Composting

Composition of the compost

The organizations responsible for composting need three parameters to live:

of balanced food, composed of a mixture of carbonaceous matters (brown-hard-dry) and nitrogenized matters (green-soft-wet);
of moisture, contained particularly in the nitrogenized matters;
of air, whose circulation is supported by the structuring carbonaceous matters (hard).

The organic residues compostables are:

waste known as nitrogenized: vegetation wastes, of Gardening (sizes of Hedge S, shearings of Lawn…), of the green Sheet S, perishable household waste (waste of the Vegetable S and Fruit S). It is thus possible to decrease by 30 50% its quantity of household refuse and to decrease by as much the size of the Décharge S and volumes of waste transported towards the Incinérateur S;
waste known as carbonaceous: crushed branches, died sheets, the Straw (one will preciously store these matters for always having some at its disposal to mix them with the nitrogenized matters);
the Shell S of egg, shells of Nut;
the Litter S biodegradable of the animals Herbivorous S;
the Paper by avoiding those which are printed, the paperboard (it is used as refuge with the worms of ground);
pieces of fabric S out of natural matters (Wool, Cotton), etc
waste of house (paper handkerchiefs, paper towel, Ash of wood, sawdusts, chips, nonsick plants of interior).

Caution: certain matters as marcs of Café have a very slow decomposition.

Description of the process

Composting is an operation which consists in degrading, under controlled conditions, organic waste in the presence of the Oxygène of the air.

Two phenomena follow one another in a process of composting. The first, bringing the residues to the state of fresh compost, is a degradation intense Aérobie: it is primarily about the decomposition of the fresh organic matter at high temperature (50-70 °C) under the action of bacteria; the second, by a less supported degradation, will transform the fresh compost into a ripe compost, rich in Humus. This phenomenon of maturation, which occurs at lower temperature (35-45 °C), led to the Biosynthèse of humic compounds by Champignon S.

Degradation

The change of the temperature during the process of degradation is carried out in three phases:

the temperature goes up quickly to 40-45°C following the breathing of the micro-organisms Mésophile S aerobes. The compounds more dégradables the such Sugar S and the Amidon are initially consumed.

A preliminary phase with this first phase is sometimes described. During this phase one notes, after a short latency, a light increase in the temperature. It results from the endogenous respiratory activity of alive cells present in the mass to perforate. This phase is thus very short and is observed only in laboratory when the mixture to be perforated contains a strong proportion of fresh fabrics.

the Respiration gradually raises then the temperature up to 60-70 °C, leading to the replacement of the micro-organisms Mésophile S by Thermophile S and thetolerant ones.

by their breathing, the micro-organisms exhaust the oxygen of the mass in composting and make the medium anaerobic. Anaerobic germs develop then, leading to a fall in the temperature because their metabolism is less thermogenic. They are more responsible for the release of nauseous volatile compounds (Méthane, Ammoniac, Hydrogen sulfide …).

To avoid this putrefaction, it is necessary to restore the aerobic conditions of the medium (see ventilation below). Thus it will be possible to prolong fermentation at high temperature. Pathogenic the, parasitic ones and seeds of bad grasses will be destroyed by the high temperature, the bad smells will be avoided, the decomposition will be faster. As soon as the temperature does not increase any more after ventilation, one can consider that degradation is finished.

Maturation

At this time, the quantity of matter easily usable by the Microflore rarefies and the Biosynthèse humic compounds becomes prevalent. One attends the disappearance of the thermophilous micro-organisms to the profit of more common species and new species mésophiles as the temperature decrease during one long period of ripening to stabilize itself on the level of the room temperature.

It still should be announced that the transition between each phase quoted previously results from a continuous evolution: there is no border marked between the species mésophiles and thermophilous. Each species has a range of vital temperatures with an optimum in the medium.

