composting process

Composting process and techniques

Composting is the natural process of 'rotting' or decomposition of organic matter by microorganisms under controlled conditions. Raw organic materials such as crop residues, animal wastes, food garbage, some municipal wastes and suitable industrial wastes, enhance their suitability for application to the soil as a fertilizing resource, after having undergone composting.
Compost is a rich source of organic matter. Soil organic matter plays an important role in sustaining soil fertility, and hence in sustainable agricultural production. In addition to being a source of plant nutrient, it improves the physico-chemical and biological properties of the soil. As a result of these improvements, the soil: (i) becomes more resistant to stresses such as drought, diseases and toxicity; (ii) helps the crop in improved uptake of plant nutrients; and (iii) possesses an active nutrient cycling capacity because of vigorous microbial activity..
Types of composting
Composting may be divided into two categories by the nature of the decomposition process. In anaerobic composting, decomposition occurs where oxygen (O) is absent or in limited supply. Under this method, anaerobic micro-organisms dominate and develop intermediate compounds including methane, organic acids, hydrogen sulphide and other substances. In the absence of O, these compounds accumulate and are not metabolized further. Many of these compounds have strong odours and some present phytotoxicity. As anaerobic composting is a low-temperature process, it leaves weed seeds and pathogens intact. Moreover, the process usually takes longer than aerobic composting. These drawbacks often offset the merits of this process, viz. little work involved and fewer nutrients lost during the process.
Aerobic composting takes place in the presence of ample O. In this process, aerobic microorganisms break down organic matter and produce carbon dioxide (CO2), ammonia, water, heat and humus, the relatively stable organic end product. Although aerobic composting may produce intermediate compounds such as organic acids, aerobic micro-organisms decompose them further. The resultant compost, with its relatively unstable form of organic matter, has little risk of phytotoxicity. The heat generated accelerates the breakdown of proteins, fats and complex carbohydrates such as cellulose and hemi-cellulose. Hence, the processing time is shorter. Moreover, this process destroys many micro-organisms that are human or plant pathogens, as well as weed seeds, provided it undergoes sufficiently high temperature. Although more nutrients are lost from the materials by aerobic composting, it is considered more efficient and useful than anaerobic composting for agricultural production. Most of this publication focuses on aerobic composting.
The aerobic composting process
The aerobic composting process starts with the formation of the pile. In many cases, the temperature rises rapidly to 70-80 °C within the first couple of days. First, mesophilic organisms (optimum growth temperature range = 20-45 °C) multiply rapidly on the readily available sugars and amino acids (Figure 1). They generate heat by their own metabolism and raise the temperature to a point where their own activities become suppressed. Then a few thermophilic fungi and several thermophilic bacteria (optimum growth temperature range = 50-70 °C or more) continue the process, raising the temperature of the material to 65 °C or higher. This peak heating phase is important for the quality of the compost as the heat kills pathogens and weed seeds.
The active composting stage is followed by a curing stage, and the pile temperature decreases gradually. The start of this phase is identified when turning no longer reheats the pile. At this stage, another group of thermophilic fungi starts to grow. These fungi bring about a major phase of decomposition of plant cell-wall materials such as cellulose and hemi-cellulose. Curing of the compost provides a safety net against the risks of using immature compost such as nitrogen (N) hunger, O deficiency, and toxic effects of organic acids on plants.
Eventually, the temperature declines to ambient temperature. By the time composting is completed, the pile becomes more uniform and less active biologically although mesophilic organisms recolonize the compost. The material becomes dark brown to black in colour. The particles reduce in size and become consistent and soil-like in texture. In the process, the amount of humus increases, the ratio of carbon to nitrogen (C:N) decreases, pH neutralizes, and the exchange capacity of the material increases.
Temperature changes and fungi populations in wheat straw compost
Note:Solid line = temperature; broken line = mesophilic fungi population; dotted line = thermophilic fungi population; left y-axis = fungal populations (logarithm of colony forming units (cfu) per gram of compost plated onto agar); right y-axis = temperature in centre of compost. a, b, c and d = heating phases.Source.
Factors affecting aerobic composting
Aeration
Aerobic composting requires large amounts of O, particularly at the initial stage. Aeration is the source of O, and, thus, indispensable for aerobic composting. Where the supply of O is not sufficient, the growth of aerobic micro-organisms is limited, resulting in slower decomposition. Moreover, aeration removes excessive heat, water vapour and other gases trapped in the pile. Heat removal is particularly important in warm climates as the risk of overheating and fire is higher. Therefore, good aeration is indispensable for efficient composting. It may be achieved by controlling the physical quality of the materials (particle size and moisture content), pile size and ventilation and by ensuring adequate frequency of turning.
Moisture
Moisture is necessary to support the metabolic activity of the micro-organisms. Composting materials should maintain a moisture content of 40-65 percent. Where the pile is too dry, composting occurs more slowly, while a moisture content in excess of 65 percent develops anaerobic conditions. In practice, it is advisable to start the pile with a moisture content of 50-60 percent, finishing at about 30 percent.
Nutrients
Micro-organisms require C, N, phosphorus (P) and potassium (K) as the primary nutrients. Of particular importance is the C:N ratio of raw materials. The optimal C:N ratio of raw materials is between 25:1 and 30:1 although ratios between 20:1 and 40:1 are also acceptable. Where the ratio is higher than 40:1, the growth of micro-organisms is limited, resulting in a longer composting time. A C:N ratio of less than 20:1 leads to underutilization of N and the excess may be lost to the atmosphere as ammonia or nitrous oxide, and odour can be a problem. The C:N ratio of the final product should be between about 10:1 and 15:1.
