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<Municipal Solid Waste Management> Composting 1.4.2 Sound technical options Composting in industrialized, transition, and developing countries The process of composting is similar in all areas of the world, but there are practical differences in industrialized, transition, and developing countries. The main differences relate to the waste stream to be composted, the agricultural traditions relating to production and use of compost, and the physical infrastructure of the built and natural environment. In relation to composting, transition countries are more like industrialized countries in relation to their infrastructure, but more like developing countries in relation to their waste streams. Siting and scale of composting General siting considerations
Backyard composting
Decentralized neighborhood-, block-, or business-scale composing Sound practice for siting neighborhood composting sites requires that they:
For neighborhood compost piles to work, there must usually be a compost monitor or supervisor within the user community who takes responsibility for maintaining order and cleanliness. Sound practice generally requires backup from the municipal government in terms of technical and logistical support for removal of undesired items or turning of the piles. Decentralized composting on the village and community scale Siting is important, and sound practice requires that the compost operation follow the siting guidelines listed above for neighborhood composting. At this scale, it may also be necessary for the site to accommodate more turning, processing, screening, and storage of the compost than at smaller scales. Centralized composting at the municipal scale Municipal-scale composting plants receive wastes from a single jurisdiction, usually a city, sometimes including associated suburbs or squatter settlements. Municipal-scale centralized plants are distinguished from regional facilities by differences in scale, management, financing, and siting. At this scale, sound practice for siting of the compost facility in industrialized countries must usually be a formal process that includes:
Centralized composting at the regional scale
Composting at landfill and incinerator sites Sound practice here differs in industrialized and developing countries. In industrialized countries, sound practice will usually require that the composting operations be separate from the landfill, have their own scale or separate entrance, and that the resulting compost be split between low-quality product used in landfill operations as daily and final cover, and high-quality for other uses. In developing countries, where the waste stream has a sufficiently high proportion of organic materials, wastes may be left to decompose at the landfill or dump. In such cases of Ònatural composting,Ó sound practice requires a clear decision about the role of decomposition processes in landfill management, including a decision whether or not to remove the top layers of material once they are partially decomposed, either for further composting or for use in agriculture, and whether to allow farmers to remove compost from the landfill or dump. Waste stream and separation protocols Waste stream of industrialized, transition, and developing countries People in industrialized and transition countries are also more likely to keep their yard wastes separate, while in developing countries these are likely to be mixed with other household wastes. Yard waste composting Sound practice in preprocessing of yard wastes, if required at all, is usually limited to shredding or chipping of larger woody wastes. For most yard wastes, sound practice dictates composting in active windrows (large, elongated piles which are watered and turned to maintain adequate levels of oxygen and moisture). In temperate climates, collection and composting of the annual leaf fall from deciduous trees and bushes represents a significant logistical and operational task for municipal governments. Sound practice, as in any composting operation, integrates the set-out and collection requirements with the composting process, to make sure that what gets collected can be composted. For example, after a number of frustrated attempts to achieve leaf composting with plastic bags, many North American municipal governments now require leaves from the autumn leaf fall to be set out in compostable paper bags. Kitchen waste and source-separated organics Depending on culinary traditions, the carbon-nitrogen ratio of kitchen wastes may be ideal for composting, or it may be too high in nitrogen (protein, from meats, dairy products, and certain green vegetables). Too high a protein content can complicate the composting activity and lead to unpleasant odors. One sound practice to minimize this is to restrict composting to only vegetable materials. While increasing the ease of management, this decreases the fertilizer value of the resulting compost. The addition of yard waste to kitchen waste streams generally increases the carbon level and decreases the moisture content, improving compostability. European biowaste composting. Northern European countries, in particular Denmark, Germany, and the Netherlands, have developed a particularly sound practice in their system approach to composting of separate kitchen and yard waste, which they call Òbiowaste.Ó This practice entails the use of a modular in-vessel composting system, followed by a period of composting, either in (a) aerated static piles (piles that are not turned, but that instead use forced aeration to bring oxygen to the waste) or (b) active windrows. In either case, several months of curing are needed prior to processing for market. The finished biowaste compost can be sold for a mix of agricultural, horticultural, and civil engineering uses, ranging from application on farmland to highway berms. Kitchen waste composting versus animal feeding in a wastemanagement system. There are many viable systems to feed kitchen waste to animals or to collect it for livestock feeding. In terms of the waste management hierarchy, this represents a higher use of kitchen wastes than composting, as more of the nutrient value is productively used. (There are, however, considerable health risks in feeding wastes to animals.) Whenever a compost system is being planned, it is important to evaluate the extent to which compostables are already being diverted to animal feed. Municipal authorities are sometimes unaware of these processes. If people need their kitchen wastes for animals they are unlikely to cooperate with centralized composting systems. In developing countries, disruption or replacement of animal feeding systems with composting does not usually represent sound practice. Wet waste Mixed solid waste Wastewater sludge and human fecal matter Anaerobic digestion of these materials can also work, and is a sound practice on farms in industrialized countries. It is operationally more difficult