Study Session 7  Solid Waste: Sources, Composition and On-site Storage

Introduction

This is the first of five study sessions that focus on solid waste management. A solid waste management system consists of a chain of linked processes. The chain begins with the generation of the waste, which is the subject of this study session. In Study Session 8 we will look at waste reduction, reuse and recycling (the ‘3 Rs’) and Study Session 9 is about collecting the remaining wastes. Study Sessions 10 and 11 cover the ways of disposing of this remaining waste.

Good management of solid waste is one of the most important ways of protecting our health. Before we can design a waste management service and monitor its effectiveness we need detailed information on the waste itself. For each type of waste produced we need to know the quantities generated, where the waste is generated and its composition.

In this study session you will learn about the types of solid wastes generated from different sources (mainly domestic, industrial and commercial), and the amounts produced. This session will also enable you to advise householders on how to store solid wastes in such a way as to minimise the risk to people in their homes.

Learning Outcomes for Study Session 7

When you have studied this session, you should be able to:

7.1  Define and use correctly each of the terms printed in bold. (SAQ 7.1)

7.2  Describe the major sources of solid wastes in urban areas. (SAQ 7.2)

7.3  Use the results from waste surveys to calculate waste generation rates. (SAQ 7.3)

7.4  Describe the potential health effects of different wastes. (SAQ 7.4)

7.5  Explain the importance of proper on-site storage of solid waste at the household level. (SAQ 7.5)

7.1  Sources of solid waste in urban areas

Solid wastes are all the wastes arising from human and animal activities that are normally solid and are discarded as useless or unwanted by the person or organisation that produces the waste.

You may recall from Study Session 1 that solid waste can be classified according to its source. The major sources of solid wastes in urban areas of Ethiopia are shown in Box 7.1.

Residential solid wastes (sometimes called household waste) usually form the largest proportion of municipal waste (the combined solid wastes in an urban area). Let us take Bahir Dar as an example. Bahir Dar is a city on the southern shore of Lake Tana, about 600 km north-west of Addis Ababa, with a population of 320,000. In 2010 a waste survey was undertaken as the first stage of a programme to find better ways of treating the city’s waste. Figure 7.1 shows the survey results for the proportion of waste from different sources. You can see that the majority of the solid waste (55%) comes from residential areas (Forum for Environment/UNEP, 2010). In Bahir Dar, the household waste is collected by a private contractor and taken to an open dump for disposal. (You will learn more about waste disposal sites in Study Session 10.)

Figure 7.1  Major sources of solid wastes in Bahir Dar and their relative proportions by mass.

Types of solid waste generated in residential areas includes food wastes, packaging (bottles and cans), papers and miscellaneous items that have been used up or broken and are thrown out as waste (e.g. ashes, old shoes, worn out clothes, broken cooking pots, paper, baskets, bags, etc.). You should also note that, in areas where latrines are not available, bags of human faeces may be found among the waste.

Plastics in the form of bags and bottles make up a relatively small proportion of residential waste, but these present a major problem. Plastics do not degrade (or degrade at a very slow rate), and so they tend to accumulate in the streets and in watercourses. Plastic bags can also choke farm animals and wildlife if they are ingested (Figure 7.2).

Figure 7.2  Waste plastic bags are a health hazard for grazing animals.

7.2  Classification of solid wastes

There are several different ways of classifying solid waste. As you have seen, one way is to classify it by where it is generated. Another way is based on whether the waste is biodegradable or not.

Biodegradable solid wastes are those thatcan be broken down (decomposed) into their constituent elements by bacteria and other micro-organisms. Food waste, manures and waste from producing crops are the main biodegradable wastes. If the decomposition process takes place in the absence of air (anaerobically), methane gas can form. Methane is a powerful greenhouse gas and can explode if enough of it accumulates and an ignition source (such as an electrical spark) is present. The decomposition may also produce offensive and irritating smells.

However, controlled anaerobic decomposition can produce biogas – a useful fuel for heating, cooking and even power generation that you learned about in Study Session 5– as well as fertilisers and soil conditioners. Waste that decomposes in the presence of an adequate air supply (aerobically) under controlled conditions can produce compost, which can be used to improve the quality of soils.

Non-biodegradable (also sometimes called inorganic) solid wastes are those thatdo not decompose by microbial action. These wastes include plastic containers, scrap metal, food and drink cans and plastic bags.

Materials in solid wastes can also be classified as combustible or non-combustible, depending on whether they will burn or not.

Depending on the inherent dangers associated with its physical and chemical properties, solid waste can be classified as either hazardous or non-hazardous. Hazardous wastespose substantial or potential threats to public health or the environment. For example, toxic, infectious and corrosive (acidic or alkaline) substances are all likely to be classed as hazardous. Non-hazardous wastes are those that do not possess hazardous characteristics, although they can still be harmful to people or the environment. (Hazardous waste is described in more detail in Study Session 10.)