Influence environment

The progression of the starting material towards the final stage, the Humus, depends on a great number of external factors like dimension of the particles, the nature of the nutrients, their structure, water the content, ventilation, pH… In addition, while multiplying, the micro-organisms constantly change their environment and make it often unsuitable to their development.

Physical conditions

Ventilation

This factor is essential since composting is a process Aérobie. It is estimated that the air should occupy at least 50% of the volume of the heap. The anaérobiose starts when the rate of oxygen of the heap is lower than 10%; it prevails with the lower part of 5% of O₂ (air = 21% O₂). Various techniques make it possible to restore aerobiosis, they will be described below.

Moisture

As for a substrate of culture, the ventilation and the moisture of the compost are dependant: a water excess decreases the quantity of air available in the volume of compost. A more effective system of ventilation will be then necessary.

The heat released by fermentation causes the evaporation of a great quantity of water. The watering of the mass in fermentation makes it possible if necessary so as to maintain a water content from 50 to 70% of the fresh mass (i.e. the equivalent of the Capacité to the field for a ground). In addition the beating rains as excessive evaporation by the sun can also slow down the process. A cover, roof or cover can answer this problem.

Dimension of the particles

In addition to its role on the Porosity with the air and the retention of water of the medium, one of the effects of preliminary dilaceration (crushing) is to increase the surface of contact between waste and the microflora. A reduction of the size of the particles thus involves an increase in the rate of decomposition but also a weaker circulation of air (risk of anaérobiose).

Temperature

By their breathing the micro-organisms release a heat such as the temperatures reached (80 and even more 90°C in a well isolated heap) can become lethal for the cells. The optimization of the process thus consists in taking care not to exceed a temperature of 70°C.

Chemical conditions

pH

Generally, the matters to be perforated present a pH ranging between 5 and 7, i.e. within acceptable limits. The pH drops during the first days and goes up then to become neutral or slightly Alcalin. Certain authors however recommend the addition of a plug or a weak base (Calcaire S or crushed Dolomie, marl, Craie phosphatic…), others refuse there because that can cause a deceleration of the process. Without addition of plug, the final pH of the compost is in the neighborhoods of 8.

Form carbon

It influences much the speed of decomposition of the compost. Certain molecules, the such simple glucids, the Starch, the hemicelluloses, the pectins and the amino-acid, are easily dégradables. The Cellulose, bulkier polymer, is more resistant. The aromatic Lignin and the other polymers, extremely solid, will be degraded more tardily, more slowly and incompletely (leading to the formation of Humus).

Report/ratio C/N

A too weak report/ratio (lower than 15) led to losses of Nitrogen; a too high C/N slows down the decomposition. The quantity of Nitrogen to be added is difficult to estimate because it is necessary to take account of the rate of fermentability of the Carbone.

Report/ratio C/P

The Phosphore is essential with the energy reactions of the micro-organisms (Adénosine Tri-Phosphate). It also enters the composition of many other macromolecules. A report/ratio C/P of the matter to be perforated close to that of the Microflora (75 to 150) led to a faster degradation of the Organic matter and to a greater production of Humus.

Other biogenic salts

The matters to be perforated must be regarded as a culture medium for microbes, where the limiting factor can be only assimilable carbon and not another component of the medium. These elements are in general present in sufficient quantity in the organic matter to perforate.

Biological conditions

The speed and the effectiveness of composting are related to the presence of an adequate microbial population. If the presence of this billion bacteria and mushrooms is essential, their sowing (“activators” or “stimulative” of composting) seems little, even not useful. The spores of these micro-organisms exist indeed in sufficient quantities in nature and it is much more important to take care to create a medium (pH, moisture, ventilation, C/N,…) favorable to their development.

The inoculation of the composts by fixing atmospheric nitrogen micro-organisms, such as Azotobacter does not seem either interesting for composting, the energy expense of these organizations to fix nitrogen being too important. The only interest of this type of inoculation could come from a possible nitrogen fixing, posterior with composting, during the culture of the plants on the composts thus inoculated. Experiments should be carried out in order to show the credibility ‚of such an assumption.