Temperature
The process of composting involves two temperature ranges: mesophilic and thermophilic. While the ideal temperature for the initial composting stage is 20-45 °C, at subsequent stages with the thermophilic organisms taking over, a temperature range of 50-70 °C may be ideal. High temperatures characterize the aerobic composting process and serve as signs of vigorous microbial activities. Pathogens are normally destroyed at 55 °C and above, while the critical point for elimination of weed seeds is 62 °C. Turnings and aeration can be used to regulate temperature.
Lignin content
Lignin is one of the main constituents of plant cell walls, and its complex chemical structure makes it highly resistant to microbial degradation (Richard, 1996). This nature of lignin has two implications. One is that lignin reduces the bioavailability of the other cell-wall constituents, making the actual C:N ratio (viz. ratio of biodegradable C to N) lower than the one normally cited. The other is that lignin serves as a porosity enhancer, which creates favourable conditions for aerobic composting. Therefore, while the addition of lignin-decomposing fungi may in some cases increase available C, accelerate composting and reduce N loss, in other cases it may result in a higher actual C:N ratio and poor porosity, both of which prolong composting time.
Polyphenols
Polyphenols include hydrolysable and condensed tannins (Schorth, 2003). Insoluble condensed tannins bind the cell walls and proteins and make them physically or chemically less accessible to decomposers. Soluble condensed and hydrolysable tannins react with proteins and reduce their microbial degradation and thus N release. Polyphenols and lignin are attracting more attention as inhibiting factors. Palm et al. (2001) suggest that the contents of these two substances be used to classify organic materials for more efficient on-farm natural resource utilization, including composting.
pH value
Although the natural buffering effect of the composting process lends itself to accepting material with a wide range of pH, the pH level should not exceed eight. At higher pH levels, more ammonia gas is generated and may be lost to the atmosphere
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Techniques for effective aerobic composting
Simple replication of composting practices does not always give the right answer to potential composters. This is because composting takes place at various locations and under diverse climates, using different materials with dissimilar physical, chemical and biological properties. An understanding of the principles and technical options and their appropriate application may be helpful in providing the optimal environment to the compost pile.
Improved aeration
In order to obtain the end product of uniform quality, the whole of the pile should receive a sufficient amount of O so that aerobic micro-organisms flourish uniformly. The methodologies deliberated in this publication made use of the techniques as presented below.
Pile size and porosity of the material
The size of the pile is of great significance and finds mention in the sections on passive composting of manure piles (Chapter 2) and turned wind-rows (Chapter 3). Where the pile or wind-row is too large, anaerobic zones occur near its centre, which slows the process in these zones. On the other hand, piles or wind-rows that are too small lose heat quickly and may not achieve a temperature high enough to evaporate moisture and kill pathogens and weed seeds. The optimal size of the piles and wind-rows should also consider such parameters as the physical property (porosity) of the materials and the way of forming the pile. While more porous materials allow bigger piles, heavy weights should not be put on top and materials should be kept as loose as possible. Climate is also a factor. With a view to minimizing heat loss, larger piles are suitable for cold weather. However, in a warmer climate, the same piles may overheat and in some extreme cases (75 °C and above) catch fire.
Ventilation
Provision of ventilation complements efforts to optimize pile size. Ventilation methods are varied. The simplest method is to punch holes in the pile at several points. The high temperature compost method of Chinese rural composting (Chapter 2) involves inserting a number of bamboo poles deep into the pile and withdrawing them a day later, leaving the pile with ventilation holes. Aeration is improved by supplying more air to the base of the pile where O deficiency occurs most often. In addition to the above-mentioned vertical poles, Ecuador on-farm composting (Chapter 2) uses a lattice of old branches at the base to allow more pile surface to come into contact with the air, and the composting period is reduced to two to three months in warm seasons. This technique is also practised in the rapid composting method developed by the Institute of Biological Sciences (IBS) in the Philippines (Chapter 2), where the platform should be 30 cm above the ground. The passively aerated wind-rows method (Chapter 3) uses a more sophisticated technique. It entails embedding perforated pipes throughout the pile. As the pipe ends are open, air flow is induced and O is supplied to the pile continuously. The aerated static pile method (Chapter 3) takes this aeration system a step further; a blower generates air flow to create negative pressure (suction) in the pile and fresh air is supplied from outside.
Turning
Once the pile is formed and decomposition starts, the only technique for improving aeration is turning. As Table 1 shows, frequency of turning is crucial for composting time. While the Indian Bangalore method requires six to eight months to mature, the Indian Coimbatore method (turning once) reduces the time to four months, and the Chinese rural composting pit method (turning three times) reduces the time to three months. An extreme example is the Berkley rapid composting method which employs daily turning to complete the process in two weeks. In some cases, turning not only distributes air throughout the pile, it also prevents overheating as it kills all the microbes in the pile and terminates decomposition. However, turning too frequently might result in a lower temperature.
