1. Basic definitions
Solid waste management includes all activities that seek to minimise the health, environmental and aesthetic impacts of solid wastes.
Solid waste can be defined as material that no longer has any value to the person who is responsible for it, and is not intended to be discharged through a pipe. It does not normally include human excreta. It is generated by domestic, commercial, industrial, healthcare, agricultural and mineral extraction activities and accumulates in streets and public places. The words “garbage”, “trash”, “refuse” and “rubbish” are used to refer to some forms of solid waste.
2. The scope of these comments
These introductory comments are concerned only with middle- and lower-income countries. The problems and opportunities relating to solid waste management in industrialised countries are very different, and will not be referred to here. Solid wastes are generated by many different activities. Very large quantities are produced by agriculture and mining, but these wastes will not be considered in this introduction. Wastes from houses, streets, shops, offices, industries and hospitals are usually the responsibility of municipal or other governmental authorities, and it is these wastes which are the subject of this article.
3. Simple or complex?
Many people feel that solid waste management is a simple affair - simply putting waste into a vehicle and unloading it at a dump. If this were true, then why do so many towns suffer from uncollected refuse blocking streets and drains, harbouring flies and rats, and degrading urban environments? Successful solid waste management is rarely achieved without thought, effort and much learning from mistakes.
Engineers may feel that any engineer, without special training or experience, can solve solid waste management problems. There is much evidence to show that this is not true, perhaps mainly because solid waste management is much more than a technological issue - it usually involves managing a large workforce and working together closely with the public. Problems with maintenance and financial aspects are common. Engineers often make mistakes in the selection of equipment, since vehicles and machines which work well in industrialised countries are often grossly inadequate in developing countries.
The preparation and management of a good solid waste management system needs inputs from a range of disciplines, and careful consideration of local conditions.
4. Risks and problems associated with solid wastes
If solid wastes are not managed properly, there are many negative impacts that may result. Some of the most important are mentioned in the following list. The relative importance of each depends very much on local conditions.
- Uncollected wastes often end up in drains, causing blockages which result in flooding and insanitary conditions.
- Flies breed in some constituents of solid wastes, and flies are very effective vectors that spread disease.
- Mosquitoes breed in blocked drains and in rainwater that is retained in discarded cans, tyres and other objects. Mosquitoes spread disease, including malaria and dengue.
- Rats find shelter and food in waste dumps. Rats consume and spoil food, spread disease, damage electrical cables and other materials and inflict unpleasant bites. (For more information see Buckle and Smith, 1994.)
- The open burning of waste causes air pollution; the products of combustion include dioxins which are particularly hazardous.
- Aerosols and dusts can spread fungi and pathogens from uncollected and decomposing wastes.
- Uncollected waste degrades the urban environment, discouraging efforts to keep streets and open spaces in a clean and attractive condition. Solid waste management is a clear indicator of the effectiveness of a municipal administration - if the provision of this service is inadequate large numbers of citizens (voters) are aware of it. Plastic bags are a particular aesthetic nuisance and they cause the death of grazing animals which eat them.
- Waste collection workers face particular occupational hazards, including strains from lifting, injuries from sharp objects and traffic accidents.
- Dumps of waste and abandoned vehicles block streets and other access ways.
- Dangerous items (such as broken glass, razor blades, hypodermic needles and other healthcare wastes, aerosol cans and potentially explosive containers and chemicals from industries) may pose risks of injury or poisoning, particularly to children and people who sort through the waste.
- Heavy refuse collection trucks can cause significant damage to the surfaces of roads that were not designed for such weights.
- Waste items that are recycled without being cleaned effectively or sterilised can transmit infection to later users. (Examples are bottles and medical supplies.)
- Polluted water (leachate) flowing from waste dumps and disposal sites can cause serious pollution of water supplies. Chemical wastes (especially persistent organics) may be fatal or have serious effects if ingested, inhaled or touched and can cause widespread pollution of water supplies.
- Large quantities of waste that have not been placed according to good engineering practice can slip and collapse, burying and killing people.
- Waste that is treated or disposed of in unsatisfactory ways can cause a severe aesthetic nuisance in terms of smell and appearance.
- Liquids and fumes, escaping from deposits of chemical wastes (perhaps formed as a result of chemical reactions between components in the wastes), can have fatal or other serious effects.
- Landfill gas (which is produced by the decomposition of wastes) can be explosive if it is allowed to accumulate in confined spaces (such as the cellars of buildings).
- Methane (one of the main components of landfill gas) is much more effective than carbon dioxide as a greenhouse gas, leading to climate change.
- Fires on disposal sites can cause major air pollution, causing illness and reducing visibility, making disposal sites dangerously unstable, causing explosions of cans, and possibly spreading to adjacent property.
- Former disposal sites provide very poor foundation support for large buildings, so buildings constructed on former sites are prone to collapse.
