There are various composting techniques, which adjust to different needs; The choice of one technique or another depends, among other things, on the quantity and type of material to be processed, available investment and availability of land, operational complexity and the final product to be obtained.[9]​

The different systems are determined by the aeration mechanisms used in the process; we can generally group them into: passive aeration, forced aeration, and aeration by turning the material.

Most industrial and commercial composting plants use active processes, because they guarantee better quality products in a shorter period of time. The greatest degree of control, and therefore the highest quality, is usually achieved by composting in a closed container with continuous control and adjustment of temperature, air flow and humidity, among other parameters.

Home composting is more varied, fluctuating between extremely passive techniques to active techniques typical of an industry. To do this, choose an outdoor location, be it patio or garden, preferably far from the house or kitchen, it should receive sun and shade during the day. Every time new organic waste is added to the compost or once a week everything is stirred with a rod. This step is very important to aerate the materials. In three or four weeks it will be observed that it is difficult to distinguish what was deposited except for the most recent waste. Deodorant products can be used, although a well-maintained pile rarely produces bad odors.

After four months it will become humus (complex and stable organic material) with a great density and variety of microorganisms that synthesize enzymes, vitamins, hormones, etc. and that have a favorable impact on the biotic balance of the soil.[11]​.

Humidity

Microorganisms need water as a medium to transport nutrients and other elements, and it is also a determining factor in gas exchange. In the composting process it is important that humidity reaches levels close to 40-60%. If the moisture content is higher, water will occupy all the pores and therefore the process would become anaerobic (without oxygen), that is, putrefaction of the organic matter would occur. If the humidity is excessively low, the activity of the microorganisms is decreased and the process is slower. The moisture content will depend on the raw materials used.[12]​.

Temperature

It is a useful parameter that allows monitoring the decomposition process of organic matter, when the material is being composted it goes through a temperature cycle that is caused by microbiological metabolic activity.[13]​ The increase in metabolic activity generates heat and as a consequence the temperature increases, conversely the decrease in activity is evidenced by the decrease in temperature. The existence of these temperature cycles divides the composting process into four stages (read: Stages of the composting process): mesophilic (less than 40 °C), thermophilic (from 40 to 60 °C), cooling phase (less than 40 °C) and maturation phase (room temperature).[12]​.

The management and control of temperatures allow a natural sanitation treatment to be carried out, especially with respect to pathogenic microorganisms, as well as destroying weed seeds, fungal spores and some phytotoxins that would later pose a problem when adding compost to agricultural crops. However, such management and control must be adequate, since otherwise it would not only destroy pathogenic organisms, but also beneficial flora before the process does so naturally at the right time. Periodic readings using the thermometer help determine when the pile should be turned, if it reaches over 70 °C. Or if a compost pile fails to raise its temperature above 48 °C after a few days, it is an indication that there is probably not enough nitrogen in the pile to continue the process.[9]​.

Temperature is conditioned by humidity and aeration, due to the relationship between these two parameters and the metabolic activity of microorganisms.[13]​.

Carbon/nitrogen ratio

Generally speaking, microorganisms absorb thirty parts carbon for every part nitrogen. Carbon is used as an energy source, with 10 parts incorporated into the cellular protoplasm and 20 parts eliminated as carbon dioxide.[14]​ In practical terms, the carbon/nitrogen ratio allows us to know the speed of decomposition and determine the composting time, as long as the humidity, aeration and temperature conditions are optimal. To obtain a good quality compost it is important that there is a balanced relationship between both elements. Theoretically, a carbon/nitrogen ratio of 25-35 is appropriate, but this will vary depending on the raw materials that make up the compost. If the ratio is greater than 35, there is not enough nitrogen for microbial growth, which will decrease biological activity and therefore delay the process. On the other hand, if it is less than 30, the nitrogen will be in excess so it can be lost as ammonia (NH), which will result in an unpleasant odor.[12]​.