Inoculation
While some composters find improved aeration enough for enhanced microbial activities, others may need inoculation of micro-organisms. Inoculum organisms utilized for composting are mainly fungi such as Trichoderma sp. (IBS rapid composting and composting weeds (Chapter 2)) and Pleurotus sp. (composting Coir Pith (Chapter 2) and composting weeds). This publication also features 'effective micro-organisms' (EMs) (EM-based quick compost production process (Chapter 2)). The inoculums are an affordable choice for those with access to the market and also for resource-poor farmers. The production cost could be reduced by using inoculums taken from compost pits (pit method of the Indian Indore method (Chapter 2)), by purchasing the commercial product and multiplying it on the farm (EM-based quick compost production process), and by utilizing native inoculums derived from soils or plant leaves.
Supplemental nutrition
The techniques mentioned above often need to be complemented by the provision of nutrients. One of the most common practices is to add inorganic fertilizers, particularly N, in order to modify a high C:N ratio. Similarly, P is sometimes applied as the C:P ratio of the material mix is also considered important (the ratio should be between 75:1 and 150:1). When micro-organisms are inoculated, they require sugar and amino acids in order to boost their initial activities; molasses is often added for this purpose.
Table 1 Salient features of selected small-scale aerobic composting techniques
Method
Salient features
Duration
Substrate size reduction
Turnings at intervals of (days)
Added aeration provision
Microbial inoculation
Supporting microbial nutrition
Indore pit
+15, +30, +60
Inoculum from old pit
4 months
Indore heap
Shredded
+42, +84
4 months
Chinese pit
+30, +60, +75
Superphosphate
3 months
Chinese high temperature compost
Shredded
+15
Aeration holes in heap through bamboo poles/maize stalks
Superphosphate
2 months
Ecuador on-farm composting
+21
Lattice of old branches/poles at heap base
2-3 months in summer;5-6 months in winter
Berkley rapid composting
Shredded to small size
Daily or alternate day turning
2 weeks with daily turning & 3 weeks with alternate day turning
North Dakota State University hot composting
Shredded
+3 or +4
4-5 holes punched in centre of pile
0.12 kg N per 90 cm dry matter
4-6 weeks
EM-based quick composting
+14, +21
EM
Molasses
4-5 weeks
IBS rapid composting
Shredded
+7, +14, then every 2 weeks
Raised platform ground/perforated bamboo trunks
Trichoderma sp.
3-7 weeks
Shredding
Downsizing, or chopping up the materials, is a sound and widely-practised technique. It increases the surface area available for microbial action and provides better aeration. This technique is particularly effective and necessary for harder materials such as wood.
Other measures
An example of other measures mentioned in this publication is the practice of adding lime. Lime is thought to weaken the lignin structure of the plant materials and enhance the microbial population. However, in some cases, liming is not recommended as the pile may become too alkaline, resulting in significant N loss.
2. Small-scale composting

Traditional methods
Anaerobic composting
Indian Bangalore method
This method of composting was developed at Bangalore in India in 1939 (FAO, 1980). It is recommended where night soil and refuse are used for preparing the compost. The method overcomes many of the disadvantages of the Indore method (below), such as the problem of heap protection from adverse weather, nutrient losses from high winds and strong sun, frequent turning requirements, and fly nuisance. However, the time required for the production of finished compost is much longer. The method is suitable for areas with scanty rainfall.

Pit preparation
Trenches or pits about 1 m deep are dug; the breadth and length of the trenches can vary according to the availability of land and the type of material to be composted. Site selection is as per the Indore method. The trenches should have sloping walls and a floor with a 90-cm slope to prevent waterlogging.
Filling the pit
Organic residues and night soil are put in alternate layers. After filling, the pit is covered with a layer of refuse of 15-20 cm. The materials are allowed to remain in the pit without turning and watering for three months. During this period, the material settles owing to reduction in biomass volume. Additional night soil and refuse are placed on top in alternate layers and plastered or covered with mud or earth to prevent loss of moisture and breeding of flies. After the initial aerobic composting (about eight to ten days), the material undergoes anaerobic decomposition at a very slow rate. It takes about six to eight months to obtain the finished product.
Passive composting of manure piles
Passive composting involves stacking the materials in piles to decompose over a long time with little agitation and management (NRAES, 1992). The process has been used for composting animal wastes. However, the simple placing of manure in a pile does not satisfy the requirements for continuous aerobic composting. Without considerable bedding material, the moisture content of manure exceeds the level that enables an open porous structure to exist in the pile. Little if any air passes through it. Under these circumstances, the anaerobic micro-organisms dominate the degradation. All of the undesirable effects associated with anaerobic degradation occur.
Where a livestock management system relies on bedding to add to livestock comfort and cleanliness, the bedding becomes mixed with the manure and creates a drier, more porous mixture. This provides some structure and, depending on the amount of bedding, enables the mixture to be stacked in true piles. The bedding also tends to raise the C:N ratio of the manure.
Aerobic composting through passive aeration
Indian Coimbatore method
This method (Manickam, 1967) involves digging a pit (360 cm long × 180 cm wide × 90 cm deep) in a shaded area (length can vary according to the volume of waste materials available). Farm wastes such as straw, vegetable refuse, weeds and leaves are spread to a thickness of 15-20 cm. Wet animal dung is spread over this layer to a thickness of 5 cm. Water is sprinkled to moisten the material (50-60 percent of mass). This procedure is repeated until the whole mass reaches a height of 60 cm above ground. It is then plastered with mud, and anaerobic decomposition commences. In four weeks, the mass becomes reduced and the heap flattens. The mud plaster is removed and the entire mass is turned. Aerobic decomposition commences in at this stage. Water is sprinkled to keep the material moist. The compost is ready for use after four months.
