Urban Organic Wastes, Urban Health And Sustainable Urban And Peri-Urban Agriculture
Linking Urban And Rural By Composting
By M S Rodrigues & J M Lopez-Real
M S Rodrigues
Mario Sergio Rodrigues
Departamento de Ciências Ambientais,
Faculdade de Ciências Agronômicas,
Universidade Estadual Paulista - UNESP,
PO Box 237, 18603-970, Botucatu/SP, Brazil.
J M Lopez-Real
Biological Sciences Dept, Wye College, University of London,
Wye, Nr Ashford, Kent,
TN25 5AH, United Kingdom.
1. Urban organic wastes
The use of urban organic waste in urban and peri-urban interface production systems has recently been extensively reviewed (Allison & Harris 1996). Traditionally, urban settlements are importers of natural resources and exporters of pollution from and to rural areas (Smit 1998). In cities in developing countries significant quantities of organic wastes are generated by the agricultural enterprises of the inhabitants, through general municipal waste, large quantities of human waste (excrement) and also vast amounts of water contaminated by organic material from agroindustrial processing or sewage. Waste management in urban areas is a serious issue. A city with one million inhabitants is estimated to consume 25,000tonnes (t) of water and 2,000t of food per day and produces 50,000t of effluent water and 2,000t of waste material daily (Deelstra 1989). There are clearly vast differences in the levels of consumption and waste production between the industrialised and developing countries. In contrast to Deelstras figures Manila (12 million people) generates 4,000t waste per day (Medina 1993), Jakarta (similar population) 5,000t per day (Simpson 1993), Dar es Salaam (3 million) approximately 1,000t per day (Lopez-Real 1995), Calcutta (10 million) 3,000t per day (Kundu 1995), Kano (1.4 million) produces 450 t per day (Lewcock 1994), São Paulo (13 million) approximately 10,000t per day.
2. Societal waste and public health
Poor refuse disposal will encourage all of the above features and may thus promote the transmission of faecal-oral infections including diarrhoeas and dysenteries (Amoebic dysentery; Campylobacter enteritis, cholera, Cryptosporidiosis, Escherichia coli diarrhoea, samonellosis, shigellosis etc.). It can also promote diseases associated with rats such as plague, leptospirosis, endemic typhus and rat bite fever. Uncollected refuse can obstruct streets and drainage channels. Mosquitoes such as Culex quinquefasciatus, Aedes aegypti, and others can breed in the wastewater retained in such blocked channels and may transmit filariasis and viral infections such as dengue and yellow fever (Cairncross & Feachem 1993). Considerable quantities of high organic containing wastes are produced from city markets posing the threats described above (Lopez-Real 1995b). Given the importance of these societal organic wastes in public health problems - and presumably the enormous 'on cost' in health terms - it is perhaps somewhat astonishing that more priority has not been given by municipalities, national governments and international agencies to the question of establishing effective organic waste collection, treatment and disposal routes.
Ideally such wastes will not only be collected but separated into an exclusively organic fraction for treatment by recycling - utilising and harnessing the power of microorganisms in order to convert the material to a safe (non- polluting and pathogen free) and useful end product (compost) for use in urban horticulture (Rodrigues 1996). The microbially mediated process of composting is the ideal route for this and is applicable at both a low and high technological level.
Composting is defined (Finstein 1995) as a microbe-based, aerobic, solid phase matrix, self-heating process. The matrix consists of the organic material which serves as a source of nutrients for microbial growth, a sink for metabolic products, a site for gas exchange, and thermal insulation. Owing to the insulating property of the material, heat generated metabolically is conserved within the system thereby elevating the matrix temperature from an ambient mesophilic starting point into the mesophilic range. As readily metabolised substances become depleted, temperatures decline and eventually return to ambient levels. The material is thus biologically oxidised to carbon dioxide and a stabilised, organic process residue. The microbes responsible for composting are various beneficial bacteria and fungi that are widespread in the environment. These are indigenous to such materials as soil, dust, vegetable matter, and wastes of all sorts. Special organisms are not required (Rodrigues, 1996). Owing to the time/temperature profile of the process the final product (the compost) will be "sanitised" - i.e. much reduced levels of human, animal and plant pathogens. Providing that the original organic waste material was low in contaminants (e.g. heavy metals) the compost may be of potential value in agriculture and horticulture (Lopez-Real, 1994).
Broadly speaking composting technology is based around the aerobic nature of the process and composting configurations reflect variations on a theme of air (oxygen) supply. Though simple stacking of organic wastes (e.g. manure heaps) is sometimes referred to as "passive composting" natural diffusion of air into the matrix cannot replace the demand exerted by the microflora leading to anaerobic conditions and anaerobic degradation pathways. True composting requires intervention methods in order to aerate and these range from simple labour intensive or mechanical turning systems (windrowing) to static forced-aerated systems (Beltsville; Rutgers) and ultimately to capital intensive "in-vessel" bioreactor type systems.
Composting is a natural process whereby microorganisms breakdown organic materials. In composting heat is generated with pile temperatures often reaching 7OoC. The high temperatures achieved are very important for sanitising the waste as they kill off almost all the pathogens including highly resistant helminth eggs. Currently the USA EPA utilises a standard of 55oC for 72 hours (bioreactor) and 55oC for 14 days (windrowing) for the composting of sewage sludges which incorporates a substantial safety marging. These temperatures are easily attained providing there is enough waste to exert a self insulating effect - this is unlikely to occur with the wastes from a single household and, therefore composting options must be based around community-based organisations (CBO) acting as collectors, separators and processors (by composting) of the organic wastes produced in their own community. Enough wastes would be accumulated to ensure pathogen reduction but not excessive quantities which would prejudicate the exercise on the basis of logistics and costs.