Acidity (pH)

This is an important parameter to evaluate the microbial environment and waste stabilization. The pH value depends on the source materials and varies in each phase of the composting process. The initial pH is normally between 5 and 7. In the first days of composting, the pH drops to 5 or less, due to the presence of simple organic acids, and the temperature rises due to the production of mesophilic organisms. After about 3 days, the temperature reaches the thermophilic stage and the pH begins to rise to about 8 to 8.5 due to the conversion of ammonium to ammonia, alkalinizing the medium for the remainder of the aerobic process. In the maturation phase, the pH value reaches a value between 7 to 8 in the compost.[9]​[15]​.

Aeration

The composting process is an aerobic process, that is, it requires the presence of oxygen for the proper development of microorganisms, therefore aeration is an important factor in the composting process since oxygen is essential for the metabolism and respiration of the microorganisms that participate in it. Aeration has a double objective, first to provide sufficient oxygen to the microorganisms and, second, to allow the maximum evacuation of CO produced.[12]​ Likewise, aeration prevents the material from compacting or becoming waterlogged. Oxygen needs vary during the process, reaching the highest consumption rate during the thermophilic phase. The oxygen saturation in the medium should not drop below 5%, with the optimal level being 10%. Excess aeration would cause a drop in temperature and a greater loss of humidity through evaporation, causing the decomposition process to stop due to lack of water. Likewise, low aeration prevents sufficient evaporation of water, generating excess humidity and an environment of anaerobiosis.[15]​.

Particle size

The activity of microorganisms generally occurs on the surface of the particles, therefore their size must be small, in order to increase the surface area and favor the activity of the microorganisms and the rate of decomposition. The ideal particle size is 2 to 5 cm. Furthermore, the smaller the particle size, the pile tends to compact, which results in less aeration and therefore less microbial activity, slowing down the process.[12]​ The density of the material, and therefore the aeration of the pile or moisture retention, are closely related to the size of the particle, with the density being approximately 150-250 kg/m³. As the composting process progresses, the size decreases and therefore, the density increases. 600-700 kg/m³.[15]​.

Other components

Sometimes other ingredients are added in order to enrich the final mixture, control the conditions of the process or activate the microorganisms responsible for it. Sprinkling lime in small quantities can control the appearance of an excessive degree of acidity that reduces the speed of fermentation. The

Algae provide important micronutrients. Some pulverized rocks provide minerals, unlike clay.

The manure fraction may come from human feces. However, the risk of not reaching temperatures high enough to eliminate pathogens means that they are not usually used on food crops. The general use of feces from carnivorous animals is also not recommended in home composting since they contain pathogens that are difficult to eliminate. Even so, they can be useful for fertilizing trees, gardens, etc.

In Mediterranean climate, compost maturity is obtained after 3 or 6 months in spring/summer and 6 to 9 months in winter.

Domestic: This category considers residual materials from food preparation (parts of fruits, vegetables, and egg shells, among others) and waste of animal origin (meat, skin, blood, bones, and others).[16]​.

Garden: Includes the remains of orchard crops, dead flowers, stems, grasses and leaf litter.

Agricultural by-products: The most used are harvest residues from practically all crops (for example, rice, wheat, barley, corn, sugar cane, beans, sunflower, etc.) as well as husks and bran obtained from threshing or grinding.

Livestock waste: Manure, urine and droppings from all types of animals are excellent for composting as they contain a high percentage of nutrients.

Forestry: The remains of trees, leaves and fallen branches are an important source of material for making compost. These wastes contain large amounts of cellulose and lignin that partially decompose in the compost pile and continue to mineralize in the soil after being applied.