5. The impact of local conditions
There are many factors that vary from place to place and that must be considered in the design of a solid waste management system. Amongst them are:
- The waste itself - Typical domestic waste from industrialised countries has a high content of packaging made of paper, plastic, glass and metal, and so the waste has a low density. (In other words one person can lift a typical bin when it is full.) The large amount of paper and the use of pre-processed food result in low proportions of moisture in the waste. In many developing countries there is a high proportion of sand (because of the materials used for paving and construction, and climatic factors) which makes the waste very dense. (If the waste is dense it means that two people may have difficulty in lifting a typical bin when it is full.) In addition, the waste may contain large amounts of moisture because of the high usage of fresh fruit and vegetables.
There are several important consequences of this density factor. Perhaps the main one is that containers, vehicles and systems that operate well with low-density wastes in industrialised countries are not suitable or reliable when the wastes are heavy. The combination of the extra weight, the abrasiveness of the sand and the corrosiveness caused by the water content, can cause very rapid deterioration of equipment.
Another important consideration is the possibility of incinerating the waste (meaning, the burning of waste under controlled conditions to minimise pollution). If the waste contains a high proportion of moisture, or is mostly inert material, it is not suitable for incineration, and so this treatment option is ruled out. Recycling or salvaging operations often reduce the proportion of combustible paper and plastic in waste before it reaches the treatment stage.
- Access to waste collection points - Many sources of waste might only be reached by roads or alleys which may be inaccessible to certain methods of transport because of their width, slope, congestion or surface.
- Public awareness and attitudes to waste - This can affect the readiness to carry waste to a shared container, the willingness to accept the proximity of a shared container, the willingness to segregate waste to assist recycling, the frequency at which wastes should be collected, the amount of litter and animal excreta that are left on the street, the willingness to pay for waste management services, the opposition to the siting of waste treatment and disposal facilities, gender issues with regard to waste recycling and collection activities, and the social groups from which waste management staff can be drawn.
- Selection of equipment - In addition to the factors already mentioned, the selection of waste collection vehicles should be influenced by the types of vehicles and chassis that are already widely used and for which spare parts and maintenance expertise are available. Taxes, duties and import restrictions should also be considered.
- Institutional issues include the current and intended legislation and the extent to which it is enforced. Standards and restrictions may limit the technology options that can be considered. The policy of government regarding the role of the private sector (formal and informal) should also be taken into account. The strength and concerns of trade unions can also have an important influence on what can be done.
6. Resource recovery
Resource recovery means the obtaining of some economic benefit from material that someone has regarded as waste. It includes
- reuse - being used for the same purpose again (such as refilling a soft drinks bottle);
- recovery - processing material so that it can be used again as the same material, such as the processing of waste paper to make pulp and then new paper;
- conversion - processing the material to make something different (such as producing padding for clothing and sleeping bags from plastic bottles, or producing compost from food waste)
- energy recovery - usually referring to the burning of waste so that the heat can be used (for example, for heating swimming pools). Another method of energy recovery is to collect the gas that is produced in very large sanitary landfills and use it as a fuel or to generate electricity.
Some key factors that affect the potential for resource recovery are the cost of the separated material, its purity, its quantity and its location. The costs of storage and transport are major factors that decide the economic potential for resource recovery. In many low-income countries, the fraction of material that is won for resource recovery is very high, because this work is done in a very labour-intensive way, and for very low incomes. In such situations the creation of employment is the main economic benefit of resource recovery. The situation in industrialised countries is very different, since resource recovery is undertaken by the formal sector, driven by law and a general public concern for the environment, and often at considerable expense.
Composting is an excellent method of recycling biodegradable waste from an ecological point of view. However, many large and small composting schemes have failed because composting is regarded as a disposal process, and not a production process. It is essential - as in any production process - to pay careful attention to the marketing and the quality of the product. Composting should be an activity of the agricultural sector, not the waste management sector.
It can be a big mistake to try to impose on low-income countries the methods of recycling that are used in industrialised countries.
7 Disposal
It appears that in most low-income countries, and many medium income countries, very little progress has been made in upgrading waste disposal operations. Open dumps, where the waste is unloaded in piles, make very uneconomical use of the available space, allow free access to waste pickers, animals and flies and often produce unpleasant and hazardous smoke from slow-burning fires.
A sanitary landfill is a site where solid wastes are placed on or in the ground at a carefully selected location by means of engineering techniques that minimise pollution of air, water and soil, and other risks to man and animals. Aesthetic considerations are also taken into account. (For more information see Rushbrook and Pugh, 1999.)
In some major cities loans or grants have been used to construct sanitary landfills on sites that have been carefully chosen, but usually little attention is paid to the training of a site manager and to the provision of sufficient financial and physical resources to allow a reasonable standard of operation. As a result, some sites quickly degenerate into open dumps. It is crucial to good operations to have a motivated and trained site manager. It is recommended that the training for this position should include practical experience on well-run sites.
