should human or pet waste.
Composting techniquesThere are a number of different techniques for composting, all employing the two primary methods of aerobic composting:
Active (or hot) composting allows aerobic bacteria to thrive, kills most pathogens and seeds, and rapidly produces usable compost. Aerobic bacteria produce less odour and fewer destructive greenhouse gases than their anaerobic counterparts. In addition, they are usually faster at breaking down material and the faster material is broken down, the faster compost is created for your garden.
Pasteurisation in a hot compost (such as the Compost Oven) will occur in any garden compost bin if the temperature reaches above 55 C (131°F) for three or more days. To achieve it, you need to keep your garden compost bin warm, insulated and damp since this encourages the cultivation of actinomycetes, a vital bacteria in the pasteurisation process.
The pasteurised soil naturally created through heat in the garden compost is valuable for the composting home gardener, since the pasteurisation process is otherwise both expensive and complicated, and adding chemicals to produce the pasteurisation effect makes the compost less healthy.
Passive (or cold) composting allows Nature to take its course in a more leisurely manner, while leaving many pathogens and seeds dormant in the pile.
Cold composting is the type of composting done in most domestic garden compost bins in which temperatures never reach above 30 C (86°F). Cold composting is characterised by individuals putting their kitchen scraps in the garden compost bin and leaving them untended. This "scrap bin," having a very high moisture content and without aeration, is likely to turn anaerobic and generate foul odours, including significant adverse greenhouse gas emissions.
When composting this way, a gardener can improve the process by adding some wood chips or small pieces of bark, leaves, twigs or a combination of these materials distributed throughout the mixture. This material helps to improve drainage and airflow.
Such composting systems may be either enclosed (home container composting, industrial in-vessel composting) or in exposed piles (industrial windrow composting).
Home compostingHome composters use a range of techniques, varying from extremely passive composting (throw everything in a pile in a corner and leave it alone for a year or two) to extremely active (monitoring the temperature, turning the pile regularly, and adjusting the ingredients over time). Some composters use mineral powders to absorb smells, although a well-maintained pile seldom has bad odors.
Microbes and heating the pileAn effective compost pile is kept about as damp as a well wrung-out sponge. This provides the moisture that all life needs to survive. Bacteria and other microorganisms fall into a variety of groups in terms of what their ideal temperature is and how much heat they generate as they do their work. Mesophilic bacteria enjoy midrange temperatures, from about 20 to 40°C (70 to 110°F). As they decompose the organic matter they generate heat, and the inner part of a compost pile heats up the most.
The heap should be about 1 m (3 ft) wide, 1 m (3 ft) tall, and as long as is practicable. This provides a suitable insulating mass to allow a good heat build-up as the material decays. The ideal temperature is around 60°C (140°F), which kills most pathogens and weed seeds while providing a suitable environment for thermophilic (heat-loving) bacteria, which are the fastest acting decomposers. The centre of the heap can get too warm, possibly hot enough to burn a bare hand. If this fails to happen, common reasons include the following:
The heap is too wet, thus excluding the oxygen required by the compost bacteria
The heap is too dry, so that the bacteria do not have the moisture needed to survive and reproduce
There is insufficient protein (nitrogen-rich material)
The solution is to add material, if necessary, and/or to turn the pile to aerate it.
Depending on how quickly the compost is required, the heap can be turned one or more times to bring the outer layers to the inside of the heap and vice versa, as well as to aerate the mixture. 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 result in a temperature rise, there is no further advantage in turning the pile. When all the material has turned into dark brown or nearly black crumbly matter, it is ready to use.