Urban and agro-industrial waste: Consists of the biodegradable fraction of garbage, such as cardboard, paper, fine food waste and natural fibers and the waste that comes from the industrialization of products such as vegetables, cocoa, coffee, rice, corn, wheat, sorghum, wood and seeds, among others. The use of non-biodegradable materials, such as glass, metals, wire, plastics, rubber, fresh ashes, synthetic fibers or fruits with thorns, should be avoided, as they can cause problems for the people in charge of handling them.

coffee composting

Coffee composting is carried out with the collection of organic coffee waste, which aims to serve as an organic contribution for plants and green areas, since it provides nutrients to generate sustainable development.[17]​[17]​.

The procedure to make coffee compost is:.

Once the coffee transformation time has passed, compost is obtained, which can be distributed in the green areas.

Composting with worms

Vermicompost can be obtained as an excretion product of the red earthworm or other members of the Lumbricidae family.[18] These organisms feed on organic waste and transform it into a product rich in nutrients and soil microbes used to fertilize or enrich the land as a growing medium. There is an activity called lumbriculture, which deals with the breeding, reproduction and survival conditions of these worms. There is even a global market to sell them.

Avicomposting

It is a systemic detritophagous use system (permaculture) in which chickens are introduced. Remains of organic matter from domestic origin and green waste from the orchard and garden are successively and daily contributed to the composter, which serve as food for the chickens and other microfauna. After about two months, the first composter is completed with the chicken manure that provides nitrogen, sealed and watered to allow and accelerate the thermal phase. At the end of the thermal phase, access is again allowed to the hens who take advantage of the high density of microfauna and worms as a protein supplement, removing it weekly until the compost matures. This composting process introduces ecological complexity and allows for increased yield and use. The composter therefore acts as a feeder (which can be made with pallets, forming a 1 m² base) leaving entrances for the hens on two sides, and/or at the top with a ladder. Generally, depending on the type and amount of waste, it is necessary to supplement the hens' feed with some grain-feed and calcium (shells) and proteins.

In Torremocha de Jarama, an experience began in 2010 with two community poultry composters, each with 13 chickens and used by 7 families who contribute their organic remains. Each family had a fixed day a week to collect the eggs. In this way, about three cubic meters of quality compost and about 3,000 eggs per year are obtained. In 2012, a larger process began in the municipality of Noáin in Navarra in a municipal park and with the involvement of the city council (park of the senses) in which 30 families participate, dividing up the days of the month to contribute biowaste from the brown container where neighbors deposit their biowaste. Noáin's case was used for the design of a prefabricated composting chicken coop model—avicompo model—by the Vermican company.

Starting in 2012, the TERRAE network of municipalities promoted the model in several municipalities of Toledo (Campillo de la Jara), Madrid (Redueña) and Cáceres (Carcaboso), with the municipalities providing the facilities and/or materials for their self-construction.

El compostaje es un proceso durante el cual la temperatura de los residuos varía dependiendo de la actividad metabólica de los microorganismos. De acuerdo con este parámetro, el proceso de compostaje se puede dividir en cuatro etapas: mesófila, termófila, enfriamiento y maduración.[12]​.

Phase I or mesophilic

In this phase we have a fresh material, without humidifying and at room temperature. The mesophilic microorganisms present in the materials begin to develop using simple sources of carbon and nitrogen, in this way they grow and multiply, decomposing the materials. This microbial activity causes the temperature to increase to 40-45 °C in a few days (between two and eight). Furthermore, the decomposition of simple carbon sources, such as sugars, produces organic acids and, therefore, the pH can drop (to about 4.0 or 4.5).[12]​[15]​.

If the initial material has little moisture, the degradation will be slow or negligible. If, on the other hand, it has excess moisture, it will tend to putrefaction instead of the aerobic decomposition that characterizes composting. This degradation with excess humidity facilitates the proliferation of anaerobic bacteria and fungi that, in addition to giving off bad odors, convert organic matter into a product that is not suitable for the soil. The Carbon/Nitrogen ratio must be adequate (between 25/1 and 30/1) to guarantee energy and protein sources for mesophilic microorganisms.[3]​.