than aerobic composting. Alternatively, sludges can be left to dry into cakes which may be used as fertilizer (but see cautions, below). Sound practice in handling these materials requires adequate attention to the health and safety of the workers and surrounding environment. The use of shoes and gloves, and the provision of facilities for washing after work, are important elements of sound practice. Government guidelines for safe composting of these materials often requires that each batch of compost be allowed to ÒcookÓ at temperatures of 50Ð55¡C for three or more days. The main problem with composting of wastewater sludge is that the wastewater usually includes both industrial and residential discharges. In most countries, wastewater from urban sources may include metals and contaminants that affect the quality and safety of the resulting compost. This can result in a situation where the compost is too contaminated and must be landfilled, at a high cost. Sound practice in composting these materials is almost always dependent on the presence of an industrial wastewater pre-treatment regimen, and on careful monitoring of the compost. Manures and animal wastes Separation and collection systems In most developing countries, lack of economic and environmental motivation means that it will be very difficult, if not impossible, to explicitly promote source separation of compostable materials. Nevertheless, since the success of composting systems and the quality of the compost depend heavily on the materials that are composted, a separate collection system for compostables would facilitate the production of high-quality compost. Community collection bins for compostables are one possibility and may, under certain circumstances, be easier to implement than household collection of compostables. To date, success is reported primarily for the collection of organic wastes from large food markets. Support for or enhancement of existing materials recovery systems, both formal and informal, will enhance the compostability of the urban waste streams. Sound compost technologies and technical options All composting processes consist of bacteriological decomposition of the waste materials. The differences in the systems described here relate to the management of that biological process, to its speed and temperature, and to the strategies for controlling moisture and aeration, and not to the decomposition process itself. Technical aspects of the composting process Duration of composting. Composting is completed when the compostable mate-rials have been completely converted to humus. Stability of compost can be tested by re-wetting the material and observing if it heats up again, which indicates that there are still uncomposted materials in the pile. Most aerobic composting systems include a period of active composting, generally from 21 to 60 days, and a period of curing, generally from 6 to 24 months. Composting can be accelerated by intensive aeration and inoculation of the piles with suitable bacteria. More land is required when the period of composting is longer because the throughput of wastes is slower. In places where land for siting is scarce, sound practice may entail selection of more intensive management practices instead of more extensive land use. Small-scale composting of animal wastes Small-scale aerobic composting of animal wastes, including manures, hide scrapings, and tannery and slaughterhouse wastes can also produce fertilizers, but carry some pathogenic risks. All of these types of small enterprises generate leachate and associated bad odors, and are typically associated with poor working conditions and risks to worker health, but may be profitable and provide subsistence income. Sound practice could include introducing technical and health improvements, rather than eradicating the activities themselves. Coordinated backyard compost systems In recent years, a number of governments in industrialized countries have treated backyard composting as a means of waste reduction, since the materials which are composted remain at home and do not enter the municipal waste stream. Sound practice in organized backyard composting systems includes the following:
Such backyard composting and mulching programs, which have operated successfully in Northern Europe, North America, Australia, and New Zealand, are much less costly to a community than centralized compostable collection programs. They have participation rates approaching 30%, with significant results in terms of wastes diverted from the municipal waste stream. Particularly in developing countries, rodents and vectors are a concern in cities with high pest populations. Consequently, municipal health officers frequently advise against backyard composting, and in some places it is prohibited by the health code. The screens on vermicompost pits in Bangalore to keep out rodents are a visible reminder to the public of the need for careful management of backyard and neighborhood composting. Pre-processing Pre-processing consists of three separate types of operations:
Mechanical pre-processing is often the most costly part of a composting system, as well as the most vulnerable to breakdown. For this reason, sound practice in composting involves minimizing pre-processing to the greatest extent possible by pre-selecting the waste streams to be composted through source separation and separate collection. Windrow and active pile systems
Active pile systems require manual or mechanical turning of the windrows, with crews using shovels or rakes, or with equipment such as a bulldozer, tractor, or windrow turning machine. Turning aerates the piles, blends the materials, brings about additional size reduction, and prevents excessive buildup of temperature to the point of spontaneous combustion. In developing countries, waste pickers may be allowed to work over the windrows to remove recyclables and pieces of wood, as was done in a windrow system in Kathmandu in the early 1990s. An active pile system:
Windrow turning machines. Specially designed windrow turning machines have been developed in the US, Asia, and Europe. These vary in size from a wildcat tractor attachment to the very large scarab-type turner, which straddles the windrows. The ones built in India are low cost and work effectively with the waste stream. They may provide an opportunity for South-South technology transfer, although the problems associated with imported technology persist: the need to train repair persons and to obtain spare parts. Windrow turning machines allow for production of a more uniform compost. They decrease labor costs but increase the capital costs of active pile systems. They may increase land requirements, as the design of turning machines limits the size of the piles and imposes pile spacing requirements. Compared to bulldozers, however, specialized windrow turning machines are more effective in aerating windrows and may therefore be a cost-effective alternative.
