24 Composting Types, Methods and Controlling-Factors
The three (3) composting types are; aerobic, anaerobic, and vermicomposting.
Composting methods are static-piling, in-vessel, windrow, open-air, tumbler, EMO, direct, combination. pit, industrial, onsite, and mechanical methods.
Factors that affect, influence or control composting are temperature, humidity/moisture content, volume, C:N ratio, particle size, aeration (oxygen-supply), microorganisms, additives and pH.
This article extensively discusses the types, methods, and controlling-factors of composting, as follows;
-Factors that Affect and Control the Composting Process
-Types of Composting
The types of composting are categorized based on the physicochemical and biological conditions involved.
1). Aerobic Composting as one of the Types of Composting
Aerobic composting is a type of composting which involves the decomposition of organic matter in the presence of air [9].
The ideology behind aerobic composting is that biodegradation of organic matter occurs more rapidly in the presence of air, since several of the important microorganisms that facilitate biodegradation, depend on oxygen for respiration and survival.
Examples of aerobic microorganisms include Mycobacterium tuberculosis, Norcadia sp., Citrobacter, Salmonella, and Pseudomonas aeruginosa.
Aerobic composting is a good option in scenarios where the organic matter is composed of complex compounds which require large microbe populations and biochemical activity to be degraded effectively.
There are various methods or mechanisms that can be used to carry out aerobic composting. These include the use of open composting, concrete structures and tumbler mechanism.
2). Anaerobic Composting as one of the Types of Composting
Anaerobic composting is a type of composting in which organic matter decomposes in the absence of any significant amounts of oxygen [16].
Compared to the aerobic type, anaerobic composting occurs over a prolonged period of time. It does not involve conditions that support the survival and activity of aerobic microorganisms therefore, anaerobic microorganisms are generally responsible for the biodegradation that occurs in this type of composting.
These microorganisms can survive in oxygen-deficient environments, and include examples like Clostridium, Actinomyces, Fusobacterium and Bacteroides.
A major disadvantage of this type of composting is the production of irritating gases. Because of the longer duration of decomposition, anaerobic composting involves some biochemical intermediates that emit unpleasant gases which may reduce air quality when released in large quantities [15].
Aside the unpleasant odor produced by the practice of anaerobic composting, greenhouse emissions may also occur.
A good example of a greenhouse gas produced from anaerobic composting is methane. Under well-controlled conditions, such as those present in waste-to-energy projects like anaerobic digestion, this gas can be collected and used as a fuel.
However, in composting, gaseous byproducts are often released into the atmosphere, causing environmental degradation.
3). Vermicomposting
Also referred to as ‘worm farm composting’, vermicomposting is a type of composting that involves the use of worms to digest, degrade and convert organic matter to compost fertilizer [6].
The basic ideology behind this type of composting is the fact that worms can facilitate the process of biodegradation by extracting nutrients from organic matter, thereby speeding up the breakdown of the compounds which make up these materials.
It is important to note that vermicomposting is most effective for some types of organic matter such as plant biomass, which may occur in the form of garden waste and food waste. These materials contain compounds like cellulose which can be ingested by the organisms.
Conditions required for vermicomposting are similar to those required for aerobic composting, because worms also require oxygen for survival. It is arguable that this is one of the best types of composting with regards to soil conservation, as the activities of worms typically convert biomass to organic fertilizer that is highly compatible and beneficial to the soil [7].
The species of worms that can be used in vermicomposting depends on the physicochemical conditions of the compost pile. This includes humidity, pH, and temperature.
Examples of worms used in vermicomposting are tiger worms, night crawlers, and red worms.
Methods of Composting
The methods of composting are categorized based on the basic mechanisms involved.
1). Static-Piling as one of the Methods of Composting
The static pile method of composting is an aerated method of composting in which the organic matter is accumulated in a pile and allowed to decompose [12].
Static piling is one of the most rapid methods of composting because it occurs under aerobic conditions, with some additional measures taken to ensure that air is sufficiently available for biodegradation.
As earlier stated, there are various measures which are taken to aerate the compost pile. One of this is the use of intermediate bulking materials like wood chips to create spaces that allow for entry and exit of air.
Because the compost pile is static. air may need to be provided through a vacuum system or a set of perforated air-pipes.
The suitability and effectiveness of static pile composting depend on the environmental conditions, as well as the characteristics of the organic matter involved.
Generally, the method is best suited to an environment with moderate climatic, biological and physicochemical conditions, since the organic matter will be exposed to these conditions. In high-temperature conditions, the compost pile may require covering to control the rate of moisture loss.
Types of organic matter which are suited to this method include food scraps, plant and animal waste. The method is good for agricultural settings where waste can be directly converted to organic fertilizer for soil amendment.