Indian Indorepit method
An important advance in the practice of composting was made at Indore in India by Howard in the mid-1920s. The traditional procedure was systematized into a method of composting now known as the Indore method (FAO, 1980).

Raw materials
The raw materials used are mixed plant residues, animal dung and urine, earth, wood ash and water. All organic material wastes available on a farm, such as weeds, stalks, stems, fallen leaves, prunings, chaff and fodder leftovers, are collected and stacked in a pile. Hard woody material such as cotton and pigeon-pea stalks and stubble are first spread on the farm road and crushed under vehicles such as tractors or bullock carts before being piled. Such hard materials should not exceed 10 percent of the total plant residues. Green materials, which are soft and succulent, are allowed to wilt for two to three days in order to remove excess moisture before stacking; they tend to pack closely when stacked in the fresh state. The mixture of different kinds of organic material residues ensures a more efficient decomposition. While stacking, each type of material is spread in layers about 15 cm thick until the heap is about 1.5 m high. The heap is then cut into vertical slices and about 20-25 kg are put under the feet of cattle in the shed as bedding for the night. The next morning, the bedding, along with the dung and urine and urine-earth, is taken to the pits where the composting is to be done.
Pit site and size
The site of the compost pit should be at a level high enough to prevent rainwater from entering in the monsoon season; it should be near the cattle shed and a water source. A temporary shed may be constructed over it to protect the compost from heavy rainfall. The pit should be about 1 m deep, 1.5-2 m wide, and of a suitable length.
Filling the pit
The material brought from the cattle shed is spread in the pit in even layers of 10-15 cm. A slurry made from 4.5 kg of dung, 3.5 kg of urine-earth and 4.5 kg of inoculum from a 15-day-old composting pit is spread on each layer. Sufficient water is sprinkled over the material in the pit to wet it. The pit is filled in this way, layer by layer, and it should not take longer than one week to fill. Care should be taken to avoid compacting the material in any way.
Turning
The material is turned three times while in the pit during the whole period of composting: the first time 15 days after filling the pit; the second after another 15 days; and the third after another month. At each turning, the material is mixed thoroughly and moistened with water.
Indian Indore heap method
Heap site and size
During rainy seasons or in regions with heavy rainfall, the compost may be prepared in heaps above ground and protected by a shed. The pile is about 2 m wide at the base, 1.5 m high and 2 m long. The sides taper so that the top is about 0.5 m narrower than the base. A small bund is sometimes built around the pile to protect it from wind, which tends to dry the heap.
Forming the heap
The heap is usually started with a 20 cm layer of carbonaceous material such as leaves, hay, straw, sawdust, wood chips and chopped corn stalks. This is covered with 10 cm of nitrogenous material such as fresh grass, weeds or garden plant residues, fresh or dry manure or digested sewage sludge. The pattern of 20 cm of carbonaceous material and 10 cm of nitrogenous material is repeated until the pile is 1.5 m high and the material is normally wetted until it feels damp but not soggy. The pile is sometimes covered with soil or hay to retain heat and it is turned at intervals of 6 and 12 weeks. In the Republic of Korea, the heaps are covered with thin plastic sheets to retain heat and prevent insect breeding.
Where materials are in short supply, the alternate layers can be added as they become available. Moreover, all the materials can be mixed together in the pile provided that the proper proportions are maintained. Shredding the material speeds up decomposition considerably. Most materials can be shredded by running a rotary mower over them several times. Where sufficient nitrogenous material is not available, a green manure or leguminous crop such as sun hemp is grown on the fermenting heap by sowing seeds after the first turning. The green matter is then turned in at the time of the second mixing. The process takes about four months to complete.
Tipping organic wastes into a pit; they are spread out into an even layer[FAO]
Chinese rural composting - pit method
Under this method, the composting is generally carried out in a corner of a field in a circular or rectangular pit (FAO, 1980). Rice straw, animal dung (usually pig), aquatic weeds and green manure crops are used. Silt pumped from river beds is often mixed with the crop residues. The pits are filled layer by layer, each layer being 15 cm thick. Usually, the first layer is a green manure crop or water hyacinth, the second layer is a straw mixture (Plate 1) and the third layer is animal dung. These layers are alternated until the pit is full, when a top layer of mud is added. A water layer of about 4 cm deep is maintained on the surface to create anaerobic conditions, which helps to reduce N losses. The approximate quantities of the different residues in terms of tonnes per pit are: river silt 7.5, rice straw 0.15, animal dung 1.0, aquatic plants or green manure 0.75, and superphosphate 0.02. In total, there are three turnings. The first turning is given one month after filling the pit and, at this time, the superphosphate is added and mixed in thoroughly. Water is added as necessary. The second turning is done after another month and the third two weeks later. The material is allowed to decompose for three months and produces about 8 tonnes of compost per pit.



Rapid methods
Aerobic high temperature composting
Chinese rural composting - high temperature method
This form of compost is prepared mainly from night soil, urine, sewage, animal dung, and chopped plant residues at a ratio of 1:4. The materials are heaped in alternate layers starting with chopped plant stalks and followed by human and animal wastes; water is added to an optimal amount.