Static pile systems In static pile composting systems, the windrows are not turned. Instead, they are aerated continuously or periodically using blown air systems. Static piles typically require a site with aeration channels built into the pad on which the piles sit. Piles are built over this channel, and a network of perforated piping is introduced during placement into piles of the materials to be composted. During composting, air is blown or drawn by pipe systems driven by electric or gas motors through the static piles to provide aeration. In-vessel systems Almost all in-vessel systems require a residence time (time physically in the vessel) of 3-30 days, followed by a period of 21 to 180 days of active composting in an active or static pile. Once the active composting is completed, the material is stored in piles or windrows for curing for up to two years. In-vessel systems offer protection from weather conditions, better odor control, and shorter periods of active processing, but they are expensive to build and operate. Their status as a sound practice for developing countries is open to question, especially since equipment and parts typically have to be imported and paid for with foreign exchange. They may represent a sound practice for industrialized or transition countries, or for industrialized areas of developing countries, for specific, at least partially source-separated waste streams and as part of an integrated waste management strategy. Common variations on in-vessel systems include: Modular in-vessel systems. Modular in-vessel systems represent the best practice in most cases where in-vessel composting is desired. These systems have a series of smaller vessels or divisions within the vessel. The modules can generally be purchased separately or added on to the system later. They are also set up to compost more than one waste stream at a time. Two variations on modular systems represent particularly sound practice. The agitated bay system uses a vessel divided into a series of bays or stalls, which are turned by an overhead agitator. Residence time in an agitated bay system is usually 10-21 days. The second type of modular system uses multiple closed boxes, each up to 20 meters in length. From the outside, they resemble trailer bodies. Inside, the walls contain a forced aeration and temperature control system. Residence time in the vessels ranges from 5 to 30 days, depending on the available land for final composting or curing. Modular systems have moderate capital costs and low-to-moderate operating costs. They control odor and leachate very well. Modular systems are a sound approach in densely populated areas where siting is difficult and land is scarce.
Drum or "Dano-type" systems. In drum systems, compostables are introduced into a rotating horizontal drum for a relatively short residence time, followed by a long period of active-pile composting. These systems are called Dano-type systems after the original Danish design, now no longer protected by patent. The large metal drums are up to 30 meters long and may be divided into separate chambers. Some have trommel screens in the first chamber to remove designated materials during composting. Dano-type systems require a large amount of land for the active pile composting. Tower systems. In tower systems, the compostable material is introduced into a vertical tower and composted under forced aeration. Some tower systems also mechanically turn or agitate the material during its residence. Residence time in tower systems is typically 2-5 weeks, and composting is essentially complete when the material is removed to curing piles. Tower-type systems offer more odor control during composting and require much less land, since the period of active composting takes place in the vessel. They represent a particularly sound practice for sludge composting or co-composting of sludge and yard wastes. Field composting and using compost from dumps The best known example of garbage farming is at CalcuttaÕs Dhapa dump, where the municipality leases out dump land for vegetable farming. The combination of dirt, dust, organics, human and animal feces, and ash in CalcuttaÕs garbage produces a fertile growing medium that requires no additives; the dump is in a wetland and there are numerous ponds between the ridges of garbage that provide water. At BeijingÕs main dump, the authority has provided sifting machines to encourage farmers to remove compost and thereby extend the life of the dump. In Yangon, Myanmar, the City Development Corporation allows small enterprises to mine the oldest inner-city dump (now closed) for metals (materiel dating from World War II) and screen the compost. In this way they hope to gradually remove the hill of compost so that land can be available for redevelopment. The remaining dump land is farmed in the growing season. These largely undocumented and informal practices in many places throughout the developing world use valuable organic matter and help in waste reduction. However, they carry a risk of bacterial, glass, or chemical contamination which can present health hazards during the work of gardening or in the consumption of the crops in some places. At old dumps in cities with low levels of industry, the subsurface compost probably contains few heavy metals and would likely pass tests for use in farming. If land, compost, and crops are monitored and judged safe for use, these traditions of natural composting and garbage farming could be regarded as sound practices for many places. Given uncertainties about quality and contamination, these practices can be endorsed as sound practice if soil and product testing is done regularly, or in places like Yangon where there is little industry. Sound practice over the long term will necessitate improving the practices themselves, rather than supplanting them and displacing their beneficiaries.