2). In-Vessel Composting
In-Vessel Composting (or ‘In Vessel Composting) (IVC) is a collective term which refers to all methods of composting in which the organic material is confined within a vessel or any other defined area.
A building (concrete enclosure), trench, or silo may also be used [19].
In-vessel composting can be considered similar to other techniques of waste treatment that involve the confinement of the substrate in an enclosure. Pyrolysis is a good example of such similar techniques, as pyrolysis reactors are designed to play a similar role as the vessels in IVC method.
IVC is also similar to ex situ environmental remediation methods that involve the use of closed systems, such as the bioreactor.
The purpose of the enclosure in IVC method is to regulate the conditions of biodegradation, of the organic matter. This method is also used where there is need to comply with regulations regarding the composting process (biodegradation control, environmental exposure, odor control).
A major advantage of the IVC method is the fact that it is conservative. Compared to other methods of composting, in-vessel method helps to conserve space, moisture, air and soil quality. This is the case when an optimal approach to the practice of IVC is applied.
In-vessel composting also ensures that the composting process is mostly unaffected by external factors like climatic conditions. The enclosure in which composting occurs, is often insulated and maintained under a predefined set of conditions that enable the organic matter to decompose as required.
Owing to its effectiveness, IVC can be used to convert a broad range of organic materials to compost, including animal and plant matter, as well as other forms of organic waste. It is also relatively rapid, because of the uniformity of degradation of organic matter within the enclosed and regulated environment.
In-vessel composting is also a flexible method. It can be scaled to meet various requirements in terms of volume, by simply varying the size of the vessel or enclosure. Similarly, the conditions (physicochemical and biological) of biodegradation can be varied to suit the types of organic matter involved.
3). Windrow Composting
Also known as turned windrow composting or aerated windrow composting, this is a large-scale method of composting whereby organic matter is piled in long rows and allowed to decompose under aerobic conditions [8].
While it may not be categorized among the best methods of composting, the windrow method is suitable and effective in situations where it is necessary for large volumes of organic waste to be treated simultaneously. This may be the case in large-scale municipal waste treatment or agricultural waste management projects.
Within a municipal context, windrow composting is good for managing yard waste [14] and can convert heterogenous organic-matter accumulations to fertilizer.
In windrowing, the rows of compost pilings should be turned occasionally, to facilitate aeration. This may be done either mechanically or manually.
The size of the compost rows is generally massive, and must permit the passage of oxygen through the organic matter while generating enough heat to support effective microbial decomposition.
There are some potential problems associated with the windrow composting method. One of these is the risk of environmental degradation.
The compost piles may produce leachate as they decompose, which can infiltrate the soil, causing both soil and water pollution. Air pollution may also result from the large-scale biodegradation process, as gaseous byproducts are released.
To address these challenges, windrow composting is usually carried out under specified conditions that reduce the environmental impact of the method.
4). Open-Air Degradation as one of the Methods of Composting
Also known as ‘outdoor composting’, open air composting is a compound term which refers to all methods of composting that involve the exposure of organic matter to unregulated aerobic conditions.
Based on this description, there are various methods which fall under the open-air category. They include windrow method, which is simply a large-scale form of open-air composting.
Open air composting may be utilized at a small scale as well. Garden waste and yard waste can be converted to organic fertilizer using this method. The setup may be simply an open space in which the organic matter is accumulated to produce compost pile, or a vessel such as a bin with apertures for aeration of the organic matter.
It is usually necessary to turn the compost pile occasionally, so as to allow for effective aeration. Owing to exposure of the compost pile, worms, microbes and insects may be easily introduced. While it is important to control the activities of such organisms to prevent unfavorable effects, they generally facilitate the composting process.
5). Tumbler Method
Tumbler composting is an enclosed method that involves the use of mechanical equipment to turn the compost in its enclosure.
The tumbler method is a small-scale method which utilizes a tumbler bin that is equipped with a turning mechanism to simplify the aeration process. Using tumblers provides a means of regulation of the composting process, but can be a labor-intensive method. It also is unusable for large scale composting.
6). EMO Composting
EMO refers to Effective Microorganisms; a composting method that uses anaerobic biodegradation and fermentation to convert organic matter to compost [11].
In order to carry out EMO composting (or ‘Bokashi composting’), the compost is placed under conditions that favor the growth and activity of anaerobic microorganisms.
Organic materials are usually accumulated in an oxygen-deficient closed vessel and layered with bran which has been inoculated with anaerobic microorganisms like lactobacillus.
The vessel is then sealed for a period of time (depending on the type and volume of organic matter) to allow the microorganisms grow and breakdown the organic matter. EMO method can be effective across a wide range of organic materials, due to the active introduction of microbes in this method.
7). Direct Composting
Direct composting is one of the simplest methods of composting, and involves burying organic waste directly in the soil without any intermediate process of compost-fertilizer production.
