Sustainable management of solid waste is a major challenge being faced by municipal authorities across the world, both in the North and the South. In developing countries, urban waste remains a serious problem that causes contamination of soil and water bodies and endangers human health and the environment. Much of the solid waste consists of organic matter that can be recycled into a profitable input (compost) for urban agriculture. Composting the large quantities of organic matter provides a win-win strategy by reducing waste flows, enhancing soil properties, recycling valuable soil nutrients and creating livelihoods, but there remain several constraints that explain why this opportunity is seldom exploited. This chapter discusses the benefits of constraints to composting and presents a framework for analysis and planning of composting interventions. The arguments and models contained in the chapter are supported with case study material from Ghana, Philippines and Kenya.

Recycling of Urban Organic Waste for Urban Agriculture

Olufunke Cofie
A. Adam-Bradford
Pay Drechsel

The Urban Waste Challenge

The accelerated growth of the global urban population implies an increasing demand for public services. Yet, urban centres in developing countries are unable to meet such demand – services such as sanitation are poor or inadequate to cope with the increasing rates of urbanisation and the associated higher standards of living. According to the UN 2002 Human Development Report, 2.4 billion people in the developing world lack access to basic sanitation. In Africa, Asia and Latin America, the sustainable management of waste is a major challenge for municipal authorities. Waste is a product or material that does not have a value anymore for the first user and is therefore thrown away; however, it could have value for another person in a different circumstance or even in a different culture (van de Klundert and Anschutz, 2001). Municipal authorities have insufficient financial, technical, and institutional capacities to collect, transport, and safely treat and dispose of municipal wastes, consequently waste management remains one of the major urban problems (Drechsel and Kunze, 2001). In Ghana for example, 58 percent of the solid waste (SW) generated is dumped by households in designated dumping sites, 25 percent is dumped elsewhere in non-designated sites, and only 5 percent is actually collected. The quantity uncollected varies from place to place and could be as high as 20 percent as in the two largest cities of Accra and Kumasi. (GSS, 2000). The situation in other African cities is hardly different. In many cities household waste collection is restricted to wealthy neighbourhoods, while in the remaining areas waste is dumped along road sides, in illegal dumps and in storm water drains (Mbuyi, 1989). The city authorities in Tanzania collect only 24 percent of the refuse (Kulaba, 1989) while in Nigeria, 35 percent of Ibadan's households, 33 percent of Kaduna's, and 44 percent of Enugu's do not have access to waste collection. (Asomani-Boateng and Haight). In Ougadougou, Burkina Faso, about 23 percent of household wastes are deposited in small drains (Ousseynou, 2000). In India, about 50 percent of the refuse generated is collected. As much as 90 percent of the Municipal Solid Waste (MSW) collected in Asian cities end up in open dumps. (Medina, 2002). The failure of city authorities to collect waste leads to unpleasant conditions and decomposing wastes constitute a serious health and environmental hazard (Ali, 2004)

Block-built triple chamber compost bin being used at Apeadu Junior Secondary School, Kumasi

Urban waste could be solid or liquid, organic or inorganic, recyclable or non-recyclable. A considerable quantity of urban waste is biodegradable and hence of immediate interest in recycling (see Box 8.1).

Very large quantities of SW are generated in urban areas; the average SW generation is 0.6 kg per person per day. Based on the composition of solid waste of cities of low- and middle income countries (from Algiers, Alexandria, Cairo, Sao Paolo, Obeng and Wright, 1987), easily bio-degradable fractions range between 44 percent and 87 percent in weight (see Figure 8.1). Similar ranges (40-85 percent) are also reported by Cointreau et al. (1985) for low-income countries. Levels of urbanisation and modernisation have a profound effect on the production and composition of municipal waste; however, some general trends such as the high content of organic matter (50-90 percent) provide an opportunity for exploitation through composting processes (Allison et al., 1998; Asomani-Boateng and Haight, 1999). The percentages of organic matter in municipal solid waste in selected African cities were recorded as 56 percent in Ibadan, 75 percent in Kampala, 85 percent in Accra, 94 percent in Kigali and 51 percent in Nairobi (Asomani-Boateng and Haight, 1999). The volume and composition may however be subject to large seasonal variations (GFA-Umwelt, 1999). A detailed report on the organic waste flow in integrated sustainable waste management has been written by Dulac (2001). In short, the waste stream is not a homogenous mass but a collection of different materials (organic material, plastics, metal, textiles etc.) that can be handled in different ways to maximise recovery. The organic waste fraction remains the largest proportion to be recovered.

