SUSTAINABLE PLASTIC WASTE MANAGEMENT A CASE OF ACCRA, GHANA. MICHAEL MENSAH WIENAAH

SUSTAINABLE PLASTIC WASTE MANAGEMENT – A CASE OF ACCRA, GHANA.

MICHAEL MENSAH WIENAAH

April, 2007

TRITA-LWR Master Thesis ISSN 1651-064X LWR –EX-07-10

Michael Mensah Wienaah

TRITA LWR Masters

SUSTAINABLE PLASTIC WASTE MANAGEMENT – A CASE OF AACRA, GHANA

MICHAEL MENSAH WIENAAH

S UPERVISOR : Dr Erik Levlin

EXAMINATOR & SUPERVISOR

Associate professor Jan-Erik Gustafsson

Stockholm 2007 TRITA-LWR Master ISSN 1651-064X LWR-EX-07-10 ii

Sustainable Plastic Waste Management- A Case of Accra, Ghana

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ACKNOWLEDGEMENTS I am extremely grateful to my supervisors Dr Erik Levlin and Associate Professor Jan-Erik Gustafsson for their invaluable support, guidance and direction. It is through their diligent guidance that this thesis work ended successfully. I specially express my profound gratitude to Associate Professor Jan-Erik Gustafsson for showing personal interest in this thesis work. I also thank Mr. Manoj Lakhiani, managing director of Blowplast Ltd for his online contribution to this research work and not forgetting Mr. Francis Javier Vilaplana, a PhD student at the department of Polymer Technology, KTH. To all my other colleagues in the EESI program, I say thank you for your support and encouragement. Lastly, my since gratitude goes to my parents, brothers and sisters whose support brought me this far. I specially dedicate this thesis work to my beloved mother for her invaluable contribution to my education. May God richly bless her.

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TABLE

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CONTENT

Acknowledgements ..................................................................................................................................................... iv Table of Content .......................................................................................................................................................... vi Abstract ......................................................................................................................................................................... 1 Introduction....................................................................................................................................................... 1

1 1.1 1.2 1.3 1.4 1.5 1.6 1.7

Background and statement of the problem ...................................................................................................... 1 Purpose and Objectives of the Study ............................................................................................................... 2 Scope of the Study .......................................................................................................................................... 2 Literature Sources............................................................................................................................................ 2 Limitation of the Study.................................................................................................................................... 2 Justification of the Study ................................................................................................................................. 3 Description of the Study Area ......................................................................................................................... 3 plastic materials and production ...................................................................................................................... 3

2

The History of Plastics .................................................................................................................................... 3 What are Plastics ............................................................................................................................................. 4 Types of Plastics.............................................................................................................................................. 5 2.3.1 Polyethylene (PE)................................................................................................................................ 5 2.3.2 Polypropylene (PP).............................................................................................................................. 5 2.3.3 Polystyrene (PS) .................................................................................................................................. 5 2.3.4 Polyvinyl chloride (PCV). .................................................................................................................... 6 2.4 Identifying the types of Plastics ....................................................................................................................... 6 2.5 Plastic Waste ................................................................................................................................................... 7 2.6 Sources of waste plastics.................................................................................................................................. 8 2.6.1 Industrial waste ................................................................................................................................... 8 2.6.2 Commercial waste ............................................................................................................................... 8 2.6.3 Municipal waste................................................................................................................................... 8 2.7 Hazardous effects of Plastics ........................................................................................................................... 8 2.7.1 Polluting Substances............................................................................................................................ 8 2.7.2 Air pollution........................................................................................................................................ 9 2.1 2.2 2.3

Plastics Waste Recycling Processes............................................................................................................... 10

3 3.1 3.2 3.3

Mechanical Recycling .................................................................................................................................... 10 Feedstock or Chemical Recycling .................................................................................................................. 10 Energy Recovery ........................................................................................................................................... 10 Initial Upgrading Techniques........................................................................................................................ 12