At the time of making the heap, a number of bamboo poles are inserted for aeration purposes. Once the heap formation is complete, it is sealed with 3 cm of mud plaster. The bamboo poles are withdrawn on the second day of composting, leaving the holes to provide aeration. Within four to five days, the temperature rises to 60-70 °C and the holes are then sealed. The first turning is usually done after two weeks and the moisture is made up with water or animal or human excreta; the turned heap is again sealed with mud. The compost is ready for use within two months.
In some locations, a modified method of high temperature composting is used. The raw materials, crop stalks (30 percent), night soil (30 percent) and silt (30 percent), are mixed with superphosphate at the rate of 20 kg of superphosphate per tonne of organic material. The compost heaps have aerating holes made by inserting bundles of maize stalks instead of bamboo poles.
Ecuador on-farm composting
Under this method, the raw materials utilized for compost making are:
· animal manure: from cows, pigs, poultry, horses, donkeys, ducks, etc.;
· crop residues and weeds: maize, bean, broad bean, groundnut, coffee and weeds;
· agro-industrial wastes, ash and phosphate rock;
· wood cuttings;
· topsoil from the forest or from an uncultivated or sparingly cultivated area;
· freshwater.
The raw materials are put in layers in the following sequence (Figure 2):
· a layer of crop residues (20 cm);
· a layer of topsoil (2 cm);
· a layer of manure (5-10 cm).
Ash or phosphate rock (50 g/m2) is then spread on the surface, and freshwater is sprinkled on the material.




Ecuador heap composting
The above steps are repeated until a height of about 1-1.2 m is reached. It is recommended to begin the heap by constructing a lattice of old branches, and to place two or three woodcuttings vertically along the lattice in order to facilitate ventilation. The heap should be 2 m × 1-1.2 m × 1-1.2 m. Once a week water should be added to the heap. However, too much water could lead to the leaching of nutrients. After three weeks, the heap must be mixed to ensure that all materials reach the centre. During the process, the temperature rises to 60-70 °C, and most weed seeds and pathogens are killed. While it may take about two to three months to prepare the compost in a warm climate, in cold regions it could take five to six months.
Aerobic high temperature composting with inoculation
EM-based quick composting
Effective micro-organisms (EM) consist of common and food-grade aerobic and anaerobic micro-organisms: photosynthetic bacteria, lactobacillus, streptomyces, actinomycetes, yeast, etc. The strains of the micro-organisms are commonly available from microbe banks or from the environment. There are no genetically engineered strains that are in use. Since 1999, seven small-scale organic fertilizer units have been using the EM-based quick production process in Myanmar (FAO, 2002). They are owned and operated by women's income generation groups. A unit consists of nine pits measuring about 180 cm (length) × 120 cm (width) × 90 cm (depth), enclosed by low walls and covered with a roof (Plate 2).
Raw materials
The raw materials for organic fertilizer production are:
· cow dung - 2 portions;
· rice husk - 1 portion;
· rice husk-charcoal - 1 portion;
· rice bran, milled - 1 portion;
· accelerator - 33 litres of EM solution or Trichoderma solution per pit.
Compost pits
Raw materials
The raw materials for compost production are:
· rice straw;
· farmyard manure;
· urea fertilizer;
· EM solution.
Procedure
Straw is stacked in layers of 20 cm height, 1 m width, and 5 m length to form a pile. A unit pile is about 5 m (length) × 1 m (width) × 1 m (height) in size. The pile is sprinkled with water (Plate 4) for adequate moisture content, followed by addition of a manure layer 5 cm high, and the sprinkling of a few handfuls of urea (100-200 g). EM solution, prepared in the same way as described in the Myanmar example, is sprinkled to accelerate aerobic decomposition.

IBS rapid composting
The IBS rapid composting technology (Virginia, 1997) involves inoculating the plant substrates used for composting with cultures of Trichoderma harzianum, a cellulose decomposer fungus. The fungus, grown in a medium of sawdust mixed with the leaves of a leguminous tree called ipil ipil (Leucaena leucocephala), is termed compost fungus activator (CFA). The technology is a development of the wind-row type of composting. Using this procedure, the composting time ranges from 21 to 45 days depending on the plant substrates used.
The procedure consists of two parts: the production of the CFA, and the composting process.
Preparation of substrates
Substrates such as rice straw, weeds and grasses should be chopped. Chopping helps speed up decomposition by increasing the surface area available for microbial action and providing better aeration. Where large quantities of substrates are to be used (i.e. several tonnes), a forage cutter/chopper is needed. Chopping can be dispensed with where the compost is not needed in the near future.
Adjustment of moisture content
Substrates should be moistened with water. Plant substrates can be soaked overnight in a pond, which reduces the need for water. Where a large volume of substrates are to be composted, a sprinkler is more convenient.
The compost mixture
Carbonaceous substrates should be mixed with nitrogenous ones at a ratio of 4:1 or less, but never lower than 1:1 (on a dry weight basis). Some possible combinations are:
· 3 parts rice straw to 1 part ipil ipil;
· 4 parts rice straw to 1 part chicken manure;
· 4 parts grasses to 1 part legume materials + 1 part manure;
· 4 parts grasses to 1 part Chromolaena odorata (a common broad-leaf weed) or Mikania cordata (a herbaceous climbing plant) + 1 part animal manure; it is important to use grasses and weeds that do not have flowers or seeds.