Most sanitary landfill designs attach considerable importance to preventing polluted water (leachate) from escaping from the site. It has been shown that large quantities of leachate can be produced by landfills, even in semi-arid climates. (Scheu, 2001) Most designs include expensive and carefully constructed impermeable layers which prevent leachate moving downwards into the ground and drainage systems to bring the leachate to a treatment plant or a storage tank. However, if the tank is not emptied before it overflows, or if the plant is not working, the leachate control system actually makes the pollution worse than from an open dump, because all the leachate is concentrated in one place, giving natural purification systems very little chance of reducing the pollution impact. This example shows that good design and construction can achieve nothing if they are not followed by good operation.
8 Involving the private sector
In many countries there is great interest in the participation of private companies in solid waste management. Sometimes this is driven by the failures of municipal systems to provide adequate services, and sometimes by pressure from national governments and international agencies. Arrangements with private companies have not all been successful, and as a result some opposition to private sector involvement is now in evidence.
An important factor in the success of private sector participation is the ability of the client or grantor - usually a municipal administration - to write and enforce an effective contract. Many municipalities do not know what it has been costing them to provide a service, so they cannot judge if bids from the private sector are reasonable. The contract document must be well written to describe in quantitative terms what services are required and to specify penalties and other sanctions that will be applied in case of shortcomings. Monitoring and enforcement should be effective. It is also important that the rights of both parties are upheld by the courts. Three key components of successful arrangements are competition, transparency and accountability. (For more information see Cointreau et al., 2000.)
As an alternative to large (often international) companies that can provide most or all of the solid waste services in a city, microenterprises or small enterprises (MSEs) can be involved. They often use simple equipment and labour-intensive methods, and therefore can collect waste in places where the conventional trucks of large companies cannot enter. These MSEs may be started as a business, to create income and employment, or they may be initiated by community members who wish to improve the immediate environment of their homes. (Further information on MSEs can be found in Arroyo et al., 1999 and Haan et al., 1998. An interesting example of MSEs in Dar es Salaam can be found in Bakker et al., 2000.)
A recurring problem with collection schemes that operate at the community level is that the community scheme generally takes the waste a relatively short distance to a transfer point, from where the waste is supposed to be collected by another organisation - often a municipality. Problems of co-ordination and payment often result in the waste being left at transfer points for a long time. An example where this co-ordination was achieved in Bangladesh is given by Ahsan and Habib (2000). Another solution is to recycle as much of the waste locally so that there is very little need for on-going transport of collected waste.
9 Hazardous wastes from hospitals
Healthcare wastes are generated as a result of activities related to the practice of medicine (including veterinary medicine and dentistry). Often this term is used to refer only to solid wastes (i.e. not to wastewater). Some of the healthcare wastes coming from any particular hospital or institution are similar in nature to domestic solid wastes, and may be called “general healthcare wastes”. The remaining wastes pose serious health hazards because of their physical, chemical or biological nature, and so are known as “hazardous healthcare wastes” or “healthcare risk wastes”. Wastes which are particularly offensive because of their appearance or smell may also be classed with the hazardous wastes.
Healthcare wastes have attracted considerable attention because of the emotional impact of seeing body parts amidst solid waste, and because of the increasing concern about AIDS and hepatitis. In many cases the most dangerous items in healthcare wastes are needles from syringes and drips, because the needles shield the viruses from chemical disinfectants and a harsh external environment, and the sharp point allows easy access for the viruses into the blood stream of anyone who is pricked by the needle.
Many attempts to upgrade healthcare waste management rely solely on the provision of incinerators or other treatment technologies. Such a strategy has several weaknesses in that
- often the hospitals and healthcare facilities are not able to afford the operating costs of the plant, and so the plants are left unused or not repaired when the break down;
- many of the risks occur before the waste gets to this final stage, and so they are not reduced by the provision of treatment equipment;
- the real need is often provide better methods of storage to train the staff to adopt safer working practices.
(More information is available from Prüss et al., 1999, Coad and Christen, 1999, and Coad 2001.)
10 Other hazardous and special wastes
Some waste materials need special care because their properties make them more hazardous or problematic than general wastes. Used oil can be refined for reuse or burned in properly equipped furnaces. Slaughterhouse wastes should be buried in special trenches at suitable sites. Car tyres should be reused as much as possible, and carefully protected from open burning. Chemical wastes from some industries (including tanning, dry-cleaning, photographic processing and many chemical production industries) and unwanted pesticides and other agricultural chemicals, should be collected under close supervision and treated in appropriate ways. The management of hazardous chemicals is not only a matter of technology and legislation, but also of enforcement, funding and financial instruments. Some wastes are so hazardous and expensive to treat that priority attention should be focussed on changing to processes that use substitutes that are less hazardous, and to minimising the quantities that are discarded. Indeed, minimisation and substitution should be seen as the preferred options in dealing with any difficult waste.
The Basel Convention seeks to control the movement of hazardous wastes across international boundaries. This instrument is necessary because the high cost of treating hazardous wastes in industrialised countries makes it financially attractive to ship the wastes to another country where no special requirements for their disposal will be applied.
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