Phase II or thermophilic

This phase is also known as the sanitization phase, the material reaches temperatures higher than 45 °C, the mesophilic microorganisms are replaced by those that grow at higher temperatures (between 45 °C and 70 °C), mostly bacteria (thermophilic bacteria), which act by facilitating the degradation of more complex carbon sources, such as cellulose and lignin.[15]​.

Initially, thermophilic bacteria and fungi begin to degrade cellulose and partially lignin, with which the temperature increases. In addition, they act by transforming nitrogen into ammonia, so the pH of the medium rises. Above 60 °C, thermophilic fungi cease their activity and bacteria appear that produce spores and actinobacteria, which are responsible for decomposing waxes, hemicelluloses and other complex carbon compounds. For several days (or a few months, depending on the starting material, climatic conditions and other factors), the high temperature is maintained and the biological activity decreases, the pasteurization of the medium occurs, allowing the destruction of bacteria and contaminants of fecal origin such as Escherichia coli and Salmonella spp. Likewise, this phase is important because temperatures above 55 °C eliminate cysts and helminth eggs, spores of phytopathogenic fungi and weed seeds that can be found in the starting material, resulting in a sanitized product. At this stage, frequent turning must be carried out in order to provide oxygen, which is quickly consumed by microorganisms.[12]​[15]​.

Phase III or cooling

When the sources of carbon and, especially, nitrogen have practically been exhausted in the organic material, the temperature drops again to 40-45 °C, since the heat generated inside the pile is less than that lost. As a consequence of this drop in temperature, mesophilic bacteria and fungi (some visible to the naked eye) reinvade the compost, restart their activity and degrade the remaining cellulose and lignin, slightly lowering the pH. This phase will be recognized when after turning the battery there is no subsequent increase in temperature.[12]​[15]​.

Phase IV or maturation

It is a period that takes months at room temperature, during which secondary reactions of condensation and polymerization of carbon compounds occur for the formation of humic and fulvic acids in the environment.[15]​.

There are various composting techniques, which adjust to different needs; The choice of one technique or another depends, among other things, on the quantity and type of material to be processed, available investment and availability of land, operational complexity and the final product to be obtained.[9]​

The different systems are determined by the aeration mechanisms used in the process; we can generally group them into: passive aeration, forced aeration, and aeration by turning the material.

Most industrial and commercial composting plants use active processes, because they guarantee better quality products in a shorter period of time. The greatest degree of control, and therefore the highest quality, is usually achieved by composting in a closed container with continuous control and adjustment of temperature, air flow and humidity, among other parameters.

Home composting is more varied, fluctuating between extremely passive techniques to active techniques typical of an industry. To do this, choose an outdoor location, be it patio or garden, preferably far from the house or kitchen, it should receive sun and shade during the day. Every time new organic waste is added to the compost or once a week everything is stirred with a rod. This step is very important to aerate the materials. In three or four weeks it will be observed that it is difficult to distinguish what was deposited except for the most recent waste. Deodorant products can be used, although a well-maintained pile rarely produces bad odors.

After four months it will become humus (complex and stable organic material) with a great density and variety of microorganisms that synthesize enzymes, vitamins, hormones, etc. and that have a favorable impact on the biotic balance of the soil.[11]​.

Humidity

Microorganisms need water as a medium to transport nutrients and other elements, and it is also a determining factor in gas exchange. In the composting process it is important that humidity reaches levels close to 40-60%. If the moisture content is higher, water will occupy all the pores and therefore the process would become anaerobic (without oxygen), that is, putrefaction of the organic matter would occur. If the humidity is excessively low, the activity of the microorganisms is decreased and the process is slower. The moisture content will depend on the raw materials used.[12]​.