Composting procedure
The substrates should be piled loosely in a compost pen to provide better aeration within the heap. The material should not be too compact and no heavy weights should be placed on top. Compost heaps should be located in shady areas, e.g. under large trees. The platform should be raised about 30 cm from the ground in order to provide adequate aeration at the bottom. Alternatively, aeration can be provided by placing perforated bamboo trunks horizontally and vertically at regular intervals.
The CFA is broadcast onto the substrates during piling. The amount of activator used is usually 1 percent of the total weight of the substrates (i.e. about 1 kg compost activator per 100 kg substrate). Decomposition is faster where the activator is mixed thoroughly with the substrate. A larger amount of activator can be used should faster decomposition be desired.
The heap should be covered over completely. This maintains the heat of decomposition, and minimizes water evaporation and ammonia volatilization. White plastic sheets, or plastic sacks with their seams opened and sewn together, can serve as a cover. The compost heap usually heats up in 24-48 hours.
The temperature should be maintained at 50 °C or higher, and the heap should be turned every five to seven days for the first two weeks, and thereafter once every two weeks. After the first week, the volume of the pile should be reduced by one-third. After two weeks, the volume of the pile should be reduced to one-half the original volume.
The mature compost should be removed from the pen and dried in the sun for two days. It should then be put into sacks and stored in a shaded area. Decomposition should be allowed to continue until the substrate is finely fragmented, so that the finished product has a powdery texture. When decomposition is complete, the compost should be sun-dried again until the moisture content is 10-20 percent.
Where mature compost is needed at once, it should be sun-dried for one day as soon as its temperature drops to 30 °C. Drying removes excess moisture and makes the compost much easier to handle. Although the compost still retains some fibres, it can be applied immediately as fertilizer.
In the large-scale commercial production of compost, the following operations need to be mechanized (other steps remaining the same):
· Chopping of substrates - a forage cutter/chopper could be used.
· Mixing/turning - where there are several tonnes of substrate, a pay loader facilitates mixing of substrates and turning of heaps.
· A hammer mill should be used to break up large lumps of mature compost before drying.
· During rainy months, it is more economical to dry compost mechanically rather than in the sun.
Composting organic materials with high lignin content - coir pith
Coir pith is a waste from the coir industry (TNAU, 1999). This is a major industry that produces coconuts on a large scale. During the process of separating fibre from the coconut husk, a large volume of pith is collected. The pith, containing about 30 percent lignin and 26 percent cellulose, does not degrade rapidly, posing a major disposal problem. However, it can be composted by using the fungus Pleurotus sp. and urea. To compost 1 tonne of coir pith, the materials required are: five spawn bottles (250 g) of Pleurotus sp. and 5 kg of urea
The first step in the compost preparation is to select an elevated shaded place, or to erect a thatched shed. The surface is then levelled and an area 500 cm × 300 cm is marked out. To start with, about 100 kg of coir pith is spread. About 50 g of Pleurotus spawn is spread over this layer. About 100 kg of coir pith is spread on that. On this layer, 1 kg of urea is spread uniformly. The process is repeated until all the pith (1 tonne) is utilized. Water is sprinkled repeatedly so as to maintain the moisture optimum of 50 percent. Well-decomposed black compost is ready in about a month. The C:N ratio falls to about 24:1 and the N content rises from 0.26 to 1.06 percent.
Composting weeds
This method has been developed for composting weeds such as parthenium, water hyacinth (Eichornia crassipes), cyperus (Cyperus rotundus) and cynodon (Cynodon dactylon). The materials required are: 250 g of Trichoderma viride and Pleurotus sajor-caju consortia, and 5 kg of urea. An elevated shaded place is selected, or a thatched shed is erected. An area of 500 cm × 150 cm is marked out. The material to be composted is cut to 10-15 cm in size. About 100 kg of cut material is spread over the marked area. About 50 g of microbial consortia is sprinkled over this layer. About 100 kg of weeds are spread on this layer. One kilogram of urea is sprinkled uniformly over the layer. This process is repeated until the level rises to 1 m. Water is sprinkled as necessary to maintain a moisture level of 50-60 percent. Thereafter, the surface of the heap is covered with a thin layer of soil. The pile requires a thorough turning on the twenty-first day. The compost is ready in about 40 days.
Compost enrichment
Farm compost is poor in P content (0.4-0.8 percent). Addition of P makes the compost more balanced, and supplies nutrient to micro-organisms for their multiplication and faster decomposition. The addition of P also reduces N losses
Large-scale composting
Wind-row composting
Turned wind-rows
Wind-row composting consists of placing the mixture of raw materials in long narrow piles called wind-rows (Plate 7) that are agitated or turned on a regular basis (NRAES, 1992). The turning operation mixes the composting materials and enhances passive aeration. Typically, the wind-rows are from 90 cm high for dense materials such as manures to 360 cm high for light, voluminous materials such as leaves. They vary in width from 300 to 600 cm. The equipment used for turning determines the size, shape and spacing of the wind-rows. Bucket loaders with a long reach can build high wind-rows. Turning machines produce low, wide wind-rows.
Wind-rows aerate primarily by natural or passive air movement (convection and gaseous diffusion). The rate of air exchange depends on the porosity of the wind-row. Therefore, the size of a wind-row that can be aerated effectively is determined by its porosity. A wind-row of leaves can be much larger than a wet wind-row containing manure. Where the wind-row is too large, anaerobic zones occur near its centre. These release odours when the wind-row is turned. On the other hand, small wind-rows lose heat quickly and may not achieve temperatures high enough to evaporate moisture and kill pathogens and weed seeds.