The application of societal organic wastes and the role of composting in urban horticulture and agriculture has been reviewed by Allison and Harris (1995). Over the years numerous municipalities throughout the world have resorted to high tech "composting" operations involving unsorted municipal urban wastes - garbage. In reality many of these operations have been in effect materials handling facilities (attempting to mechanically separate glass, metals, paper etc.) with a composting "add-on" and have consequently failed to deliver their promise and have become expensive materials reduction exercises with wastes ultimately being landfilled. Unfortunately many have become labelled incorrectly 'composting' plants with a consequent erroneous label of failure attached to the microbial process (Lopez-Real 1994a).
The recent support of Community Based organisations (CBO) has led to the establishment in many countries of smaller scale and successfully operated organic waste composting operations suited to the needs of urban horticulturists (Lopez-Real 1995).
There is a considerable body of literature concerning the efficacy of compost use in agriculture and horticulture though it is extremely difficult to analyse such data as their is enormous variation in the studies with respect to: quantities used, details of composting often absent, poor quality compost (unstable); soil types; placement of material; crops used; climatic region etc. (Rodrigues, 1996).
In general most researchers publish positive findings in terms of yield levels, moisture retention, organic matter input, improvement in soil physical characteristics etc. Negative findings are usually traceable or deliberately target poor quality compost as a result of waste stream mismanagement (i.e. failure to separate wastes leading to heavy metal and other contamination). Compost production and use in an urban setting has many attractions. As outlined above it is a process of direct public health importance as it enables potentially polluting and disease ridden materials to be sanitised and stabilised. Such materials (if clean) are of value to crop growing but of particular value to quick growing high value horticultural crops - there is less value is using composts for say cereals where the full benefits may not transfer to the grain yield (Rodrigues 1996).
Though such materials could be used in rural farming the economic costs of transportation of such bulky materials is often prohibitive. It therefore makes real economic and common sense to collect, process and utilise as close to the organic waste origins as possible. The urban/peri-urban sector is therefore ideal for the utilisation of composts in particular for horticultural crops which are more amenable to high density planting. The use of composts will undoubtedly have an important knock on effect in terms of water use - an area much criticised in UA - as one of the widely recognised benefits of composting is its ability to hold and retain moisture (Parr & Papendick, 1982).
4. Urban and Peri-urban Agriculture
In recent years, there has been a revolution in terms of the traditional occupations in both urban and rural environments. Rural areas are increasingly offering a myriad of non-agricultural jobs in both high-income and less-developed countries (da Silva, 1997). At the same time, urban areas witness an ever growing offer of opportunities in traditionally rural activities: agriculture, horticulture, orchards, and animal husbandry. Historically, however, urban agriculture has played an important role in many civilisations. Throughout the world, there is a long tradition of farming intensively within and at the edge of cities (Smit et al., 1996). Urban agriculture offers to urban citizens the benefits of an alternative source of income, hunger reduction, nutrition improvement, environment enhancement and sustainable management. It can also contribute to a better public health, sustainable waste management, and stimulate new patterns of social organisation and community participation.
Contrary to the common sense, urban and peri-urban agriculture are an important economic activity, essential to the livelihood of millions of families around the globe (Mougeot, 1994), and the trend indicates an accelerated increase. According to the Smit report (1996) the number of urban farmers producing for the market is expected to double from about 200 million in the early 1990s to 400 million by 2005. Many are the examples of successful urban agriculture schemes around the globe, from Dar es Salaam to Singapore, from Vancouver to Curitiba. A more comprehensive view on this can be found in the book by Smit et al. (1996) sponsored by the UN Development Program.
In addition to the benefits of increasing food supply and income to urban families, urban agriculture has an important significance to global sustainability. The production of food close to the consuming market reduces the need for transportation, therefore reducing the consumption of fossil fuels and the associated emissions of CO2. There is also a reduction in packaging, refrigeration and the use of preserving additives (Rees, 1997).
Producing food in or close to urban settlements allows the utilisation of wastes as inputs to the process, therefore recycling the nutrients that would be otherwise discharged to the environment in landfills or surface water. Societal organic wastes - municipal solid waste and sewage sludge - can be processed into organic fertiliser, returning to nearby garden or farmland (Smit et al., 1996).
It is in the context of urban and peri-urban agriculture that composting has its most important role to play, producing a rich soil conditioner while diverting from dumping sites and landfills the organic fraction of the urban waste thus reducing health hazards and methane emissions.
There is also an educational interface of urban agriculture to be explored. Since they involve experimental and co-operative learning, school gardens are effective learning tools to students of all ages. Likewise many other countries, Brazil has a long tradition of school gardens. For quite sometime, however, this tradition has been put in disuse for reasons ranging from lack of support by the authorities to a kind of prejudice against agricultural jobs by the children. More recently, there has been a re-birth of school gardens all around.
In Botucatu, a town of 110,000 people, 230 Km from São Paulo City, São Paulo State, the local Campus of the São Paulo State University/UNESP is dealing with a growing demand for both school and community based gardens. In the past, a system of grants allowed the allocation of students from the Faculty of Agricultural Sciences to give technical assistance to primary and secondary schools. Today, as an effort to rescue the program and answering to a demand by the children, there is a pilot scheme running in a voluntary basis in two state-owned schools. Two university students offer at least four hours per week labour, the schools give tools and land, and the University offers seeds, seedlings and support to the program. The University is now producing compost, which will be used in the beds. The vegetables produced enrich the daily meal given to the students at school. Every exceeding production is given to the student's families and to the schools' employees.
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