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Composting is the purposeful biodegradation of organic matter, such as yard and food waste. The decomposition is performed by micro-organisms, mostly bacteria, but also yeasts and fungi. In low temperature phases a number of macro-organisms, such as springtails, ants, nematodes, isopods and red wigglers also contribute to the process, as well as soldier fly, fruit flies and fungus gnats. There are a wide range of organisms in the decomposer community.
- A biodegradable material is capable of being broken down under the action of microorganisms into carbon dioxide, water and biomass. It may take a very long time for some material to biodegrade depending on its environment (e.g. wood in an arid area versus paper in water). Many contaminating materials not dealt with in common composting are in fact "biodegradable", and may be dealt with via bioremediation, or other special composting approaches.
- A compostable material biodegrades substantially under specific composting conditions. It is metabolized by the microorganisms, being incorporated into the organisms or converted into humus. The size of the material is a factor in determining compostability, and mechanical particle size reduction can speed the process. 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. Some biodegradable materials are only compostable under very specific conditions, usually with an industrial process.
Composting upcycles organic kitchen and yard waste and manures into an extremely useful humus-like, soil end product, permitting the return of vital organic matter, nutrients, and particularly bacteria, that are vital to plant nutrition to the soil. Managed aerobic composting arranges environmental conditions so they are optimal for the natural processes to take place. There is a popular expression: "compost happens", but it is helpful to engineer the best possible circumstances for large amounts of organic waste to decompose quickly and efficiently, with the greatest conservation of useful nutrients and mass. Uncontrolled composting is when compost "happens", and although that may be functional in some circumstances, as with forest floor detritus, a neglected heap of kitchen and yard wastes will more likely result in "smells happen", or "rodents happen" long before useful compost does.
Long used in subsistence farming and home gardening for creating garden-ready soil, composting is becoming increasingly important and better understood as a tool for reducing municipal solid waste, and reducing the amount of green waste going into landfills. The decomposition of organic material sent to landfills is a principal cause of methane, an important greenhouse gas, making reduction of organic waste being landfilled a key element in the fight against climate change. In general, net greenhouse gas emissions for landfills tend to be higher than that for composting facilities. In suburban and rural areas, much of the organic waste could be removed from the waste stream by promoting home composting, where consumers compost their yard waste and kitchen scraps on their own land, regardless of whether the material is ever actively re-used as "soil". In urban areas with dwellings predominantly lacking individual yard space, there are indoor small scale composting alternatives, such as vermicomposting and bokashi composting.
Composting organisms require four equally important things to work effectively:
- Carbon ("C" or carbohydrates), for energy - the microbial oxidation of carbon produces the heat.
- High carbon materials tend to be brown and dry.
- Nitrogen ("N" or protein), to grow and reproduce more organisms to oxidize the carbon.
- High nitrogen materials tend to be green (or colorful, like fruits and vegetables) and wet.
- Oxygen, for oxidizing the carbon, the decomposition process.
- Water, in the right amounts to maintain activity without causing anaerobic conditions.
Certain ratios of these elements will provide beneficial bacteria with the nutrients to work at a rate that will heat up the pile. In that process much water will be released as vapor ("steam"), and the oxygen will be quickly depleted, explaining the need to actively manage the pile. The hotter the pile gets, the more often added air and water is necessary; the air/water balance is critical to maintaining high temperatures until the materials are broken down. At the same time, too much air or water also slows the process, as does too much C (or too little N).
The most efficient composting occurs with a C:N mix of about 30 to 1. All organics have both carbon and nitrogen, but amounts vary widely, with characteristics noted above (dry/wet, brown/green). Fresh grass clippings have an average ratio of about 15 to 1 and dry autumn leaves about 50 to 1 depending on species. Mixing equal parts by volume approximates the ideal C:N range. Few individual situations will provide the ideal mix of materials at any point in time - in this respect, home composting is like horseshoes, perfect is great, but close still works. Observation of amounts, and consideration of different materials as a pile is built over time, can quickly achieve a workable technique for the individual situation.
Ingredients that are primarily carbon include:
- Dry, straw-type material, such as cereal straws and corn stalks
- Dry leaves (best shredded, as with a rotary mower, to prevent matting)
- Wood, as coarse or fine (may compact) sawdust, or ground wood waste
Paper and card board, both unprinted and printed are not recommended as both the inks and paper contain materials such as pigments, clays, binders, etc that are not biodegradable. While these insoluble ingredients are not toxic, they will not readily break down as other biodegradable materials. In addition, paper will decompose very slowly interfering with the composting process.
