Compost

Organisms

Organisms can break down organic matter in compost if provided with the correct mixture of water, oxygen, carbon, and nitrogen.^[3] They fall into two broad categories: chemical decomposers, which perform chemical processes on the organic waste, and physical decomposers, which process the waste into smaller pieces through methods such as grinding, tearing, chewing, and digesting.^[3]

Phases of composting

Under ideal conditions, composting proceeds through three major phases:^[16][17]

1. Mesophilic phase: The initial, mesophilic phase is when the decomposition is carried out under moderate temperatures by mesophilic microorganisms. 2 to 8 days
2. Thermophilic phase: As the temperature rises, a second, thermophilic phase starts, in which various thermophilic bacteria carry out the decomposition under higher temperatures (50 to 60 °C (122 to 140 °F).)
3. Cooling phase (also called Mesophilic II)
4. Maturation phase: As the supply of high-energy compounds dwindles, the temperature starts to decrease.

Semicomposting is the degradation process that handles volumes of organic waste lower than that recommended for composting and therefore does not present a thermophilic stage, because mesophilic microorganisms are the only responsible ones, for the degradation of organic matter.^[18][19]

Hot and cold composting – impact on timing

The time required to compost material relates to the volume of material, the particle size of the inputs (e.g. wood chips break down faster than branches), and the amount of mixing and aeration.^[3] Generally, larger piles reach higher temperatures and remain in a thermophilic stage for days or weeks. This is hot composting and is the usual method for large-scale municipal facilities and agricultural operations.

The Berkeley method produces finished compost in 18 days. It requires assembly of at least 1 cubic metre (35 cu ft) of material at the outset and needs turning every two days after an initial four-day phase.^[20] Such short processes involve some changes to traditional methods, including smaller, more homogenized particle sizes in the input materials, controlling carbon-to-nitrogen ratio (C:N) at 30:1 or less, and careful monitoring of the moisture level.

Cold composting is a slower process that can take up to a year to complete.^[21] It results from smaller piles, including many residential compost piles that receive small amounts of kitchen and garden waste over extended periods. Piles smaller than 1 cubic metre (35 cu ft) tend not to reach and maintain high temperatures.^[22] Turning is not necessary with cold composting, although a risk exists that parts of the pile may go anaerobic as it becomes compacted or waterlogged.

Pathogen removal

Composting can destroy some pathogens and seeds, by reaching certain temperatures. Dealing with stabilized compost – i.e. composted material in which microorganisms have finished digesting the organic matter and the temperature has reached between 50 and 70 °C (122 and 158 °F) – poses very little risk, as these temperatures kill pathogens and even make oocysts unviable.^[23] The temperature at which a pathogen dies depends on the pathogen, how long the temperature is maintained (seconds to weeks), and pH.^[24]

Compost products such as compost tea and compost extracts have been found to have an inhibitory effect on Fusarium oxysporum, Rhizoctonia species, and Pythium debaryanum, plant pathogens that can cause crop diseases.^[25] Aerated compost teas are more effective than compost extracts.^[25] The microbiota and enzymes present in compost extracts also have a suppressive effect on fungal plant pathogens.^[26] Compost is a good source of biocontrol agents like B. subtilis, B. licheniformis, and P. chrysogenum that fight plant pathogens.^[25] Sterilizing the compost, compost tea, or compost extracts reduces the effect of pathogen suppression.^[25]

Diseases that can be contracted from handling compost

When turning compost that has not gone through phases where temperatures above 50 °C (122 °F) are reached, a mouth mask and gloves must be worn to protect from diseases that can be contracted from handling compost, including:^[27]

• Aspergillosis
• Farmer's lung
• Histoplasmosis – a fungus that grows in guano and bird droppings
• Legionnaires' disease
• Paronychia – via infection around the fingernails and toenails
• Tetanus – a central nervous system disease

Oocytes are rendered unviable by temperatures over 50 °C (122 °F).^[23]

Organic solid waste

The two broad categories of organic solid waste are green and brown. Green waste is generally considered a source of nitrogen and includes pre- and post-consumer food waste, grass clippings, garden trimmings, and fresh leaves.^[1] Animal carcasses, roadkill, and butcher residue can also be composted, and these are considered nitrogen sources.^[32]

Brown waste is a carbon source. Typical examples are dried vegetation and woody material such as fallen leaves, straw, woodchips, limbs, logs, pine needles, sawdust, and wood ash, but not charcoal ash.^[1][33] Products derived from wood such as paper and plain cardboard are also considered carbon sources.^[1]