7.3  Composition of residential solid waste

The composition of solid waste means the different types of waste material that it consists of and their characteristics. As an urban WASH worker you may need to understand the factors that affect the composition of solid waste in your area. These factors include the season of the year, the habits or culture of the community, people’s educational or economic status and the geographical location.

Returning to the Bahir Dar survey mentioned earlier, Table 7.1 shows the major components of municipal solid wastes generated in Bahir Dar according to the 2010 study.

Figure 7.3  Waste from an Ethiopian kitchen.

The composition of solid waste affects the length of time that it can be safely stored before disposal. If kept uncovered for too long, some waste will become a health hazard or cause some other nuisance. The recommended time for storage is shown in Table 7.2 for typical Ethiopian conditions where the waste is stored at normal temperatures in a container that is not sealed.

7.3.1 Chemical composition of solid wastes

Knowing the chemical composition of solid waste is very important when planning for different waste management options. Key aspects of chemical composition are:

• moisture content – the percentage of water in the waste
• ash content – the amount of material remaining after burning the waste
• heat content – also known as calorific value, which is the amount of heat energy produced when the waste is burned.

Full analysis could also identify the different chemical elements present in the waste.

In practice, the composition of a mixture of wastes is found by separating the components and testing each different material to find out its moisture, ash content and chemical composition, and combining the values to get the overall composition of the waste.

The moisture content of municipal solid wastes varies depending on the season of the year, humidity and weather conditions, as well as on the composition of the waste. It is calculated by weighing an amount of waste, drying it in an oven which causes the moisture to evaporate, then weighing it again. The mass lost in the drying process is equivalent to the moisture content and is calculated as a percentage of the initial mass. Table 7.3 shows typical moisture content of the major solid waste components generated in Bahir Dar during the 2010 study. As you can see from the data, organic wastes such as food, paper and garden wastes have high moisture contents.

Analysing waste to determine its chemical composition and calorific value is highly specialised work that needs laboratory equipment to prepare the samples and then carry out the analyses. As an urban WASH worker you are unlikely do this type of work, but you should be aware that the results can be used to assess the suitability of a waste for composting, anaerobic digestion, recycling and energy recovery.

7.4  Quantities of residential waste

The total quantity of waste generated by a town or city is obviously directly related to the size of population. The amount of solid waste produced in a given area is usually expressed in terms of tonnes per day (or tonnes per month or year) and sometimes as kilograms per person per day. (You may see this written as ‘per capita waste generation’.) For some purposes, the volume of waste, expressed in cubic metres (m³), produced in a given time period may be reported.

Countrywide average rates of waste generation in most industrialised countries lie between 0.8 and 1.4 kg per person per day. In developing countries, the average generation rate is more likely to be in the range of 0.3 to 0.5 kg per person per day. Reliable information is not widely available, but Table 7.4 shows the values of different solid waste generation rates in different towns and cities in Ethiopia based on separate studies.

In areas where the waste is collected (perhaps by the kebele authorities) it is necessary to know how much is produced to make sure that the bins and skips for the waste are large enough. If the waste is then transported to a disposal site, the information on the amount of waste produced can be used to find out how many trips per week the transport vehicle will have to make.

Example

A small town produces an average of 1.5 m³ of waste per day and the accumulated waste is to be taken in a truck to a disposal site a few kilometres away once a week. If a storage container can hold 1.1 m³ of waste, how many containers will be needed?

Answer

The amount of waste produced each week is

The number of containers required =

It only makes sense to have a whole number of containers, so the town would need ten containers.

The generation rate data allows current waste collection and treatment needs to be planned, and also helps to predict the amounts that may be seen in the future.

7.5  Handling and on-site storage of solid wastes

In some areas, residential waste is taken from a collection point to the disposal site by the kebele authority or by a contractor employed by the authority. In other places the householder either takes their waste to a communal disposal point or disposes of it in a pit in their yard. This is also true for commercial and industrial organisations.

Unless the waste is placed in a pit as soon as it is produced, there will be a need for some kind of storage. Storage is the first stage of the waste management process. It is important because poorly handled and stored waste can be sources of nuisance, flies, smells and other hazards. The other stages of the waste management process are covered in Study Sessions 8 to 11.

7.5.1  Types of storage containers

According to the Solid Waste Management Proclamation (FDRE, 2007), solid wastes should be stored indoors in a closed container that is animal-proof and insect-proof. The containers should also be washable and strong enough to withstand normal day-to-day treatment. The container should be emptied every day into an outdoor storage container or directly into a disposal pit.

However, a recent situation assessment of waste management in 28 Ethiopian cities and towns indicated that 88% of the households use sacks to collect solid waste in their household (Addis Continental Institute of Public Health, 2015). So clearly, the majority of households store their solid waste in a container that does not meet the standard (Figure 7.4).