There is a need for assistance to developing countries in adequate testing and long-term monitoring, and in advice on crops that may be safely grown on old garbage dumps or on composted garbage removed from them. Sound practice improvements to garbage farming include:
Promising innovations The use of vermicomposting in centralized or village-scale composting systems is currently being explored in pilot projects. Considerable work was done in Manila in the 1970s but the markets for the resulting worm castings did not develop. Vermiculture can be carried out by small-scale enterprises, in a cottage-industry manner. Worms are easily affected by impuri- ties, so the organic wastes should be source-separated domestic wastes, or from markets. Vermiculture produces a superior fertilizer-type product. However, there is not yet enough information to indicate whether sufficient markets exist to absorb worm castings on a scale that would significantly contribute to municipal waste reduction. Vermiculture does not necessarily kill all pathogens. In particular, some viruses and parasites can survive the process. Therefore, if the input materials present a high risk of containing pathogens, the finished product could still contain pathogens. This may be of particular concern in developing countries, where wastes used in vermicomposting may not be source-separated. Anaerobic digestion. Anaerobic digestion is an approach to composting which shows promise as a sound practice in industrialized countries. A number of facilities in France and Belgium appear to be capable of composting mixed waste under pressure and are recovering both compost and methane gas. Most anaerobic systems include pre- processing similar to that in centralized aerobic systems, followed by conveyance of the compostables to a large pressurized tank or vessel. Water must usually be added as anaerobic bacteria generally require a liquid or semi-liquid environment. Sound marketing approaches Marketing of compost While sound practice in composting systems depends heavily on marketing of compost, this does not necessarily require that composting make a profit. Marketing may include any type of beneficial use, ranging from use in roads and parks to give-away programs for residents. Market development To date, Europe is the only area of the world with clear and enforceable compost quality standards, and with a history of government intervention to stimulate and support compost markets. The European emphasis on compost marketability has led its development from mixed waste composting to composting of source-separated biowaste. This has had an enormous effect on compost quality far larger than policy makers imagined.
Government stimulation of the compost market. Government action to stimulate the market has been significant and has included:
The French Association for Energy and Environment, now called ADEME (formerly ANRED) provides nearly a textbook example of how government-supported intervention essentially created composting practice in France. The program was based on three concepts:
Compost and land application standards. The second critical element in sound marketing approaches is the establishment of compost and land application standards and guidelines. The European Union is far ahead of all other countries and regions in its compost standards program, which identifies three grades of compost ranging from acceptable to very high quality based on levels of heavy metals. Land application standards connect the compost itself to its use in agriculture and allow or limit compost use on various crops. It is essential to give farmers confidence that compost is safe for use, and that they will not be penalized at a later date. Compost use in the public sector. The successful development of compost systems to date has almost always involved heavy participation of the public sector in using compost in public works projects. This is a win-win strategy, as it tends to reduce costs and increase effectiveness of public works expenditures while illustrating the value of compost for greening and landscaping. Its effect is enhanced by signs that bring the use of compost to the attention of the public. Compost as daily or final landfill cover. Another use of dump compost is as cover for fresh deposits of MSW, in cases where other cover material is scarce or expensive to transport to the dump. Landfill cover is an excellent use of lower quality compost or compost with large particles in it. The use of compost as cover for landfills has been important in some places, since the compost substitutes for soil that would otherwise have to be excavated, purchased, and transported. Compost quality control The first element in quality control is controlling the inputs to the composting process. In industrialized countries, quality control rests upon clear and enforceable source separation protocols, combined with sound collection practice and pre-processing. In developing countries, where source separation is unlikely, manual pre-processing and post- processing is required to ensure the quality of the compost produced. Market wastes make particularly good input for composting operations. Sound management of the composting process itself is also essential to ensuring the nutrient value of the compost. Process control, moisture control, aeration, and temperature all contribute to the biological processes.
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