Ingredients with relatively high nitrogen content include:
- Green plant material, like crop residues, new shoots, hay (especially alfalfa), grass clippings and weeds.
- Manure from poultry, humans, pets, and herbivorous animals such as horses, cows and llamas.
- Kitchen waste - fruit and vegetable cooked waste and trimmings, juicing-pulp residue, tea and coffee grounds, meat, bones, eggs.
For "back yard" composting, mixing the materials as they are added increases the rate of decomposition, as does reduced particle size (ie, chopped, shredded), or materials can be added in alternating layers, about 15 centimeters (6 in) thick. Keeping a carbon "cache" handy to the pile for covering and mixing with fresh wet additions (lawn clippings, kitchen scraps) is simplest. Special additions or activators are not necessary, although some sprinklings of good garden loam as a first pile is built will aid more rapid working by inoculating it with beneficial soil bacteria, and some of the material from the first finished batch can be used in the subsequent mixes. Adding soil also provides grit to help earthworms digest, as well as providing particles for finished compost to aggregate with to create humus. Agricultural lime is not necessary - the bacteria prefer a slightly acidic pH, and their processing invariably results in a near-neutral product. Seaweed meal, rock dust or rock flour, and other trace element amendments are best added to the finished compost, or directly to the garden.
Active (Intensively Managed)
Hot thermophilic composting is essential with some materials, such as meat and other animal products, dairy products, eggs, grease, cooking oil, manure of non-herbivores, and residuals from the treatment of wastewater, in order to kill pathogens; but these materials are not generally recommended in home composting because of the likelihood of creating odors and attracting rodents. Human waste can be composted by industrial methods as well as composting toilets. When high temperatures are reached, the resulting compost can be safely used for agricultural or horticultural purposes, providing local health regulations are met. Humanure fertilizer (as opposed to night soil) is used throughout the developing world and is becoming more accepted as a garden amendment in the developed world.
Hot, aerobic composting is conducted at close to the ideal conditions noted above, allowing thermophilic bacteria to thrive. These aerobic bacteria break down material faster, producing less odor, fewer pathogens, and less greenhouse gas than cool, uncontrolled, or accidental anaerobic methods. Commercial scale composting operations actively control the composting conditions (C:N ratio, moisture level and air), usually in a closed environment (in-vessel composting, tunnel composting or aerated static pile composting), where air is fan forced through the mass, and moisture added with sprayers, or conserved via the enclosure, with computer monitored probes detecting conditions.
In Thailand an aerated static pile system is in use by farmer groups at over 400 sites. The process needs only 30 days to finish without manual turning, with 10 metric tons of compost produced per month. A 38 centimetres (15 in) squirrel-cage blower with 2.2 kilowatts (3.0 hp) motor is used to force air through 10 covered static piles of compost twice a day. The raw materials consist of agricultural wastes and animal manure in the ratio of 3:1 by volume.
Sustained, high temperatures may destroy insects, larvae, and weed seeds, but no compost will be totally sterilized by high temperatures alone. This is because the reduction of available substrate (e.g. water soluble carbon) for bacteria growth must also accompany heating. In a hot compost where the temperature exceeds 55 °C (131 °F) for several weeks, the ability of most organisms to survive is of course compromised, and there are temperature standards, such as in the USA EPA-503 rule, based on early suppositions about this ( 1980-era). Nevertheless, many organisms in nature can survive extreme temperatures, including extremophiles such as Thermus thermophilus which play an important role in thermogenic composting, as well as pathogens such as Clostridium. The necessary second stage of any composting is maturation, a period allowing the dissipation of readily available substrates for pathogen proliferation and any phyto-toxins remaining from the process or contaminating ingredients (eg: chemical residues), and achieving a state of nutrient stability (low C:N ratio) that will not have an impact on Nitrogen availability in the receiving soil. It is a popular - and dangerous- misnomer of the composting industry that once a pile gets hot for a short period of time, pathogen content is eliminated. Very recent studies of E. coli O157 show survival in hot composting out to 120 days.