Figure 7.4  The most common types of containers used for on-site storage of solid wastes.

Where waste is collected for disposal, householders have to empty their waste containers into larger communal containers. These containers should also be fitted with lids to keep insects, other animals and rainfall out. An ideal container is shown in Figure 7.5, but open containers known as skips are often used (Figure 7.6).

Figure 7.5  A communal waste storage container.

Figure 7.6  Communal waste skip awaiting collection to be emptied. Regular emptying is essential to avoid the situation shown here.

7.6  Commercial and industrial solid wastes

As explained in Section 7.1, commercial wastes are those produced from businesses such as food and drink establishments, shops, banks and by public administration offices. These wastes contain similar materials to residential waste, although the proportions may vary. For example, a restaurant will produce more food waste than a normal household and an insurance office will produce more paper and less food waste.

There are also many industrial facilities in Ethiopia that process agricultural products such as cotton, flour, hides and skins. Other important industries include plastic and resin manufacturing, textiles, cement, metallurgical, foods, general chemicals and pharmaceuticals. All these industries manufacture useful products and contribute to the country’s economy but, at the same time, they can also be a major contributor to the country’s solid waste and pollution problems.

The composition of the waste produced by industry depends very much on the nature of the industry concerned. For example, animal hide processing produces large amounts of biodegradable waste (animal parts), while the construction industry produces a lot of excavated soil, rock and demolition waste (bricks, stones, wood, glass, etc.). For this reason, industrial waste is usually processed and disposed of by the industry itself, often using specialised technologies. As explained in Section 7.2, these wastes can be classed as either hazardous or non‑hazardous depending on the inherent dangers associated with their physical and chemical properties.

As you have seen from Figure 7.1, industrial and commercial organisations only produce a small proportion of a city’s waste. Because of this, less attention is given to these wastes and the amounts produced are not known accurately. However, for urban WASH workers, it is useful to understand the characteristics of the solid waste generated from these sources in order to:

• advise workers on the potential health and environmental hazards of handling the solid waste
• give advice on the transportation, treatment, and disposal systems needed
• develop precautions and procedures to protect people during collection and disposal
• understand and determine which of the solid wastes generated in any particular industry can be managed along with the household and commercial wastes.

7.7  Healthcare wastes

Healthcare waste is any waste produced by healthcare activities. It is sometimes known as medical waste, hospital waste or infectious waste. The main sources of healthcare waste are hospitals, health posts, health centres and clinics. Dental surgeries, veterinary surgeries and cosmetic establishments (ear-piercing and tattoo parlours) also produce some healthcare waste. Healthcare waste consists of both hazardous and non-hazardous waste.

Around 75–90% of the waste produced by healthcare establishments is general commercial waste. This must be kept separate from the hazardous material at all times until it is finally disposed of. The remaining 10–25% is hazardous and can contain sharps (syringe needles, knives and other surgical instruments), blood, other body fluids, human organs and tissues, used dressings, drugs, other chemicals and possibly radioactive substances.

This waste is potentially hazardous to health in a number of ways. Sharps can cause physical harm and provide a way into the body for pathogenic micro-organisms. Much of the waste contains pathogens, which can cause many types of infections. Of particular concern is the risk of transmission of the human immunodeficiency virus (HIV), which causes AIDS, and the hepatitis A and C viruses.

Healthcare waste must be collected, transported and treated under carefully controlled conditions. If not, it will present a serious risk to everyone in the community. We will be looking at the disposal of healthcare waste in more detail in Study Session 12.

Summary of Study Session 7

In Study Session 7, you have learned that:

1. The major sources of solid wastes in Ethiopia in urban areas are residential areas, commercial areas, institutions and street sweepings.
2. Solid waste can be classified by source or by its characteristics including whether or not it is biodegradable, combustible or hazardous.
3. The composition of municipal waste varies with location, season and the habits and economic status of the community.
4. Food wastes, papers, plastics, textiles, street sweepings, wood and charcoal, glass and ashes are the major solid waste types generated in Ethiopian towns and cities.
5. Waste, especially organic waste, can become a health hazard if stored for too long.
6. Analysis of moisture content, ash content and heat content of wastes are important for planning waste management options.
7. Assessment of the quantities of wastes generated are needed to plan for waste collection and disposal schemes.
8. On-site waste storage requires closed containers that are emptied frequently.
9. Commercial solid waste has a similar composition to residential waste. The nature of industrial waste depends on the type of industry.
10. Healthcare wastes should be considered hazardous and need special handling and disposal procedures.

Self-Assessment Questions (SAQs) for Study Session 7

Now that you have completed this study session, you can assess how well you have achieved its Learning Outcomes by answering these questions.