Temperature

It is a useful parameter that allows monitoring the decomposition process of organic matter, when the material is being composted it goes through a temperature cycle that is caused by microbiological metabolic activity.[13]​ The increase in metabolic activity generates heat and as a consequence the temperature increases, conversely the decrease in activity is evidenced by the decrease in temperature. The existence of these temperature cycles divides the composting process into four stages (read: Stages of the composting process): mesophilic (less than 40 °C), thermophilic (from 40 to 60 °C), cooling phase (less than 40 °C) and maturation phase (room temperature).[12]​.

The management and control of temperatures allow a natural sanitation treatment to be carried out, especially with respect to pathogenic microorganisms, as well as destroying weed seeds, fungal spores and some phytotoxins that would later pose a problem when adding compost to agricultural crops. However, such management and control must be adequate, since otherwise it would not only destroy pathogenic organisms, but also beneficial flora before the process does so naturally at the right time. Periodic readings using the thermometer help determine when the pile should be turned, if it reaches over 70 °C. Or if a compost pile fails to raise its temperature above 48 °C after a few days, it is an indication that there is probably not enough nitrogen in the pile to continue the process.[9]​.

Temperature is conditioned by humidity and aeration, due to the relationship between these two parameters and the metabolic activity of microorganisms.[13]​.

Carbon/nitrogen ratio

Generally speaking, microorganisms absorb thirty parts carbon for every part nitrogen. Carbon is used as an energy source, with 10 parts incorporated into the cellular protoplasm and 20 parts eliminated as carbon dioxide.[14]​ In practical terms, the carbon/nitrogen ratio allows us to know the speed of decomposition and determine the composting time, as long as the humidity, aeration and temperature conditions are optimal. To obtain a good quality compost it is important that there is a balanced relationship between both elements. Theoretically, a carbon/nitrogen ratio of 25-35 is appropriate, but this will vary depending on the raw materials that make up the compost. If the ratio is greater than 35, there is not enough nitrogen for microbial growth, which will decrease biological activity and therefore delay the process. On the other hand, if it is less than 30, the nitrogen will be in excess so it can be lost as ammonia (NH), which will result in an unpleasant odor.[12]​.

Acidity (pH)

This is an important parameter to evaluate the microbial environment and waste stabilization. The pH value depends on the source materials and varies in each phase of the composting process. The initial pH is normally between 5 and 7. In the first days of composting, the pH drops to 5 or less, due to the presence of simple organic acids, and the temperature rises due to the production of mesophilic organisms. After about 3 days, the temperature reaches the thermophilic stage and the pH begins to rise to about 8 to 8.5 due to the conversion of ammonium to ammonia, alkalinizing the medium for the remainder of the aerobic process. In the maturation phase, the pH value reaches a value between 7 to 8 in the compost.[9]​[15]​.

Aeration

The composting process is an aerobic process, that is, it requires the presence of oxygen for the proper development of microorganisms, therefore aeration is an important factor in the composting process since oxygen is essential for the metabolism and respiration of the microorganisms that participate in it. Aeration has a double objective, first to provide sufficient oxygen to the microorganisms and, second, to allow the maximum evacuation of CO produced.[12]​ Likewise, aeration prevents the material from compacting or becoming waterlogged. Oxygen needs vary during the process, reaching the highest consumption rate during the thermophilic phase. The oxygen saturation in the medium should not drop below 5%, with the optimal level being 10%. Excess aeration would cause a drop in temperature and a greater loss of humidity through evaporation, causing the decomposition process to stop due to lack of water. Likewise, low aeration prevents sufficient evaporation of water, generating excess humidity and an environment of anaerobiosis.[15]​.

Particle size

The activity of microorganisms generally occurs on the surface of the particles, therefore their size must be small, in order to increase the surface area and favor the activity of the microorganisms and the rate of decomposition. The ideal particle size is 2 to 5 cm. Furthermore, the smaller the particle size, the pile tends to compact, which results in less aeration and therefore less microbial activity, slowing down the process.[12]​ The density of the material, and therefore the aeration of the pile or moisture retention, are closely related to the size of the particle, with the density being approximately 150-250 kg/m³. As the composting process progresses, the size decreases and therefore, the density increases. 600-700 kg/m³.[15]​.