For backyard composters, carbon and nitrogen ratios in various ingredients and the calculations required to get the ideal mixture can be intimidating, so rules of thumb exist for approximating it by ingredient types and condition, as mentioned in Materials above. If the pile is built in a short period, and has a good mix of materials (C:N) and a coarse structure, with about 50% moisture ("like a squeezed out sponge"), the temperature should rise within days to as high as 60 °C (140 °F). When the temperature begins to fall, more air is needed, usually added by turning the pile or using an agitating tool, and moisture may be needed at the same time. Turning or other aeration is usually needed about every 6–10 days to maintain the highest heat levels until the material is fairly uniformly broken down to unrecognizability, and temperatures no longer rebound. A pile that has been maintained at peak temperatures may be ready for maturing in as little as 30 days, but rarely sooner than 60 days. Another 30–60 days maturing should suffice to allow passing the a plant growth test- depending on the plant species chosen.
To achieve thermophilic decomposition, a compost bin is best about 1 cubic metre (1.3 cu yd), or 1 metre (3 ft) wide, 1 metre (3 ft) tall, and as long as desired for windrow composting. This provides enough insulating mass to build up heat but also allows oxygen infiltration. The center of the pile heats up the most, so regular turning/mixing is needed for insuring all material spends some time in the hottest area. When turning the pile results in no further temperature rise, the active aerobic phase is complete, and the mass may be turned out to a maturing pile. When the matured material has a dark brown crumbly appearance and the smell of rich damp earth, it is ready to use.
The natural sequence of the decomposition community involved will be:
- 0–15 °C (32–59 °F) - psychrophiles predominate, beginning the heating process as they multiply
- 15–40 °C (59–104 °F) - mesophiles take over, psychrophiles die off or are relegated to the borders
- 40–70 °C (104–158 °F) - thermophiles work at their peak, including consuming many other bacteria
At the lower temperatures and around the borders, there will also be various fungal activity, as well as larger organisms, getting their share - a very dry, cooler pile may be attractive to ants, and gastropods may visit very wet piles. As the temperature returns to ambient at the end of the process, the sequence reverses, including new organisms that prefer the more degraded materials. Added heat and pile insulation may be useful in the coldest weather, but is not ordinarily necessary, and is not desirable if it interferes with aeration or natural convective evaporation. Keeping the top dry and burying fresh additions in the center of a pile will be effective during winter conditions until heating resumes in spring.
Odor is generally the most frequent and serious complaint from neighbors of compost facilities. The first and most important task for the operator is to determine what problematic odors are present and where they are being generated. Only then can appropriate remedial actions be taken.
Compost that is properly made under aerobic conditions will have an earthy aroma that is not offensive. However, partly decomposed feedstocks or poor composting techniques can generate problematic odors including ammonia, hydrogen sulfide (the smell of rotten eggs) and volatile fatty acids (VFAs). VFAs are compounds for which most people have very little tolerance.While determining that there is an odor may seem simple, identifying the source and cause of problematic odors at a compost facility can be complex. Sites are large open areas with many potential odor-producing sources, and odors travel in unpredictable ways.
It is essential to determine whether odors are generated by piles of incoming material that have not yet been incorporated, or from a specific compost pile, standing water, holding pond or another source. Identification of the source of the problem is important because the actions required to remedy each of these problems will differ.
Determine the CauseOdors occur at compost facilities for several reasons. Identifying the type of odor can give an indication of the root cause of the problem.
AmmoniaAn ammonia smell is usually generated in a compost pile that contains too much nitrogen-rich material such as fresh grass. Incoming material can already generate ammonia odors if it has been closed up in plastic bags for very long.Ammonia can also be generated when carbon has been supplied to the piles in particles that are too large, such as uncut brush. In either of these examples, there is too much nitrogen in the original mix for the amount of available carbon (low C:N ratio). An ammonia odor can also sometimes indicate a pH level that is too high.