Aptitude for composting (CNFP)

The aptitude for composting is a formed parameter of a code of four letters, capital letters or tiny, it represents the four fundamental aspects to join together to carry out a good compost: “It or “it for Carbon, “or “for Nitrogen, “F” or “F” for the degree of fermentability (C. - with-D. the aptitude to be fermented of the product), and “P” or “p” for total porosity. A small letter indicates a correct contribution for this aspect, a capital letter indicating of the improving properties. The absence of a letter (“-”) means lack, with complémenter by product having properties improving for the same factor. The realization of the compost will thus be done by combining two by-products (three with the rigor) having complementary properties in such a way that the four letters of the code are present in the mixture carried out.

“it or “It

“it indicates a product having a correct report/ratio C/N (15 to 30). “It indicates a product to strong content of Carbone, i.e. having a C/N higher than 75. Such a product will have to be mixed with a product of the type “or will not receive a supplement of Azote in the form of mineral Engrais (Urée for example).

“or “

Complementary to the preceding factor, “does not indicate a correct C/N; “Does not indicate a weak C/N (lower than 10) requiring a mixture with a product of the type “It; a material with high C/N will be of type “-” for this factor.

“F” or “F”

Give an indication on the shape of carbon present: “F” represents a suitable balance between the molecules with accelerated fermentation (sugars) and the molecules with slow degradation (lignins). The molecules with fast degradation are necessary for the starting of fermentation and obtaining an high temperature in the mass of compost (“Pasteurization” of the compost). A material rich in these molecules will be of type “F”, a poor material of type “-”. The molecules with slow degradation as for them will be used as a basis for biosynthesis of the humic compounds.

“p” or “P”

Porosity with the air of material is important for its role on the ventilation of the compost and the retention of water (porosity with water, expressed pourcent total porosity of it, is the complement with 100 of porosity to the air). It is influenced mainly by the dimension of the particles. A material whose porosity with the air is high (“P”, material of structuring) will make it possible for example to carry out heaps of composts of more important volume without risking a compressing which would prevent the air circulation. It could also be used as basic material to mix with materials without structure (“-”: muds of purification plants or water of industrial process for example). “p” represents a material having a good balance between porosity to the air and porosity with water.

It should be noticed that composting, in oneself, does not require one structuring of organic origin. Rubber chips (resulting from old tires) can be used, for example for the mud composting of purification plants.

Various methods of composting

The methods described below relate to only the phase of active fermentation. The phase of maturation as for it usually proceeds with the free air in heap of great dimension.

With the free air

One will however build a hood above the composts in fermentation in order to protect them from the excessive rains or the desiccation by the wind and the sun.

In pit

The method of composting in pit is the practice in the past employed but led quickly to anaerobic conditions. The pit is dug in a sheltered place and isolated well. Organic waste is laid out there in layers of a score of centimetres thickness, alternating the products rich in nitrogen (standard “) and those rich in carbon (standard “It). They are then covered with a thick layer of straw (insulation) then of a layer of ground of approximately 10 cm thickness. This method is very slow and partially Anaérobie because no later contribution of water or air is carried out. It is reserved for amateurism and the fresh climates (better insulation) or dry (reduction of the water losses). Under a moderated climate, this method causes the appearance of bad smells (anaerobic decomposition).

In heap

It is the most common method of composting. Waste is gathered in Andain S indefinite length and of which the height depends at the same time on porosity to the air of the compost (more it is raised, standard “P”, more the heap can be high) as well as frequency and method of selected ventilation (a high frequency and/or a ventilation by forced ventilation authorize more important heaps).

In corridor

This method is extremely similar to the preceding one, but the wind-rows lie here between two side low walls. It allows sometimes an easier installation of the devices of ventilation but requires a more important investment. One also has less flexibility for the organization or the modification of the building site of composting.