Animal manure and bedding

On many farms, the basic composting ingredients are animal manure generated on the farm as a nitrogen source, and bedding as the carbon source. Straw and sawdust are common bedding materials. Nontraditional bedding materials are also used, including newspaper and chopped cardboard.^[1] The amount of manure composted on a livestock farm is often determined by cleaning schedules, land availability, and weather conditions. Each type of manure has its own physical, chemical, and biological characteristics. Cattle and horse manures, when mixed with bedding, possess good qualities for composting. Swine manure, which is very wet and usually not mixed with bedding material, must be mixed with straw or similar raw materials. Poultry manure must be blended with high-carbon, low-nitrogen materials.^[34]

Human excreta

Human excreta, sometimes called "humanure" in the composting context,^[35][36] can be added as an input to the composting process since it is a nutrient-rich organic material. In soil, humanure helps to build organic matter, foster micronutrients, and develop healthy microbial communities.^[37]: 220 Nitrogen, which serves as a building block for important plant amino acids, is found in solid human waste.^[38][39] Phosphorus, which helps plants convert sunlight into energy in the form of ATP, can be found in liquid human waste.^[40][41] It is also an excellent source of potassium as a soil nutrient.^[37]: 220

The term "humanure" is a neologism coined by organic farmer Joseph Jenkins.^[37]: 219

Solid human waste can be collected directly in composting toilets, or indirectly in the form of sewage sludge after it has undergone treatment in a sewage treatment plant. Both processes require capable design, as potential health risks need to be managed. In the case of home composting, a wide range of microorganisms, including bacteria, viruses, and parasitic worms, can be present in feces, and improper processing can pose significant health risks.^[42]

In the case of large sewage treatment facilities that collect wastewater from a range of residential, commercial and industrial sources, there are additional considerations. The composted sewage sludge, referred to as biosolids, can be contaminated with a variety of metals and pharmaceutical compounds.^[43][44] Insufficient processing of biosolids can also lead to problems when the material is applied to land.^[45]

Humanure's potential to spread disease can be managed through proper fermentation process, which can kill or neutralize pathogens and parasite eggs within two weeks to six months, depending on climate.^[37]: 221

Urine can be put on compost piles or directly used as fertilizer.^[46] Adding urine to compost can increase temperatures, so can increase its ability to destroy pathogens and unwanted seeds. Unlike feces, urine does not attract disease-spreading flies (such as houseflies or blowflies), and it does not contain the most hardy of pathogens, such as parasitic worm eggs.^[47]

Animal remains

Animal carcasses may be composted as a disposal option. Such material is rich in nitrogen.^[48]

Human bodies

Industrial-scale composting

In-vessel composting

This section is an excerpt from In-vessel composting.[edit]

In-vessel composting generally describes a group of methods that confine the composting materials within a building, container, or vessel.^[54] In-vessel composting systems can consist of metal or plastic tanks or concrete bunkers in which air flow and temperature can be controlled, using the principles of a "bioreactor". Generally the air circulation is metered in via buried tubes that allow fresh air to be injected under pressure, with the exhaust being extracted through a biofilter, with temperature and moisture conditions monitored using probes in the mass to allow maintenance of optimum aerobic decomposition conditions. Many In-vessel composters also use augers or rotation to agitate the material, speeding up degradation by continuously homogenizing and mechanically aerating the material.^[55] Common examples of this include rotary drum composters from manufacturers like XACT^[56] and auger based in-vessel technology like the Earth Flow from Green Mountain Technologies^[57] and Biomax G by Sorain Cecchinni.^[58]

In-vessel composting is generally used for putrescible municipal scale organic waste processing, including final treatment of sewage biosolids, food waste, or the organic fraction of municipal solid waste. In-vessel composting allows for sanitization of pathogens so the waste can be reclaimed as a soil amendment, while minimizing the environmental impact in the form of odors, runoff or emissions. In-vessel composting can also refer to aerated static pile composting within a building or under removable covers that enclose the piles, as with the system in extensive use by farmer groups in Thailand, supported by the National Science and Technology Development Agency there.^[59] In recent years, smaller scale in-vessel composting has been advanced. These often use custom or common roll-off waste dumpsters as the vessel. They may also use retrofitted shipping containers or custom drums.

Aerated static-pile composting

This section is an excerpt from Aerated static pile composting.[edit]

Aerated static pile (ASP) composting refers to systems using forced aeration to keep compost material aerobic without physical manipulation. This is typically done by placing mixed feedstocks on perforated piping where air is then either pumped up (positive aeration), sucked down (negative aeration), or pumped and sucked (reversing aeration) through the material. If air is sucked through the material (negative and reversing aeration) it is typically then run through a scrubber like a biofilter to remove odor and emissions.^[60]

ASP's are typically constructed outdoors in mass beds or bunker configurations (see image for example of bunker style ASP). They can also be constructed within open-sided buildings, enclosed bunkers known as tunnels, or windrows. Many ASPs are protected from rain and precipitation with a roof, reducing leachate generation and allowing operators to evaporate detained water through biodrying.