Other components

Sometimes other ingredients are added in order to enrich the final mixture, control the conditions of the process or activate the microorganisms responsible for it. Sprinkling lime in small quantities can control the appearance of an excessive degree of acidity that reduces the speed of fermentation. The

Algae provide important micronutrients. Some pulverized rocks provide minerals, unlike clay.

The manure fraction may come from human feces. However, the risk of not reaching temperatures high enough to eliminate pathogens means that they are not usually used on food crops. The general use of feces from carnivorous animals is also not recommended in home composting since they contain pathogens that are difficult to eliminate. Even so, they can be useful for fertilizing trees, gardens, etc.

In Mediterranean climate, compost maturity is obtained after 3 or 6 months in spring/summer and 6 to 9 months in winter.

Domestic: This category considers residual materials from food preparation (parts of fruits, vegetables, and egg shells, among others) and waste of animal origin (meat, skin, blood, bones, and others).[16]​.

Garden: Includes the remains of orchard crops, dead flowers, stems, grasses and leaf litter.

Agricultural by-products: The most used are harvest residues from practically all crops (for example, rice, wheat, barley, corn, sugar cane, beans, sunflower, etc.) as well as husks and bran obtained from threshing or grinding.

Livestock waste: Manure, urine and droppings from all types of animals are excellent for composting as they contain a high percentage of nutrients.

Forestry: The remains of trees, leaves and fallen branches are an important source of material for making compost. These wastes contain large amounts of cellulose and lignin that partially decompose in the compost pile and continue to mineralize in the soil after being applied.

Urban and agro-industrial waste: Consists of the biodegradable fraction of garbage, such as cardboard, paper, fine food waste and natural fibers and the waste that comes from the industrialization of products such as vegetables, cocoa, coffee, rice, corn, wheat, sorghum, wood and seeds, among others. The use of non-biodegradable materials, such as glass, metals, wire, plastics, rubber, fresh ashes, synthetic fibers or fruits with thorns, should be avoided, as they can cause problems for the people in charge of handling them.

coffee composting

Coffee composting is carried out with the collection of organic coffee waste, which aims to serve as an organic contribution for plants and green areas, since it provides nutrients to generate sustainable development.[17]​[17]​.

The procedure to make coffee compost is:.

Once the coffee transformation time has passed, compost is obtained, which can be distributed in the green areas.

Composting with worms

Vermicompost can be obtained as an excretion product of the red earthworm or other members of the Lumbricidae family.[18] These organisms feed on organic waste and transform it into a product rich in nutrients and soil microbes used to fertilize or enrich the land as a growing medium. There is an activity called lumbriculture, which deals with the breeding, reproduction and survival conditions of these worms. There is even a global market to sell them.

Avicomposting

It is a systemic detritophagous use system (permaculture) in which chickens are introduced. Remains of organic matter from domestic origin and green waste from the orchard and garden are successively and daily contributed to the composter, which serve as food for the chickens and other microfauna. After about two months, the first composter is completed with the chicken manure that provides nitrogen, sealed and watered to allow and accelerate the thermal phase. At the end of the thermal phase, access is again allowed to the hens who take advantage of the high density of microfauna and worms as a protein supplement, removing it weekly until the compost matures. This composting process introduces ecological complexity and allows for increased yield and use. The composter therefore acts as a feeder (which can be made with pallets, forming a 1 m² base) leaving entrances for the hens on two sides, and/or at the top with a ladder. Generally, depending on the type and amount of waste, it is necessary to supplement the hens' feed with some grain-feed and calcium (shells) and proteins.