In closed enclosure or “digester”

The common principle of the processes of “accelerated” fermentation known as is based ‚on the more or less fast stay of waste in devices called digesters. A Digesteur is a closed enclosure inside which it is possible to control the course of fermentation while acting primarily on ventilation. Waste enters in general by an end of the device and arises, at the end of the fermentation, at the other end. The mixing and the ventilation of materials are generally carried out uninterrupted.

Vertical silo (turn)

Many devices exist, more or less complex, but their principle remains the same one. Waste is conveyed, via a conveying belt, at the top of the tower of digestion. They will go down either by means of endless screw or of scrapers while following a succession from plates, or by gravity. On each level, or in the mass of the compost, flexible ventilation ducts are installed making it possible to oxygenate the medium. At the end of the fermentation, the compost is recovered at the base of the tower.

Biostabilisator

The digester is laid out here, either vertically, but horizontally. They are in fact a rotary cylinder a length 25 to 35 meters and a diameter 3 to 4 meters. Rotation continues cylinder, inside whose are fixed helicoid deflecting plates, allows to ensure at the same time the mixing and the ventilation of the product like its progression towards the end of the device. The duration of stay of waste inside the biostabilisator is about 4 to 6 days, after which they are transferred on the surface from maturation.

Mode of ventilation

As we already indicated, the ventilation of the mixture in composting is essential during the phase of active fermentation. Several methods exist, adapted better to one or the other method of composting or to a more or less large scale of work.

Passive ventilation and Chinese method

In the traditional systems of composting in heap, only the porosity of this one ensures the ventilation of the mass. One is thus limited to heaps of low dimensions and composts with very high porosity ‚(standard “P”, large particles). The Chinese improved this system by installing beams of bamboos at the time of the constitution of the heap. These bamboos are then withdrawn after 1 or 2 days, leaving free openings plunging until the medium of the heap and by which ventilation can be done more actively.

Mixing of materials

The most effective oxygenation of a mass in hot fermentation is obtained by its reversal. Complete mixing also makes it possible to ensure a more homogeneous fermentation of all the mass, each particle evolving/moving sufficient time in the center of the compost, where the temperature is highest. Between the reversals, the part external of the heap evolves/moves in aerobiosis by passive ventilation (see preceding paragraph) while the oxygen rate in the center of the heap decreases quickly. The frequency and the quality of the reversals are thus the fundamental parameters of this technique.

According to the dimension of the building site of composting, mixing will be done with the fork (manual labor), by means of a machine of public works (shovel tyred loader), or by means of specialized machines.

Ventilation activates by blower

Contrary to the preceding techniques, the contribution of Oxygène during fermentation is continuous here. The wind-rows to be aired recover a network of pipings perforated over all their length and connected to a booster. The power of the booster is function of the volume and the compressing of the mass to be aired.

Ventilation activates by aspiration (Beltsville method)

The contribution of fresh air is carried out here by aspiration through the wind-rows according to a diagram identical to that of the preceding method (if it is not that the booster is replaced by a vacuum cleaner). The device by aspiration is to be preferred with that by blower because the aspired air tends less to take preferential ways, which would be prejudicial with the effectiveness of ventilation. One will however envisage a filter, which can be simply a mature heap of compost, on the outlet side of the vacuum cleaner in order to eliminate the odors. Ventilation can be combined, if it is wished, with the mixing of materials describes higher.

Determination of the end of composting

A good compost is a product whose organic components underwent a biological conversion into less aggressive and more stable substances. The processes of degradation persist however ata rate more reduced to beyond same of the phase of fermentation. It thus should be known when and for which use one will be able to use a compost without risk of Phytotoxicité.