Worm CompostingRecycling the organic waste of a household into compost allows us to return badly needed organic matter to the soil. In this way, we participate in nature's cycle, and cut down on garbage going into burgeoning landfills. Worm composting or vermicomposting is a method for recycling food waste into a rich, dark, earth-smelling soil conditioner. The great advantage of worm composting is that this can be done indoors and outdoors, thus allowing year round composting. It also provides apartment dwellers with a means of composting. The worm then excretes a soil-nutrient material called worm castings. This is why wise farmers have historically wanted to have healthy worm populations living in their fields. Worms are at the bottom level of the food chain, and thus are critical to healthy soil. In a nutshell, worm compost is made in a container filled with moistened bedding and redworms. Add your food waste for a period of time, and the worms and micro-organisms will eventually convert the entire contents into rich compost. Some good gardeners have developed a radical composting product, made through a brewing process which runs distilled water through Red Wiggler worm castings. The nutritious elements and microorganisms of the castings are captured in a concentrated liquid form, named worm tea. By using worm tea on your plants and gardens, you put healthy microorganisms back into the soil where they thrive and multiply, creating a much healthier growing environment for your plants.
Industrial compostingIndustrial composting systems are being increasingly installed as an alternative form of waste management to landfill along with other advanced waste processing systems. The industrial composting or anaerobic digestion can be combined with mechanical sorting of mixed waste streams and is given the term mechanical biological treatment. Industrial composting helps prevent global warming by treatment of biodegradable waste before it enters landfill. If this waste is landfilled it breaks down anaerobically producing landfill gas that contains methane, a potent greenhouse gas.
Most commercial and industrial composting operations use active composting techniques. This ensures a higher quality product and produces results in the shortest time (see compost windrow turner). The greatest control, and therefore the highest quality, is generally achieved by composting inside an enclosed vessel which is monitored and adjusted continuously for optimal temperature, air flow, moisture, and other parameters. See In-vessel (also 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 bio-solids. The world's largest co-composter is in Edmonton, 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 using a facility 38,690 square metres in size (equivalent to 8 football fields). The aeration building alone is the largest stainless steel building in North America (the size of 14 NHL rinks).
Other ingredientsSome users like to put special materials and activators into their compost. Adding commercially available Effective MicroorganismsTM helps to keep the balance between "good" and "bad" bacteria. A light dusting of agricultural lime (not on the animal manure layers) can curb excessive acidity that can slow down the fermentation. Seaweed meal can provide a ready source of trace elements. Finely pulverized rock (rock flour or rock dust) can also provide needed minerals, as opposed to clay (which is trace mineral-poor and/or leached rock dust).
Animal manure should only be collected from vegetarian animals, such as horses, cows, sheep, llamas, etc. Pet waste, human waste, and non-vegetarian animal waste should not be used in the average compost heap.
Human waste can be collected by composting toilets (in this case, human feces). However, such compost is usually not used as a fertilizer for plants that are directly edible (e.g., salad crops). Most composting toilets do not allow for the thermophilic activity needed to completely kill off the pathogens and bacteria. However, if these high temperatures are reached, there is no danger of contamination, and the resulting compost can be safely used on food crops. Most composts heaps are unable to reach those temperatures. Composting toilets should only be used as a way to reduce waste in the environment, not as a fertilizer; in the case that they are used with crops, they should only use human waste for non-food crops, or with careful filteration, food based crops.
And from the G Living Network, comes more insight (http://gliving.tv/theroots.php):
Container CompostingThere are sound reasons for composting in containers, although there is debate as to whether slatted or closed sided bins are preferable, for this will affect air circulation within the compost pile, as well as the potential for heat loss. The Indore method developed by Sir Albert Howard and the Shewell Cooper method favour slats, while the New Zealand Box method advocates the use of closed sides.
Mike Morrison's (Australia)approach has been a departure from traditional methods with the Compost Oven comprising of patented bubble film with an embedded self aerating and heating device to keep the compost warm, damp and insulated against heat loss.
There are also differences between these techniques in terms of activators (that is, high nitrogen content organic substances to stimulate high bacterial activity within the heap, e.g., urine, grass mowings, comfrey leaves, etc.) and materials used. However, most agree that a good mixture of carbon and nitrogenous materials, usually created in layers and on a base consisting of rougher, stemmy material (to encourage air circulation) that is in contact with the soil are essential to all successful composting processes.
For those who do not have a lot of space, composting can be carried out with good results by using cylindrical bins provided that attention is paid to the all-important issues of aeration and C:N ratios. Such bins are available proprietarily, and are often supplied by local authorities at low cost to encourage recycling.