Figure 8.1 Solid Waste characteristics in selected cities (Drawn using data from: Hughes, 1986; Obeng and Wright, 1987; WASTE 1997; Zurbrügg, 2003; Ali, 2004)

Urban Waste Management Strategy

Many approaches to waste management exist. Generally, solid waste is managed through landfills, incineration and recycling or reuse. However in developing countries, properly engineered landfills are not common while the cost of modern incineration is too exorbitant to bear. Hence, the most common method of waste disposal is some form of landfill, including variants such as uncontrolled dumping in undefined areas, collection and disposal on unmanaged open dumps, collection/disposal on controlled dumpsites (UNEP, 2004). It is common to find scavengers moving from door to door or sorting through communal bins to pick dry recyclable materials. However, these pickers are more interested in inorganic recyclable materials such as plastics and glass, but not in organic wastes.

Agenda 21, adopted in Rio in 1992, states that environmentally sound waste management should include safer disposal or recovery of waste and changes to a more sustainable pattern introducing integrated life cycle management concepts (UNEP, 2004). It introduced a stepwise approach to waste management in order of environmental priority. The general principle of the waste management hierarchy consists of the following steps:

• Minimising wastes;
   
• Maximising environmentally sound waste reuse and recycling;
   
• Promoting environmentally sound waste disposal and treatment;
   
• Extending waste service coverage.

After Rio most countries have generally accepted this hierarchy as a strategy towards an environmentally sound waste management system. In the last ten years the concept of Integrated Waste Management (IWM) has evolved and is slowly becoming accepted by decision makers (UNEP 2004). IWM relies on a number of approaches to manage waste, including all aspects of waste management, from generation to disposal, and all stages in between with proper consideration of technical, cultural, social, economic and environmental factors. Resource recovery is critical and is embedded in this strategy.

Recycling of Urban Organic Waste

Current urban organic waste recycling practices include the following:

• The use of fresh waste from vegetable markets, restaurants and hotels, as well as food processing industries as feed for urban livestock (Allison et al. 1998);
   
• Direct application of solid waste on and into the soil;
   
• Mining of old waste dumps for application as fertiliser on farmland (Lardinois and van de Klundert, 1993);
   
• Application of animal manure such as poultry/pig manure and cow dung;
   
• Direct application or human excreta or bio-solids to the soil (Cofie et al., 2005)
   
• Organised composting of SW or co-composting of SW with animal manure or human excreta.

Whichever method is used, a process of microbial degradation releases the useful nutrients in organic waste for soil improvement and plant growth. Composting is the process of decomposing or breaking down organic waste materials (by micro-organisms such as bacteria, protozoans, fungi, invertebrates) into a valuable resource called compost. Composting is done at different scales (large, medium, small) by various people (municipalities, NGOs, communities, individuals) and for various purposes (gardening, landscaping, farming) in the urban areas. In the 1970s, large scale centralised composting was prominent especially in the Western world. However, this has proved to be a failure (Onibokun, 1999). The collection and transportation of organic waste to centrally managed sites is expensive, time consuming and energy intensive; these processes are also dependent on fossil fuel inputs that are often heavily subsidised in order to enable maintenance of fuel inputs, therefore extending economic inefficiency at the macro-level. In situations where funding is secured from donor agencies, the conditions accompanying such funds are often disincentives to good practice. Technological know-how on financial analysis, engineering design of composting facilities and transport schedule modelling has been very limited in developing countries (Cointreau-Levine, 1997). In addition, technological transfers of composting processes and equipment from developed countries were often done in the past without considering local constraints (Hoornweg et al., 1999; Etuah-Jackson et al., 2001) and the technologies transferred were often not applicable in the receiving country. Also comprehensively planned composting stations, based on a demand-supply analysis, are not common. In fact, waste management authorities in many developing countries hardly have the "luxury" of planning for recycling; instead they focus their limited resources on the priority needs of "waste collection" and "safe disposal" which consume an immense share of the municipal budgets in low-income countries as cost recovery is low (Drechsel et al., 2004). The irony is that if well planned, the costs of waste disposal could be reduced through composting. However, what appears to be a logical win-win- situation for city authorities and farmers, is seldom a reality in the developing world (see the case study by Duran et al on Marilao, Philippines, for an example of an innovative win-win solution). This is due to several factors such as lack of affordable equipment, technical personnel, frequent mechanical breakdowns, and financial restrictions (Drechsel et al., 2004; Asomani-Boateng et al., 1996).