A fresh compost, i.e. having undergone a beginning of fermentation (about 2 weeks), could be used in Paillage (Mulch ing) or in Champignonnière S. At the end of the fermentation, the compost is stabilized and could be useful like organic Engrais/amendment. A use as substrate of culture requires as for it a compost having undergone one long period of maturation (all the more long as the plants are sensitive: young people Sowing, Lettuce,…).

Change of the temperature

Simple means of following the course of the process of composting consist, as mentioned previously, to use thermometric probes plunging in the mass in fermentation. This method gives information on the stage of fermentation but little on the level of maturity of the compost. It will have to thus be supplemented by one or more other methods.

Report/ratio C/N

The consumption of organic carbon by the Microflore releases a great quantity of CO₂. The progressive reduction in the percentage of carbon of the medium has as a consequence a considerable reduction in the value of the report/ratio C/N. Indeed the Nitrogen, fixed in the microbial proteins, remains in the mass of the compost (except possible losses by release of Ammoniac). According to the degree of fermentability of carbon composing the residues, one will regard as favorable a report/ratio C/N from 20 to 40 at the end of the maturation.

Many authors quote a report/ratio C/N from 15 to 30 like ideal. The practical experiment shows that, for substrates rich in lignins or other forms of not very fermentable Carbone, a ratio of 40 even 50 does not cause a deficiency by immobilization of the Azote. The degradation of these carbonaceous compounds by the Micro-organismes is indeed so slow that low fuel consumption of Azote which results from it does not compete with the culture.

Let us announce finally that C/N is chemically given (Method of analysis of the C/N). However, the chemical reagents do not correspond to the enzymatic luggage of the microflora present in the compost. In addition, the chemical Analyze degrades the particles of the sample completely, i.e. much more than the surface of attack immediately accessible to the microbial enzymes. Ideal C/N will be thus to determine in each case.

Circular chromatography on paper

It was developed by PFEIFER (Biochemical Research Laboratory, Spring Valley - the USA) and was tested by HERTELENDY. Its principle is based on the fact that different, dissolved substances in the same solvent (NaOH solution with 5%), present variable affinities of capillary migration on an absorbing surface (round filter paper sheet). This absorbing surface is pretreated with the Silver nitrate, which serves as revealing. The alkaline solution of compost is brought drop by drop to the center of the filter paper. After migration of the solution, one lets the chromatogram develop in attenuated light. A not very advanced compost is characterized by a dark image with the periphery and a clear central spot; a ripe compost shows on the contrary a dark central spot with clear trails towards the periphery.

Germination of plants test

It is incontestably about the simplest method and most reliable. The plants test most usually used are the garden Cresson ( Lepidium sativum ) and the Laitue ( Lactuca sativa ). Seeds of the plant test are sown on the compost humidified out of hermetically closed bottle. After 3 days, maturity is evaluated according to the percentage of germination and, possibly, the quantity of green matter obtained.

Other methods

Many other methods exist, more or less fast, more or less reliable, requiring a more or less expensive equipment and a more or less qualified personnel. Among those, let us quote:

of the empirical criteria: more or less dark color, aspect of the compost to the touch, the odor,…
of the respirometric methods: Consumed O₂, emitted CO₂,…
of the physicochemical methods: proportioning of the Chemical Demand for Oxygen, proportioning of the shapes of mineral nitrogen (NH4⁺ and NO3^-), proportioning of the S.O.D and the S.O.R. (Decomposable Organic Substance and Resistant Organic Substance), measurement of the pH,…
of the biological methods: proportioning of the ATP (Adenosine Tri-Phosphate), growth of plants or micro-organisms tests,…

Uses of the compost

The compost can be used like Engrais on meadow or before Labor. Its use improves the structure of the ground S (contribution of Organic matter), as well as the Biodisponibilité in nutritive elements (Azote). It also increases the Biodiversité of the Pédofaune.

With the garden, it is used to fertilize the plat bands, the Fruit trees and the Potager. It can also be used like Terreau for the plants out of pot.

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