For small- to moderate-scale operations, turning can be accomplished with a front-end loader or a bucket loader on a tractor. The loader lifts the materials from the wind-row and spills them down again, mixing the materials and reforming the mixture into a loose windrow. The loader can exchange material from the bottom of the wind-row with material on the top by forming a new wind-row next to the old one. In order to minimize compaction, this needs to be done without driving onto the wind-row. Wind-rows turned with a bucket loader are often constructed in closely spaced pairs and then combined after the wind-rows shrink in size. Where additional mixing of the materials is desired, a loader can be used in combination with a manure spreader.
Wind-rows on a farm
There are a number of specialized machines for turning wind-rows that reduce the time and labour involved considerably, mix the materials thoroughly, and produce a more uniform compost. Some of these machines attach to farm tractors or front-end loaders, others are self-propelled. A few machines can also load trucks and wagons from the wind-row.
It is very important to maintain a schedule of turning. The frequency of turning depends on the rate of decomposition, the moisture content and porosity of the materials, and the desired composting time. Because the decomposition rate is greatest at the start of the process, the frequency of turning decreases as the wind-row ages. Easily degradable or high N mixes may require daily turnings at the start of the process. As the process continues, the turning frequency can be reduced to a single turning per week.
In the first week of composting, the height of the wind-row diminishes appreciably and by the end of the second week it may be as low as 60 cm. It may be prudent to combine two windrows at this stage and continue the turning schedule as before. Consolidation of wind-rows is a good wintertime practice for retaining the heat generated during composting. This is one of the advantages of wind-row composting. It is a versatile system that can be adjusted to different conditions caused by seasonal changes.
With the wind-row method, the active composting stage generally lasts three to nine weeks depending upon the nature of the materials and the frequency of turning. Eight weeks is usual for manure composting operations. Where three weeks is the goal, the wind-row requires turning once or twice per day during the first week and every three to five days thereafter.


Passively aerated wind-rows
Under the passively aerated wind-row method, air is supplied to the composting materials through perforated pipes embedded in each wind-row, thereby eliminating the need for turning. The pipe ends are open. Air flows into the pipes and through the wind-row because of the chimney effect created as the hot gases rise upward out of the wind-row.
The wind-rows should be 90-120 cm high, built on top of a base of straw, peat moss or finished compost to absorb moisture and insulate the wind-row. The covering layer of peat or compost also insulates the wind-row, discourages flies, and helps to retain moisture, odour and ammonia. The plastic pipe is similar to that used for septic-system leach fields with two rows of 1.27-cm diameter holes drilled in the pipe. In many aerated pile applications, the pipe holes are oriented downward to minimize plugging and allow condensate to drain. However, some researchers recommend that the holes face upwards.
The wind-rows are generally formed by the procedures described for the aerated static pile method. Because the raw materials are not turned after the wind-rows are formed, they must be mixed thoroughly before they are placed in the wind-row. It is important to avoid compaction of materials while constructing the wind-row. Aeration pipes are placed on top of the peat/compost base. When the composting period is completed, the pipes are removed, and the base material is mixed with the compost.
This method has been studied and used in Canada for composting seafood wastes with peat moss, manure slurries with peat moss, and solid manure with straw or wood shavings. Manure from dairy, beef, swine and sheep operations has been used.
Aerated static pile layout



Aerated static pile
The aerated static pile method takes the piped aeration system a step further, using a blower to supply air to the composting materials. The blower provides direct control of the process and allows larger piles. No turning or agitation of the materials occurs once the pile is formed. When the pile has been formed properly and where the air supply is sufficient and the distribution uniform, the active composting period is completed in about three to five weeks.
With the aerated static pile technique, the raw material mixture is piled over a base of wood chips, chopped straw or other very porous material (Figure 3). The porous base material contains a perforated aeration pipe. The pipe is connected to a blower, which either pulls or pushes air through the pile.
The initial height of the piles should be about 150-245 cm high, depending on: material porosity, weather conditions, and the reach of the equipment used to build the pile. Extra height is advantageous in the wintertime as it helps retain heat. It may be necessary to top off the pile with 15 cm of finished compost or bulking agent. The layer of finished compost protects the surface of the pile from drying, insulates it from heat loss, discourages flies, and filters ammonia and potential odours generated within the pile.

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In-vessel composting
In-vessel composting refers to a group of methods that confine the composting materials within a building, container or vessel (NRAES, 1992). In-vessel methods rely on a variety of forced aeration and mechanical turning techniques to accelerate the composting process. Many methods combine techniques from the wind-row and aerated pile methods in an attempt to overcome the deficiencies and exploit the attributes of each method.
There are a variety of in-vessel methods with different combinations of vessels, aeration devices, and turning mechanisms. The methods discussed here have either been used or proposed for farm composting.
Bin composting
Bin composting is perhaps the simplest in-vessel method. The materials are contained by walls and usually a roof. The bin may simply be wooden slatted walls (with or without a roof) (Plate 8), a grain bin, or a bulk storage building. The buildings or bins allow higher stacking of materials and better use of floor space than free-standing piles. Bins can also eliminate weather problems, contain odours, and provide better temperature control.