4

Collection...................................................................................................................................................... 12 Cleaning ........................................................................................................................................................ 12 4.2.1 Washing ............................................................................................................................................ 12 4.2.2 Drying............................................................................................................................................... 12 4.3 Sorting .......................................................................................................................................................... 12 4.3.1 Manual sorting................................................................................................................................... 12 4.1 4.2

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4.3.2 4.3.3

Density-based sorting methods.......................................................................................................... 13 Sorting by selective dissolution .......................................................................................................... 13

Size Reduction Techniques ............................................................................................................................ 13

5 5.1 5.2 5.3

Cutting .......................................................................................................................................................... 13 Shredding...................................................................................................................................................... 14 Agglomeration............................................................................................................................................... 14 Further Reprocessing Techniques................................................................................................................ 17

6

Pelletizing...................................................................................................................................................... 17 6.1.1 The Pelletizing Process...................................................................................................................... 17 6.1.2 Quality improvement......................................................................................................................... 17 6.2 Product Manufacturing.................................................................................................................................. 18 6.2.1 Extrusion .......................................................................................................................................... 18 6.2.2 Injection Moulding ............................................................................................................................ 22 6.2.3 Blow moulding .................................................................................................................................. 23 6.2.4 Film Blowing..................................................................................................................................... 23 6.1

Importance and bottlenecks of Plastic Recycling......................................................................................... 23

7

Importance.................................................................................................................................................... 23 7.1.1 Environmental .................................................................................................................................. 23 7.1.2 Economic.......................................................................................................................................... 25 7.1.3 Social................................................................................................................................................. 25 7.2 Bottlenecks.................................................................................................................................................... 25 7.2.1 Technological .................................................................................................................................... 25 7.2.2 Market............................................................................................................................................... 25 7.1

Conclusion and Recommendations ............................................................................................................... 26

8 8.1 8.2

Conclusion .................................................................................................................................................... 26 Recommendations......................................................................................................................................... 26

Reference..................................................................................................................................................................... 27 9

Appendix.......................................................................................................................................................... 28

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Sustainable Plastic Waste Management- A Case of Accra, Ghana

A B S T RA C T Sustainable Solid Waste Management is a critical problem not only for developing countries but for the developed countries as well. Considerable amount of waste is generated in the municipality of Accra, Ghana due to increased urbanization. There is indiscriminate littering of the city of Accra and its environs with plastic waste. This research discusses sustainable ways of managing plastic waste in Accra, Ghana in order to minimize their adverse environmental impacts. The need for such a study is justified as it is desirable to change the unsustainable pattern of consumption, production and disposal associated with these materials. Plastic waste especially ‘sachet water bags’ appear in very high proportion in the municipal solid waste stream in Accra and is causing environmental problems such as choking of animals and soils; blockage of waterways and rivers; blight of landscapes and trees; and resource depletion. Of the various options considered, mechanical recycling was deemed appropriate because it is less expensive and does not demand special expertise or skills for implementation. It is also best suited for developing countries and Accra, Ghana, is no exception. Recycling was also preferred to the other methods of waste management since it has the potential of leading to resource recovery and the creation of jobs for the unemployed. The various mechanical recycling processes such as initial upgrading techniques, size reduction techniques to product manufacturing are all elaborated in this thesis. The theoretical background of the problem and that of possible remedies was investigated from literature sources. Experiences of other developing countries on the issue were studied and incorporated into this thesis work. The compiled information and suggestions are expected to be helpful in managing the plastic waste menace in Accra, Ghana, and the related challenges due to unsustainable patterns of consumption, production and disposal. Key words: Cutting, Shredding, Agglomeration, Recycling, Depolymerisation, Pellets, Extrusion, Injection moulding, Blow moulding and Film blowing.