Hydrogen sulfideA smell of hydrogen sulfide (rotten eggs) indicates that anaerobic conditions are present. Anaerobic conditions form if there are not enough air spaces through which air can flow. This can be caused by too much moisture and/or a lack of aeration. A pile with too much moisture will lack adequate aeration because too many of the air spaces are filled with water.Anaerobic conditions can also develop when the pile becomes compacted or air flow through the pile is short circuited. Compaction occurs when there are not enough large or rigid particles such as wood chips to maintain structure and air spaces in the mass. Short circuiting is a term used to describe the tendency of air to follow the route (channels) of least resistance through the pile. Channeling most often occurs in a compost pile that is not mixed during the composting process. In short circuiting, air flows preferentially through the channels rather than being distributed evenly throughout the mass.Although air does pass through the pile when short circuiting occurs, there are areas within the pile with no air, creating localized anaerobic conditions. Rapidly falling temperatures within a compost pile can be an indication that short circuiting is causing the odor problem.
Volatile Fatty Acids (VFAs)VFA's are not only offensive to the olfactory senses, their presence can contribute to phytotoxicity problems in finished compost. Like hydrogen sulfide odors, VFA odors are generated by microbial decomposition that occurs under anaerobic conditions. Because VFAs are generated under anaerobic conditions, it is necessary to determine why those conditions exist and eliminate them.
Remedy the situation
AmmoniaIf the ammonia odor is related to an imbalance in the amount of carbon and nitrogen in the recipe (C:N ratio), add more carbon-rich material such as leaves or wood chips. It is important that the particle size of the carbon source is small enough that it can be used by microbes. A carbon source such as brush or very large wood chips may need to be chipped or ground to a smaller particle size.High pH (greater than 8) can cause excessive ammonia loss and kill bacterial decomposers. If the pH level of a compost pile is greater than 8, add acidic material such as leaves or sulfates and avoid adding more alkaline material to the pile.
Hydrogen sulfideHydrogen sulfide odors (rotten eggs) indicate that anaerobic conditions are present within the compost pile. This is either because the material is too wet or because there is insufficient aeration.If the hydrogen sulfide odor is caused by wetness, add dry bulking agent, remix and place piles in an area where they won't be located in standing water. If insufficient aeration is related to poor structure or compaction, add bulking agent and remix the pile. If airflow through the pile is uneven (short circuiting), turning piles more frequently should eliminate the problem.
VFAsOccurrence of VFAs also indicates that anaerobic conditions are present within the compost pile. The remedy applied should be the same as when hydrogen sulfide odors are present.If malodorous conditions persist despite taking the actions previously described, it may be necessary to time the turning of compost piles carefully. Turning is best when done with sensitive receptors in mind, such as when it is raining or when the wind direction is opposite of where sensitive receptors are located.
Passive (Non-intensive) Composting
Ambient composting, typical of "sheet composting" methods espoused in early organic farming, usually results in temperatures not climbing substantially above background temperatures, hence the expression ambient. It is slower, and is the more common type of composting in domestic gardening, particularly in home composting where compost piles are less than 1.3 cu. yd (1 cubic meter) in volume. Such composting systems may be in open or closed containers of wood or plastic, or in open exposed piles. 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 may become odorous. To improve drainage and airflow, and reduce odor, carbon-rich materials, or 'browns', such as wood chips, shredded bark, leaves, or twigs may be added to mix and cover each wet addition, or holes made occasionally in the pile. The amount of attention may vary from none through occasional to "regular".
An unusual form of composting in nature is seen in the case of the mound-builders (megapodes) of the Australasian region. These Megapodes are fowl-sized birds famous for building nests in the form of compost heaps containing organic litter, in which they incubate their eggs. The male birds work assiduously to maintain the correct incubation temperatures, by adding and removing litter from the compost pile.
Forest floor detritus is the natural fertilizing material in temperate forests. Soil dwelling organisms slowly decompose the continuously deposited litter from below, returning the nutrients to the soil for bacterial conversion to forms useful to the native vegation. This is a natural form of mulching or sheet composting.
Worm composting or vermicomposting is a method using Red Wiggler worms in a container to process kitchen waste. Moistened high-carbon bedding such as shredded paper is used as a base to which the food waste is added, and the worms and micro-organisms convert the materials to rich compost called worm castings, a nutrient and microbially rich material. Worm composting can be done indoors, allowing year-round composting, and providing apartment dwellers with a means of recycling food waste.
Industrial composting systems are increasingly being installed as a waste management alternative to landfills, along with other advanced waste processing systems. Mechanical sorting of mixed waste streams combined with anaerobic digestion or in-vessel composting, is called mechanical biological treatment, increasingly used in developed countries due to 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 potent greenhouse gas.