Windrow composting

This section is an excerpt from Windrow composting.[edit]

In agriculture, windrow composting is the production of compost by piling organic matter or biodegradable waste, such as animal manure and crop residues, in long rows – windrow.

Other systems at household level

Hügelkultur (raised garden beds or mounds)

Main article: Hügelkultur

The practice of making raised garden beds or mounds filled with rotting wood is also called Hügelkultur in German.^[61][62] It is in effect creating a nurse log that is covered with soil.

Benefits of Hügelkultur garden beds include water retention and warming of soil.^[61][63] Buried wood acts like a sponge as it decomposes, able to capture water and store it for later use by crops planted on top of the bed.^[61][64]

Related technologies

• Vermicompost (also called worm castings, worm humus, worm manure, or worm faeces) is the end product of the breakdown of organic matter by earthworms.^[66] These castings have been shown to contain reduced levels of contaminants and a higher saturation of nutrients than the organic materials before vermicomposting.^[67]
• Black soldier fly (Hermetia illucens) larvae are able to rapidly consume large amounts of organic material and can be used to treat human waste. The resulting compost still contains nutrients and can be used for biogas production, or further traditional composting or vermicomposting^[68][69]
• Bokashi is a fermentation process rather than a decomposition process, and so retains the feedstock's energy, nutrient and carbon contents. There must be sufficient carbohydrate for fermentation to complete and therefore the process is typically applied to food waste, including noncompostable items. Carbohydrate is transformed into lactic acid, which dissociates naturally to form lactate, a biological energy carrier. The preserved result is therefore readily consumed by soil microbes and from there by the entire soil food web, leading to a significant increase in soil organic carbon and turbation. The process completes in weeks and returns soil acidity to normal.
• Co-composting is a technique that processes organic solid waste together with other input materials such as dewatered fecal sludge or sewage sludge.^[10]
• Anaerobic digestion combined with mechanical sorting of mixed waste streams is increasingly being 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. The methane produced in an anaerobic digester can be used as biogas.^[70]
• Small-scale and organic farms can also produce aerobic compost piles for crop bed preparation using a "turned pile" technique in which piles are formed with the appropriate ratios of nitrous, carbonous, and neutral materials, checked with a temperature gauge, and turned by-hand at weekly intervals.^[71][72]

Agriculture and gardening

On open ground for growing wheat, corn, soybeans, and similar crops, compost can be broadcast across the top of the soil using spreader trucks or spreaders pulled behind a tractor. It is expected that the spread layer is very thin (approximately 6 mm (0.24 in)) and worked into the soil prior to planting. Application rates of 25 mm (0.98 in) or more are not unusual when trying to rebuild poor soils or control erosion.^[73] In countries such as Germany, where compost distribution and spreading are partially subsidized in the original waste fees, compost is used more frequently on open ground on the premise of nutrient "sustainability".^[74]

In plasticulture, strawberries, tomatoes, peppers, melons, and other fruits and vegetables are grown under plastic to control temperature, retain moisture and control weeds. Compost may be banded (applied in strips along rows) and worked into the soil prior to bedding and planting, be applied at the same time the beds are constructed and plastic laid down, or used as a top dressing.

Many crops are not seeded directly in the field but are started in seed trays in a greenhouse. When the seedlings reach a certain stage of growth, they are transplanted in the field. Compost may be part of the mix used to grow the seedlings, but is not normally used as the only planting substrate. The particular crop and the seeds' sensitivity to nutrients, salts, etc. dictates the ratio of the blend, and maturity is important to insure that oxygen deprivation will not occur or that no lingering phyto-toxins remain.^[75]

Compost can be added to soil, coir, or peat, as a tilth improver, supplying humus and nutrients.^[76] It provides a rich growing medium as absorbent material. This material contains moisture and soluble minerals, which provide support and nutrients. Although it is rarely used alone, plants can flourish from mixed soil that includes a mix of compost with other additives such as sand, grit, bark chips, vermiculite, perlite, or clay granules to produce loam. Compost can be tilled directly into the soil or growing medium to boost the level of organic matter and the overall fertility of the soil. Compost that is ready to be used as an additive is dark brown or even black with an earthy smell.^[1][76]

Generally, direct seeding into a compost is not recommended due to the speed with which it may dry, the possible presence of phytotoxins in immature compost that may inhibit germination,^[77][78][79] and the possible tie up of nitrogen by incompletely decomposed lignin.^[80] It is very common to see blends of 20–30% compost used for transplanting seedlings.

Compost can be used to increase plant immunity to diseases and pests.^[81]

Other

Compost can also be used for land and stream reclamation, wetland construction, and landfill cover.^[90]

The temperatures generated by compost can be used to heat greenhouses, such as by being placed around the outside edges.^[91]