In direct composting, the entire process is simplified because the organic waste is introduced into the ground where the composting process occurs. It does not require careful regulations and the use of turning mechanisms like other methods.
However, direct composting is ideal only in small-scale scenarios, where the amount of organic waste available can be easily managed by burying it in agricultural soil. The method cannot be applied in cases where there are large volumes of waste.
8). Combination Method
Combination or ‘Compot’ composting is a method of composting which combines some attributes and practices from other composting methods, like EMO, direct, open air and vermicomposting.
The ideology behind this practice, is to borrow various aspects of as many different methods of composting as required, to optimize the degradation process under a given set of conditions. Methods which are featured in any combined composting project depend totally on the conditions of the environment and the types of organic matter which are being used.
Combination composting is not a simple or well-defined method compared to others however, it introduces a significant degree of flexibility into the composting process.
9). Pit Composting
Pit composting is one of the simplest methods of composting, and is carried out by digging trenches or holes in which organic matter is buried to form compost.
The method is similar to direct composting, but differs by being more elaborate and larger in scale. It is a partially anaerobic method, due to the limited supply of air to the buried compost pile.
The length of time required to form compost fertilizer using this method, varies based on the environmental conditions and the type of organic matter, but is typically between four (4) and six (6) months.
10). Industrial Composting
Also known as ‘commercial composting’, industrial composting is a large-scale method that is designed to address cases involving large volumes of organic waste [17].
The ideology behind this method is to ensure that the effectiveness of composting is not compromised by the volume of organic matter involved. It is important to note that other methods of composting can be described as commercial or industrial, provided the volume of organic matter involved is very large.
Sources of organic matter or raw material in industrial composting, include agricultural industries, food processing plants, saw mills, farms and municipalities. To cope with the scale, large expanses of land are usually needed. Aerobic type of composting is also applied, to speed-up the process and reduce residence time for the substrate.
The compost pile may be turned at intervals of a few days (3-4) to allow for effective biodegradation.
11). Onsite Method
Onsite composting refers to any form of composting where the organic matter used is derived from the same site on which the composting process occurs.
This method is usually small-scale and requires less equipment than other methods.
Onsite composting can occur naturally in many ecosystems, such as tropical rainforests, where the remains of plants and animals may accumulate and decompose in situ (or ‘on-site’). Such a scenario is beneficial to the sustainability of the ecosystem, as it facilitates the cycling of nutrients through the energy pyramid.
The practice of onsite composting is useful in public institutions like school campuses, as well as communities and organizations, as a means to effectively and economically manage organic waste in the form of food scraps and garden residue.
12). Mechanical Composting
Mechanical composting is a method of composting which utilizes equipment powered by electricity to heat and turn organic matter, to form compost.
Compared to other methods of composting, this method is highly efficient and time-conserving. In its more sophisticated forms, mechanical composting can involve active segregation of organic waste into compatible segments that can degrade effectively [3].
Mechanical composting is usually carried out in a digester, which is equipped with mechanical components. It is a relatively-expensive method and may not be used where energy conservation is a priority. Some level of technical knowledge is also required to operate the equipment that are used for this method.
Factors that Affect and Control the Composting Process
Factors that affect and control composting are;
1). Temperature
Temperature determines the rate and effectiveness of composting.
This is mainly because the presence and activity of microorganisms (which are mostly responsible for biodegradation) depend on temperature.
Changes in temperature are required for composting to proceed through its natural sequence of mesophilic, thermophilic, cooling and curing stages. These temperature changes also control the microorganisms present in compost, and their activities.
Generally, a temperature range of 20-155°C is optimal for composting, as this supports the growth and survival of microbial populations. Low temperatures and extremely high temperatures either prevent microbe growth or kill already existing microbes in the compost pile.
2). Humidity as a Factor Affecting Composting
Humidity or moisture content is an important factor with regards to its effects on composting.
This is because the kinetics of biodegradation are heavily influenced by moisture [13], which determines the effectiveness of microbial activity.
Generally, a fair degree of moisture and environmental humidity will lead to better rates and greater efficiency of composting. Low moisture content reduces microbial activity [5] and therefore slows down (and reduces the efficiency of) composting.
The optimum moisture content for effective composting lies between 45 and 60 percent. By some assessments, this range may be placed between 45-55% or 50-60%. At moisture content of less than 30%, the effectiveness of composting declines notably.
However, the effect of moisture on composting varies with environmental and biochemical conditions of individual scenarios.
3). Organic Matter Chemistry as a Factor Affecting Composting
Because composting is basically the decomposition of organic matter, the chemical composition of this organic matter has a significant effect on the process.
Specifically, the ratio of carbon to nitrogen (C:N or C/N) in organic matter determines how the composting process will occur. Based on the concentration of these elements (carbon and nitrogen) biomass can be categorized as brown (high in carbon content) and green (high in nitrogen content).