Rectangular agitated bed composting system
Rectangular agitated beds
The agitated bed system combines controlled aeration with periodic turning. The composting takes place between walls that form long, narrow channels referred to as beds (Figure 4). A rail or channel on top of each wall supports and guides a compost-turning machine.
A loader places raw materials at the front end of the bed. As the turning machine moves forward on the rails, it mixes the compost and discharges the compost behind itself. With each turning, the machine moves the compost a set distance toward the end of the bed. The turning machines work in a similar way to wind-row turners, using rotating paddles or flails to agitate the materials, break up clumps of particles, and maintain porosity. Some machines include a conveyor to move the compost. The machines work automatically without an operator and are controlled with limit switches.
Most commercial systems include a set of aeration pipes or an aeration plenum recessed in the floor of the bed and covered with a screen and/or gravel. Between turnings, aeration is supplied by blowers to aerate and cool the composting materials. As the materials along the length of the bed are at different stages of composting, the bed is divided into different aeration zones along its length. Several blowers are used per bed. Each blower supplies air to one zone of a bed and is controlled individually by a temperature sensor or time clock.
The capacity of the system is dependent on the number and size of the beds. The width of the beds in commercially available systems ranges from about 180 to 600 cm, and bed depths are between about 90 and 300 cm. The beds must conform to the size of the turning machine, and the walls must be especially straight. To protect equipment and control composting conditions, the beds are housed in a building or a greenhouse or, in warm climates, covered by a roof.
The length of a bed and frequency of turning determine the composting period. Where the machine moves the materials 300 cm at each turning and the bed is 30 m long, the composting period is ten days with daily turning. It increases to 20 days where turning occurs every other day. Suggested composting periods for commercial agitated bed systems range from two to four weeks, though a long curing period may be necessary.
Silos
Another in-vessel technique resembles a bottom-unloading silo. Each day an auger removes composted material from the bottom of the silo, and a mixture of raw materials is loaded at the top. The aeration system blows air up from the base of the silo through the composting materials. The exhaust air can be collected at the top of the silo for odour treatment. A typical composting time for this method might be 14 days, so one-fourteenth of the silo volume must be removed and replaced daily. After leaving the silo, the compost is cured, often in a second aerated silo. This system minimizes the area needed for composting because the materials are stacked vertically. However, the stacking also presents compaction, temperature control and air flow challenges. Because materials receive little mixing in the vessel, raw materials must be well mixed when loaded into the silo.
Rotating drums
This system uses a horizontal rotary drum to mix, aerate and move the material through the system. The drum is mounted on large bearings and turned through a bull gear. A drum about 3.35 m in diameter and 36.58 m long has a daily capacity of approximately 50 tonnes with a residence time of three days. In the drum, the composting process starts quickly; and the highly degradable, O-demanding materials are decomposed. Further decomposition of the material is necessary and is accomplished through a second stage of composting, usually in wind-rows or aerated static piles. In some commercial systems, the composting materials spend less than one day in the drum. In this case, the drum serves primarily as a mixing device.
Air is supplied through the discharge end and is incorporated into the material as it tumbles. The air moves in the opposite direction to the material. The compost near the discharge is cooled by the fresh air. In the middle, it receives the warmed air, which encourages the process; and the newly loaded material receives the warmest air to initiate the process.
The drum can be open or partitioned. An open drum moves all the material through continuously in the same sequence as it enters. The speed of rotation of the drum and the inclination of the axis of rotation determine the residence time. A partitioned drum can be used to manage the composting process more closely than the open drum. The drum is divided into two or three chambers by partitions. Each partition contains a transfer box equipped with an operable transfer door. At the end of each day's operation, the transfer door at the discharge end of the drum is opened and the compartment emptied. The other compartments are then opened and transferred in sequence, and finally a new batch is introduced into the first compartment. A sill in place at each of the transfer doors retains 15 percent of the previous charge to act as an inoculum for the succeeding batch. Upon discharge, the compost can go directly into a screen to remove oversized particles, which can be returned to the drum for further composting.
On a smaller scale, composting drums can be adapted from equipment such as concrete mixers, feed mixers, and old cement kilns. Although less sophisticated than commercial models, the functions are the same: mix, aerate, and ensure that the composting process starts rapidly.

Transportable containers
A different type of in-vessel system, relies on a transportable vessel and a central composting facility. A number of local farms participate and provide manure as a raw material. Each farm receives a transportable vessel, which resembles a solid waste roll-off container. In its base, the container has aeration pipes that are connected to a blower. At the farm, the manure and dry amendments are loaded daily into the container and aerated for several days until the container is picked up and delivered to the central facility to finish composting. When the composting container is picked up, the farm is provided with another empty container to continue the cycle. The farm supplies the manure and receives bulking agent, compost and/or revenue in return.
References
Community Sanitation and Recycle Organization (CSARO). Web site: http://www.bigpond.com.kh/users/csaro/
Cracas, P. 2000. Vermicomposting Cuban style. Worm Dig. Iss., 25 - online articles.
FAO. 1980. A manual of rural composting. FAO/UNDP Regional Project RAS/75/004 Field Document No. 15. Rome.
FAO. 2002. Biofertilizer production plant, Myanmar (FAO/UNDP Project), by H. Hiraoka. Back to Office Report. Bangkok, FAO-RAP.
(: Web http://www.bigpond.com.kh/users/csaro/),