In the 1990s, small to medium scale decentralised composting based initiatives evolved (eg. see GFA-Umwelt, 1999). However, a transition from centralised composting to decentralised composting approaches is often compounded by the lack of inter-sectoral planning (waste/planning/agriculture) in waste management. Ecological approaches to waste management have only been adopted where predominant conventional waste management approaches are not challenged. Consequently, small-scale decentralised approaches are yet to receive extensive government support at national levels. Cuba is a marked exception to this general pattern in urban planning and management. In the very different geopolitical and social conditions of Havana, Cuba, substantial progress has been made in recycling urban organic waste, as nutrient recycling principles have been implemented in practice and have proven to be very successful (Cruz and Medina, 2003; Díaz and Harris, 2005; Viljoen and Howe, 2005). But generally on a global scale, at the lowest intervention level, backyard composting is practised by few individuals.

Waste ready for collection

By far, the better composting options are those that are decentralised and use organic waste as close to the source as possible. Decentralised on-site (for commercial organic waste) and on-plot (for domestic organic waste) are the preferred levels of intervention with each individual intervention requiring the appropriate technology at the appropriate scale In essence, the primary function is all about getting the nutrients and organic matter in waste back into the soil in the most efficient and effective manner; hence the priority order of backyard composting (household) and decentralised (community) approaches (see Figure 8.2). Centralised municipal approaches do not have a good track record and the potential scale-of-economy advantages have not materialised due to operational and marketing constraints.

Figure 8.2 Composting scales of intervention

Use of Urban Organic Waste for Urban Agriculture

The provision of sufficient food and the provision of basic sanitation services, two major challenges in (mega-)cities, are inter-linked as the urban food supply contributes significantly to the generation of urban waste (Drechsel and Kunze, 2001). In principle, therefore, recycling organic waste through composting could be a win-win situation for municipalities and farmers (for example see the Marilao, Philippines case study by Duran et al.). The interests of urban waste recycling go well with the promotion of urban agriculture since urban and peri-urban farmers are in need of organic matter as a soil conditioner. Cities and towns, on the other hand, wish to conserve disposal space and reduce the costs of landfills as well as municipal solid waste management. Also important is the need to incorporate informal waste collectors and the private sector that contribute to urban waste management into this process (see Box 8.2 and the Nairobi, Kenya case study by Njenga and Karanja).

Benefits and constraints

Zurbrugg and Drescher (2002) report that the potential benefits of organic waste recycling are particularly in reducing the environmental impact of disposal sites, in extending existing landfill capacity, in replenishing the soil humus layer and in minimising waste quantity. Other benefits adapted and summarised from Hoornweg et al. (1999) with particular reference to organic waste composting are that it:

• increases overall waste diversion from final disposal, especially since as much as 80 percent of the waste stream in low- and middle-income countries can be composted;
   
• enhances recycling and incineration operations by removing organic matter from the waste stream;
   
• produces a valuable soil amendment - integral to sustainable agriculture;
   
• promotes environmentally-sound practices, such as the reduction of methane generation at landfills;
   
• enhances the effectiveness of fertilizer application;
   
• can reduce waste transportation requirements;
   
• is flexible for implementation at different levels, from household efforts to large-scale centralised facilities;
   
• can be started with very little capital and operating costs;
   
• the climate of many developing countries is optimum for composting;
   
• addresses significant health impacts resulting from organic waste such as reducing Dengue Fever;
   
• provides an excellent opportunity to improve a city's overall waste collection programme;
   
• accommodates seasonal waste fluctuations such as leaf litter and crop residues;
   
• can integrate existing informal sectors involved in the collection, separation and recycling of wastes.

Although composting seems an attractive option in many respects, it is also constrained (Hoornweg et al., 1999) by the following factors:

• Inadequate attention to the biological process requirements;
   
• Over-emphasis placed on mechanised processes rather than labour-intensive operations;
   
• Lack of vision and marketing plans for the final product - compost;
   
• Poor feed stock which yields poor quality finished compost, for example when contaminated by heavy metals;
   
• Poor accounting practices which neglect that the economics of composting rely on externalities, such as reduced soil erosion, water contamination, climate change, and avoided disposal costs;
   
• Difficulties in securing finances since the revenue generated from the sale of compost will rarely cover processing, transportation and application costs.