Examples of green organic matter include food waste and agricultural residue such as manure, hay and vegetable waste. Brown organic matter is exemplified by wood chips, straw, sawdust, cotton fabric, straw and paper.
During composting, green (nitrogen-rich) organic matter enables microbial populations to derive proteins needed for their growth, while brown (carbon-rich) organic matter is relevant to their overall metabolism.
Biodegradation (and therefore, composting) is optimal at C:N ratio of 25:1 to 30:1 [10]. Relative concentrations of 20:1 and 40:1 may also yield favorable results.
For high ratios above 40:1, the excess carbon and insufficient nitrogen will inhibit microbe growth. In the case of low values (less than 20:1), the excess nitrogen may degrade air quality due to the formation of nitrous oxide (a greenhouse gas) and/or ammonia (a highly irritating gas).
As the composting process proceeds, the C:N ratio may decrease to values of 15:1 to 10:1, due to the metabolic consumption of carbon by microbes.
4). Volume of Organic Matter
Volume of organic matter affects composting, because it determines the distribution of heat, oxygen and microbes among other factors necessary for biodegradation.
While there is no specified optimal range of organic matter volume for composting, the size of the compost pile should ideally be large enough to build and retain the necessary heat for effective decomposition, yet small enough to be within manageable limits.
Organic matter volume in composting can be estimated in terms of width, length and height of the compost pile (or row). It is also important to note that this volume typically decreases as the composting process progresses.
5). Particle Size as a Factor Affecting Composting
In general, smaller particle size leads to better composting [20].
This is because the effectiveness of biodegradation is proportional to the amount of organic surface area which is available to microorganisms. Given that, smaller particles provide more surface area, this translates to faster, more effective composting.
Additionally, when organic materials in a compost pile occur in small particle sizes, it makes the entire compost pile (or organic mixture) more homogeneous, as the particles from various organic materials can mix more evenly. This reduces the biochemical complexity of the decomposition process.
Organic matter particle size can be reduced deliberately in composting, through shredding, chipping and grinding of the materials (where possible) before adding them to the compost pile.
6). Aeration
Aeration or oxygen-supply, is important in composting, simply because microorganisms often require oxygen to survive and function effectively.
The only exception to this requirement is anaerobic microorganisms. While an anaerobic approach can be used in composting, it is less common and comes along with challenges such as the complexity of oxygen exclusion and the risk of environmental degradation.
To further emphasize its importance, the amount of oxygen available during composting, determines the type of composting which will occur.
Where the oxygen concentration is less than 5mg/l, anaerobic composting occurs. Aerobic composting is effective when the concentration of oxygen is 5mg/l and beyond.
Aeration and moisture content have a roughly equal importance in aerobic composting. They are also interdependent variables in terms of their effects on the biodegradation process.
7). Microorganisms
Microorganisms which are important to composting include fungi, actinomycetes and bacteria [2].
The presence and population-size of these microbes influence the composting process in terms of rate, mechanism and effectiveness. Generally, various groups of microorganisms differ in their ability to breakdown various types of organic matter. When the required microbes are present in sufficiently-large numbers, the composting process is optimized.
8). Additives
Additives in composting can be categorized broadly into activators, lime (pH amendment) and inoculants. Individual examples are polyethylene glycol, gypsum, straw, manure, grass clippings, blood meal and fly ash.
These additives supply various physicochemical and biological stimulants to the compost pile. Additives affect composting by preferentially altering other influential factors like pH, aeration, temperature and moisture [4].
9). pH as a Factor Affecting Composting
A pH range of mildly-acidic to mildly-alkaline; such as 5.5-8.0 can.be considered optimal for composting. This is because most microorganisms needed for biodegradation do not function optimally under extreme acidic or alkaline conditions.
Microbe destruction occurs with low pH (high acidity), leading to a decline in the rate of composting [1].
Additives can be used to adjust the pH of a compost pile so as to optimize biodegradation. pH also determines the biochemical reactions and byproducts that may occur during composting, which in turn determine the environmental effects of the process.
In general, pH changes as composting progresses, and is often between 6.0 and 8.0 by the end of the process [18].
Conclusion
Types of composting are;
- Aerobic Composting
- Anaerobic Composting
- Vermicomposting
Methods of composting are;
- Static-Piling
- In-Vessel Composting
- Windrow Composting
- Open-Air Degradation
- Tumbler Method
- EMO Composting
- Direct Composting
- Combination Method
- Pit Composting
- Industrial Composting
- Onsite Method
- Mechanical Composting
Factors that affect the composting process are;
- Temperature
- Humidity
- Organic Matter Chemistry
- Volume of Organic Matter
- Particle Size
- Aeration
- Microorganisms
- Additives
- pH
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