Growing concerns relating to land degradation, the inappropriate use of inorganic fertilizers, atmospheric pollution, soil health, soil biodiversity and sanitation have rekindled global interest in organic recycling practices such as composting. The potential of composting to turn on-farm waste materials into a farm resource makes it an attractive proposition. Composting offers benefits such as enhanced soil fertility and soil health that engender increased agricultural productivity, improved soil biodiversity, reduced ecological risks and a better environment. However, many farmers, and especially those in developing countries find themselves at a disadvantage as they fail to make the best use of organic recycling opportunities. These farmers work under various constraints relating to: a lack of knowledge on efficient expeditious technology; long time spans; intense labour, land and investment requirements; and economic factors.
As there is an extensive literature on composting methodology, this review presents only a selective and brief account of the salient approaches. It makes a broad distinction between small-scale and large-scale composting practices. While small-scale production systems normally employ infrastructure and techniques that are technically and financially more feasible to farmers, large-scale systems require investment for containers and/or turning, as well as greater knowledge and skills to control the process. Therefore, the former may serve individual small-scale composters as technology packages that are fine-tuned to suit specific circumstances, and the latter as a means to meet quantum requirements of an individual or group of individuals.
The review also makes a distinction between traditional and rapid composting practices. The distinction is based mainly on the difference between those practices adopted as a convention and recent introductions for expediting the process that entail individual or combined application of treatments such as shredding and frequent turning, mineral nitrogen compounds, effective microorganisms, use of worms, cellulolytic organisms, forced aeration and mechanical turnings.
Traditional methods generally adopt an approach based on anaerobic decomposition or one based on aerobic decomposition using passive aeration through measures such as little and infrequent turnings or static aeration provisions such as perforated poles/pipes. These processes take several months. On the other hand, using the recently developed techniques mentioned above, rapid methods expedite the aerobic decomposition process and reduce the composting period to about four to five weeks. Most of these methods include a high temperature period and this adds further value to the product by eliminating pathogens and weed seeds.
Traditional methods based on passive composting involve stacking the material in piles or pits to decompose over a long period with little agitation and management. Using this approach, the Indian Bangalore method permits anaerobic decomposition for a larger part of operations and requires six to eight months to produce compost. The method is mainly used to treat urban wastes in the developing world. A similar method employed on large farms in the Western Hemisphere is passive composting of manure piles. The active composting period in this process may take one to two years.
The Indian Indore methods enhance passive aeration slightly through a few turnings, thereby permitting aerobic decomposition and enabling production in a time span of about four months.
The Chinese rural composting pit method uses a passive aeration approach through turnings to provide output in two to three months. The above methods are in widespread use in the developing world. Although the labour requirements for these methods are high, they are not capital intensive and do not require sophisticated infrastructure and machinery. Small farmers find them easy to practice, especially where manual labour is not a constraint. However, the low turnover and longer time span are the major drawbacks of these methods.
Rapid methods such as Berkley rapid composting and North Dakota State University hot composting involve accelerated aerobic decomposition using a range of measures: chopping raw materials into small pieces; using mineral compounds such as ammonium sulphate, chicken manure, and urine; and turning the material on a daily basis. While chopping without much mechanical support may be possible on a small scales, mechanization may be necessary for large-scale applications. While the Berkley rapid composting method claims an active composting period of two to three weeks owing to its extremely frequent turning, the North Dakota State University hot composting method may take four to six weeks.
The EM-based quick composting process involves aerobic decomposition of rice husk/bran, rice straw and cow dung as raw materials in pits or on a flat surface; and uses effective microorganisms (EMs) as activator to expedite the decomposition process. The use of EMs as activator reduces the composting period from 12 to 4 weeks. An example of a method based on cellulolytic culture is the rapid composting approach developed by the Institute of Biological Sciences in the Philippines. Its salient features include: chopping of vegetative organic materials, stacking of materials in wind-rows, passive aeration through air ducts, and the use of cellulose decomposing fungus (Trichoderma harzianum). The process takes about four weeks.
Turned wind-rows have been in use on large farms for some time, especially in the developed world. The wind-rows are turned periodically using a bucket loader or special turning machine. The turning operation mixes the composting materials, enhances passive aeration and provides congenial conditions for aerobic decomposition. Composting operations may take up to eight weeks. Passively aerated wind-rows eliminate the need for turning by providing air to the materials via pipes, which serve as air ducts. The active composting period lasts 10-12 weeks.
Mechanical forced aeration methods such as the aerated static pile approach reduce composting time significantly, allow for higher, broader piles, and have lower land requirements compared with turned wind-row and passively aerated wind-row methods. However, there is little experience of using aerated static piles with agricultural wastes. The technology is commonly used for treating municipal sewage sludge. The active composting period may range from three to five weeks.
Mechanical forced aeration and accelerated mechanical turning methods such as in-vessel composting are specially-designed commercial systems whose potential advantages include: reduced labour, weatherproofing, effective process control, faster composting, reduced land requirements, and quality output. Among these systems, bin composting and rectangular agitated beds have become established on some large farms in the developed world. Bin composting involves: provision for forced aeration in the bin floor; little turning of the composting material; and the transfer of material from one bin to another. Although the initial high investment and recurring operation and maintenance costs involved in bin composting could limit its adoption, there are practices such as passively aerated bin composting of municipal waste (in Phnom Penh) that are technically and financially affordable for the developing world.
In addition, there is another recently introduced approach called vermicomposting. Vermicomposting is not composting as such because it is not the decomposition of organic materials by micro-organisms, but enzymatic degradation through the digestive system of earthworms. It is the casts of the worms that are utilized. Vermicomposting results in high-quality compost and does not require physical turning of the material. In order to maintain aerobic conditions and limit the temperature rise, the bed or pile of materials needs to be of limited size. Temperatures need to be regulated to favour the growth and activity of worms. However, it has a lower turnover than other rapid methods and the composting process takes 6-12 weeks.
In some circumstances, a combination of aerobic decomposition, anaerobic decomposition and vermicomposting may be useful for the more effective production of high-quality compost. Integrating traditional composting and vermicomposting is one such example. In this approach, while the high temperature ensures better quality through the destruction of pathogens and weed seeds, worms perform the roles of turning and maintaining an aerobic condition, thereby reducing the need for investment and labour.