An evaluation of composting projects in West Africa pointed out that apart from being too expensive, a common problem leading to project failure is poor co-ordination among institutions and stakeholders due to weak institutional linkages and the lack of an enabling institutional framework, including clear legislation and policies. Experiences from six composting stations of different scales of production in five countries in West-Africa (see the overview in table 8.1 in the Annex) showed that compost stations in the sub-region suffer from a number of omissions (Drechsel et al., 2005). Lack of thorough market analysis including consideration of alternative soil inputs; transport costs; user's demand as well as willingness and ability to pay for compost prior to station set-up; lack of supportive legal frameworks and institutional arrangement to implement composting initiatives are some of these. In many cases, important stakeholders (land owners, waste collectors etc) were often not involved in planning which then constrained successful implementation. Apart from these, most composting projects are not financially viable, especially when outside funding available for the initial set up is exhausted. These points confirm the need for a comprehensive feasibility study before setting up any composting project.

Framework for Analysis and Planning of Composting

Planning is necessary to ensure a well functioning composting system. Analyses of the various segments - from waste generation, recycling to re-use - is necessary. The nutrient recycling loop concept is very helpful in this process (see Figure 8.3). The recycling loop is represented in this figure by various segments: urban consumption and waste generation, waste processing, compost demand for agriculture, along with an economic feedback mechanism and finally the legal, institutional and communal settings throughout the loop. (Drechsel et al., 2002)

The first segment of the loop, urban consumption and waste generation, addresses the supply dimensions of urban waste. It raises questions regarding organic waste production, location, ownership, quality, quantity, time, availability, value, health & safety constraints, etc. This is followed by the second segment waste processing, where questions are raised on (possibility of) organic waste transportation, appropriate processing methods (i.e. composting), production capacity, operation costs, sustainability, subsidies etc). The third segment deals with compost demand and address questions on users' demand, application, experiences, ability and willingness to pay, cultural constraints, etc. In addition to these three segments, there is an economic analysis linking the demand and composting segments that addresses economic viability, marketability and distribution. The final element looks at the legal, institutional and communal setting, in which the issues of planning, regulations, by-laws, policy constraints or support, land availability, local stakeholder participation, monitoring & evaluation, inter/intra-sectoral corporations, etc. are addressed, throughout the cycle of analysis.

Community involvement in waste collection

This nutrient recycling loop is used to scope and assess all the processes involved in recycling organic waste into a valuable resource at municipal (centralised), community (decentralised) and/or household (backyard) levels for use in urban agriculture. The model provides a diagrammatic illustration of the systematic processes that are involved in selecting an appropriate organic waste recycling technology at the appropriate scale of intervention. For an urban farmer this process may take the form of a rapid appraisal or scribbles on the back of an envelope, whereas for a community-based organisation or a municipal authority it will form a logical guide to a more detailed and rigorous assessment study.

The recycling loop gives the required framework and potential best practice for planning composting for urban agriculture (Cofie et al., 2001, Drechsel et al., 2002, 2004, Danso et al., 2005). The questions which should be addressed at each moment in this cycle are summarised in Figure 8.3. The effectiveness and usefulness of this framework was tested in Ghana (Drechsel et al., 2004) using specific methods in the analysis of each segment of the recycling loop. It is important to note however that the analysis can have various degree of sophistication depending on the specific location, scale of the intended composting project, available funds, etc.

Figure 8.3 The Nutrient Recycling Loop (modified from Drechsel et al., 2002).

Application of the Nutrient Loop

The supply of organic waste

The key question in the waste supply context is: Where is which amount of waste of what kind of quality and when is it available for composting? This will allow identification of recycling needs in terms of design and capacity. Supply studies should focus on the various types, amounts, quality, present and potential uses, current value and availability of organic municipal waste for composting. The analysis of waste supply in West Africa showed that the availability of organic waste is not the limiting factor for compost production, although, not every form of waste is always available as there are often alternative uses (fodder, fuel etc) and seasonal variations. A comparison of waste generation and availability along a south to north gradient from Accra, Ghana to Ouagadougou in Burkina Faso showed that with decreasing biomass production, the amount of organic waste and related nutrient availability per capita decreases progressively as dryer eco-zones are encountered (Danso et al., 2005)

A result of waste surveys in Ouagadougou (Eaton, 2003) indicated that 80,000 tons of organic waste is produced each year in the form of solid household waste with a nitrogen content of 26 tons. It was estimated that about 25,000 tons of organic material per year could be composted and sold to farmers for application on a relatively modest estimate of 200 ha of intensive urban horticulture plots. This would correspond to an estimated 8 tons of nitrogen. This leaves approximately 55,000 tons of organic material per year that could be spread over an area of 8,500 hectares of peri-urban staple crop fields, a flow of approximately 18 tons of nitrogen. In other words, the supply of organic material is much more than can be realistically absorbed in agriculture, at least given current economic circumstances (Tessier, A. 2004).