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, although difficulties controlling inert and plastic contamination from Municipal solid waste makes this approach less attractive. The world's largest MSW co-composter is the Edmonton Composting Facility 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. The facility is 38,690 meters2 (416,500 ft2), equivalent to 4½ Canadian football fields, and the operating structure is the largest stainless steel building in North America, the size of 14 NHL rinks.
Molasses based Distilleries all over the world generate large amount of effluent termed as spent wash or vinasse. For each liter of alcohol produced, around 8 liters of effluent is generated. This effluent has COD of 1,50,000 PPM and BOD of 60,000 PPM and even more. This effluent needs to be treated and the only effective method for conclusive disposal is by Composting. Sugar Factories generate pressmud / cachaza during the process and the same has about 30% fibers as Carbon and has large amount of water. This pressmud is dumped on prepared land in the form of 100 meters long windrows of 3 meters X 1.5 meters and spent wash is sprayed on the windrow while the windrow is being turned. Composting machines called Windrow King, Windrow Prince and Windrow Queen are used for this applications. These machines help consume spent wash of about 2.5 times of the volume of the pressmud. Which means that a 100 meters of windrow accommodates about 166 MT of pressmud and uses about 415 M3 of Spent wash in 50 days. Microbial Culture TRIO COM-CULT is used about 1 Kg per MT of pressmud for fast de-composing of the spent wash. Lakhs of M3 of spent wash is being composted all over the work in countries like India, Colombia, Brazil, Thailand, Indonesia, South Africa etc. Composting machines Windrow King/Prince/Queen of various capacities of 336 HP, 153 HP and 80 Hp are being used all over the world for this application round the clock. The technology of composting spent wash / Vinasse is called TAS BIO-COM. The compost yard has to be prepared as per the norms of local countries however it has to be prepared in such a way that the land is impervious and does not allow the liquid effluent to pass down into the earth. The compost yard has to be located at a place where the smell of composting operations is not allowed to spread, as any composting operation generates foul smell. The compost thus generated, is of excellent quality and is rich in nutrients and it takes care of balance of nature. The products derived from nature go back to nature.
Compost is an important source of nutrients commonly used in modern agriculture. Through steaming, compost can be sanitized and prepared for further use.
- Compost bin Home containers
- Container composting Small scale
- Compost The product
- Compost uses
- Compost tea
- Bokashi composting
- Comparison of anaerobic and aerobic digestion
- Ecological sanitation
- Humanure Composting
- In-vessel composting
- Jean Pain Farm-based compost bioenergy system
- List of composting systems
- Sustainable agriculture
- Biodegradable polythene film
- Soil steam sterilization
- ^ Compost microbes, Cornell Univ.
- ^ Vermicompost Guide.
- ^ Compost consumers, Cornell Univ
- ^ Gypsum board composting
- ^ Vancouver, B.C. Canada municipal program
- ^ Lou, X. F.; Nair, J. (2009). [Expression error: Missing operand for > "The impact of landfilling and composting on greenhouse gas emissions – A review"]. Bioresource Technology 100 (16): 3792. doi:10.1016/j.biortech.2008.12.006. PMID 19155172.
- ^ The Natural Resources Defense Council on Composting
- ^ home made Effective Microorganisms for Bokashi composting
- ^ Materials for composting - University of Illinois extension, retrieval date: 3/12/2009
- ^ Klickitat County WA, USA Compost Mix Calculator
- ^ Effect of lignin content on bio-availability
- ^ Lime addition and pH
- ^ Aerated Static Pile composting
- ^ Appl. Environ. Microbiol. 1996 62 1723-1727 Isolation of Thermus strains from hot composts (60°C-80°C) Beffa, T; Blanc, M; Lyon, PF; Vogt, G; Marchiani, M; Fischer, JL; Aragno, M.
- ^ Phytotoxicity and maturation
- ^ abstract: Phytotoxicity of biosolids compost at different degrees of maturity compared
- ^ Edmonton composting facility
- Commercial scale systems
- US EPA regulations for commercial compost use
- State of Florida compost info page
- Riverside County CA home compost bin plans
- Washington State Extension Service on composting
- Maturity indicators for estimating phytotoxicity in compost-amended soil
- Understanding the microbiology of compost
- British Standards Institute spec's
- Cornell University Composting pages