In Torremocha de Jarama, an experience began in 2010 with two community poultry composters, each with 13 chickens and used by 7 families who contribute their organic remains. Each family had a fixed day a week to collect the eggs. In this way, about three cubic meters of quality compost and about 3,000 eggs per year are obtained. In 2012, a larger process began in the municipality of Noáin in Navarra in a municipal park and with the involvement of the city council (park of the senses) in which 30 families participate, dividing up the days of the month to contribute biowaste from the brown container where neighbors deposit their biowaste. Noáin's case was used for the design of a prefabricated composting chicken coop model—avicompo model—by the Vermican company.

Starting in 2012, the TERRAE network of municipalities promoted the model in several municipalities of Toledo (Campillo de la Jara), Madrid (Redueña) and Cáceres (Carcaboso), with the municipalities providing the facilities and/or materials for their self-construction.

El compostaje es un proceso durante el cual la temperatura de los residuos varía dependiendo de la actividad metabólica de los microorganismos. De acuerdo con este parámetro, el proceso de compostaje se puede dividir en cuatro etapas: mesófila, termófila, enfriamiento y maduración.[12]​.

Phase I or mesophilic

In this phase we have a fresh material, without humidifying and at room temperature. The mesophilic microorganisms present in the materials begin to develop using simple sources of carbon and nitrogen, in this way they grow and multiply, decomposing the materials. This microbial activity causes the temperature to increase to 40-45 °C in a few days (between two and eight). Furthermore, the decomposition of simple carbon sources, such as sugars, produces organic acids and, therefore, the pH can drop (to about 4.0 or 4.5).[12]​[15]​.

If the initial material has little moisture, the degradation will be slow or negligible. If, on the other hand, it has excess moisture, it will tend to putrefaction instead of the aerobic decomposition that characterizes composting. This degradation with excess humidity facilitates the proliferation of anaerobic bacteria and fungi that, in addition to giving off bad odors, convert organic matter into a product that is not suitable for the soil. The Carbon/Nitrogen ratio must be adequate (between 25/1 and 30/1) to guarantee energy and protein sources for mesophilic microorganisms.[3]​.

Phase II or thermophilic

This phase is also known as the sanitization phase, the material reaches temperatures higher than 45 °C, the mesophilic microorganisms are replaced by those that grow at higher temperatures (between 45 °C and 70 °C), mostly bacteria (thermophilic bacteria), which act by facilitating the degradation of more complex carbon sources, such as cellulose and lignin.[15]​.

Initially, thermophilic bacteria and fungi begin to degrade cellulose and partially lignin, with which the temperature increases. In addition, they act by transforming nitrogen into ammonia, so the pH of the medium rises. Above 60 °C, thermophilic fungi cease their activity and bacteria appear that produce spores and actinobacteria, which are responsible for decomposing waxes, hemicelluloses and other complex carbon compounds. For several days (or a few months, depending on the starting material, climatic conditions and other factors), the high temperature is maintained and the biological activity decreases, the pasteurization of the medium occurs, allowing the destruction of bacteria and contaminants of fecal origin such as Escherichia coli and Salmonella spp. Likewise, this phase is important because temperatures above 55 °C eliminate cysts and helminth eggs, spores of phytopathogenic fungi and weed seeds that can be found in the starting material, resulting in a sanitized product. At this stage, frequent turning must be carried out in order to provide oxygen, which is quickly consumed by microorganisms.[12]​[15]​.

Phase III or cooling

When the sources of carbon and, especially, nitrogen have practically been exhausted in the organic material, the temperature drops again to 40-45 °C, since the heat generated inside the pile is less than that lost. As a consequence of this drop in temperature, mesophilic bacteria and fungi (some visible to the naked eye) reinvade the compost, restart their activity and degrade the remaining cellulose and lignin, slightly lowering the pH. This phase will be recognized when after turning the battery there is no subsequent increase in temperature.[12]​[15]​.

Phase IV or maturation

It is a period that takes months at room temperature, during which secondary reactions of condensation and polymerization of carbon compounds occur for the formation of humic and fulvic acids in the environment.[15]​.