The demand for waste-derived compost

The demand assessment includes the characterisation of all potential clients under consideration of their willingness (and ability) to pay (WTP). It is expected that a major demand for compost in rapidly expanding cities will come from landscape designers (horticulturists, parks and gardens) and real estate developers, so this sector must not be left out in the analysis. The demand analysis should also consider socio-cultural aspects, farm economics, attitudes/perceptions of users of waste compost and actual demand projections. Danso et al., (2005) reported for Ghana that many urban farmers have positive perceptions and are willing to use compost although not all have the necessary experience. Farmers' interest in compost was both for its plant-growth enhancing (fertility) effect and soil amelioration. Variations in WTP were recorded between farmers with and without compost experience, different farming systems, urban and peri-urban farms, as well as between different cities with different compost alternatives. The WTP expressed by farmers who already used compost was in several cases lower than among non-users. This was due to past experience with poor quality compost (in Accra) which resulted in poor crop performance and the negligible market demand for "organically" produced crops in Kumasi.

Waste collection in Lomé

The study further revealed that estate developers were willing to pay higher prices for compost than urban and peri-urban farmers. In comparison to agriculture, the real estate sector has much lower qualitative requirements as compost will mostly be used for lawns and ornamentals. Thus the real estate sector could be the "favourite" customer group with options for private-public partnerships with the municipality. The financial strength of the real estate sector could subsidise parts of the compost production for agriculture.

The process of waste composting

The process of waste composting includes the determination of the type of facility, optimal number, capacity, and location of compost stations per city. Most critical in this assessment is to include possible ways of composting and determine the number of potential compost stations and station capacity with due consideration of waste supply and compost demand. Composting is best achieved by providing optimal conditions for the micro-organisms through the best combination of air, moisture, temperature and organic materials (Agromisa, 1999). Composting processes can be aerobic (with oxygen) or anaerobic (without oxygen) and even alternate between the two during the decomposition process. Anaerobic composting is a low-temperature process that is not recommended for urban agriculture due to the strong odours and the inability to destroy harmful pathogens that may be present in urban organic waste. Conversely, aerobic composting is a high-temperature process due to the development of microbes that generate higher temperatures in the compost pile. The key factors affecting the biological decomposition processes and/or the resulting compost quality are listed in box 8.3

The choice of a technology for aerobic composting will depend on the location of the facility, the capital available and the amount and type of waste delivered to the site. The two main types of systems generally distinguished are: 1) open systems such as windrows and static piles and 2) closed "in-vessel" systems. These "in-vessel" or "reactor" systems can be static or movable closed structures where aeration and moisture is controlled by mechanical means and often requires an external energy supply. (see the Kumasi, Ghana case study by Adam-Bradford). Such systems are usually investment intensive and also more expensive to operate and maintain. "Open" systems are the ones most frequently used in developing countries. They can be classified as:

Windrow, heap or pile composting: The material is piled up in heaps or elongated heaps (called windrows).

Bin composting: Compared to windrow systems, bin systems are contained by a constructed structure on three or all four sides of the pile. The advantage here is a more efficient use of space. (for illustrations see the Kumasi, Ghana case study).

Trench and pit composting: Trench and pit systems are characterised by heaps which are partly or fully contained under the soil surface. Structuring the heap with bulky material or turning is usually the choice for best aeration. Control of leaching is difficult in trench or pit composting. In some cases, composting materials are completely buried in the trench which then serves as a planting bed, for example Mtshepo's home gardening in South Africa.

Compost heaps at Kumasi co-composting plant

The aerobic composting process can last from a few weeks to 3-4 months, depending on the type of composting feedstock and the method of composting.

Mtshepo's home gardening in South Africa

Locally made composting pits in Tamale, Ghana

Emerging trends include the practice of vermiculture and the use of effective micro-organisms (EM) to accelerate the composting process. Vermiculture is the use of worms to digest organic waste into rich humus, similar to compost, that can then be applied in urban agriculture. Local varieties of both surface and burrowing earthworms can be used, although the latter are particularly suited as they not only digest organic matter but also modify the soil structure. Vermiculture is particularly suited to urban agriculture because it can be applied in a variety of settings and at different scales. The practice is also used very often as part of integrated gardening in community building urban agriculture (see chapter 6). Indeed, broad-scale vermiculture is widespread in India, Indonesia and the Philippines (GFA-Umwelt, 1999), while the practice has recently been gaining ground in Cuba and Argentina (Dubbeling and Santandreu, 2003; Viljoen and Howe, 2005). In broad-scale vermiculture, the earthworms are introduced to organic waste piled in elongated rows that are covered with some form of vegetative protection to prevent water logging (Ismail, 1997).