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friendly compared to the other methods of waste disposal. Through recycling of Plastic Waste, we can have material and energy recovery and therefore value will be derived from the waste instead of regarding it as garbage or trash. According to a study conducted in Accra, Ghana by GOPA Consultants in 1983, Plastic Waste accounts for 1-5% (of net weight) of the total amount of waste generated (Lardinois and Van de Klundert, 1995). Since then, there has been a tremendous increase in plastic waste particularly sachet water bags due to increase urbanization and consumption pattern. The Accra Metropolitan Assembly (AMA) comprises of five administrated districts. The various modes of solid waste disposal by these districts are shown in Table 1(see appendix). The rather unreliable statistics released by the AMA Waste Management Department and other waste management bodies indicate that about 9000 tonnes of waste is generated daily, out of which 315 tonnes are plastic related (Amankwah, 2005). In Ghana, drinking water comes in plastic bags and not bottles. The public have developed a strong taste for such sachet water since it is portable and can easily be carried from one place to another. There is also a perception that such sachet water is cleaner and more mineralized than tap water. After gulping down the liquid content, these bags are discarded indiscriminately thereby littering the whole environment. These bags now constitute a major proportion of the plastic waste generated throughout the country. Also over the years, plastics have replaced leaves, glass and metals as a

INTRODUCTION

1.1 Background and statement of the problem Sustainable Solid Waste Management is a crucial problem not only for developing countries but for the developed countries as well. Enormous amount of Waste is generated through out the world and the most crucially posed question is how to manage these wastes effectively and efficiently to save the environment and the continuous existence of mankind. Many municipalities, cities and towns continue to grapple with the problem of Solid Waste Management and the Municipality of Accra, Ghana is no exception. The Organic component of Municipal Solid Waste may not be too much of a problem since that is biodegradable. However, the Plastic Waste component of the Municipal Solid Waste is quite problematic because this is non-biodegradable and therefore can stay in the environment for a considerable length of time causing all sorts of problems. The management of Plastic Waste through combustion (incineration) is not environmentally friendly and sustainable since this may release carbon dioxide, a major contributor to global warming (greenhouse effect). Landfilling with Plastic Waste is not also desirable since plastic is nondegradable and no economic value would have been derived from the waste in that case. The best option for Sustainable Plastic Waste Management is through recycling. This is because the benefits of recycling of Plastic Waste are numerous and also environmentally

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cheaper and more efficient means of packaging (IRIN, 2006). Soon after usage, these are randomly discarded. They then collect around the city, choking gutters, threatening small animals, damaging the soil and polluting beaches. Almost all the major gutters in Accra are currently choked with plastic waste and this has resulted in floods, loss of property and in Ghana recording high rate of malaria and cholera even in the 21st century. The whole country is gradually being swallowed up by the plastic waste menace that the Ghana government had to declare a recycling war on plastic waste in 2004. This is what was said by the minister for Local government, ‘we expect recycling to create a healthy environment for tourists, create jobs and save foreign exchange in imports of drugs to fight cholera and malaria that may result from the rubbish heaps’ (IRIN, 2006). However, very little has been yet done in this area. Hence the earlier the plastic waste menace problem is tackled the better it would be for the environment and sustainable livelihood.

Solid Waste Management all over the world is a complex one. There is a proposed waste management hierarchy which is shown in Figure 1. However, the main focus of this research work is on how to manage the plastic waste menace in Accra which is the administrative capital of Ghana through recycling. Recycling was chosen because it has numerous advantages over the other modes of waste disposal and comparatively less capital intensive. Plastic waste is only a component or fraction of the municipal solid waste generated in the city of Accra, Ghana. There are three recycling processes namely mechanical recycling, feedstock/chemical recycling and incineration/energy recovery. This thesis work however adopted mechanical recycling since that is more appropriate for a developing country such as Ghana. The thesis therefore covers the initial upgrading techniques, size reduction techniques, reprocessing techniques through to the final product manufacturing. An integrated waste management approach as shown in the waste hierarchy is however the preferred choice for an efficient and effective waste management for any country. Though the viability and feasibility of recycling would depend on a large extend to the availability of potential markets for the reprocessed products, that is not discussed within this thesis work. However considering the income levels of majority of the populace, it is obvious that reprocessed products would attract high demand.

1.2 Purpose and Objectives of the Study The over-all objectives of this thesis work are three fold. •

To investigate the actual situation of plastic waste management in Accra.

•

To investigate various ways of handling plastic waste in the municipality of Accra.

•

To identify and propose future sustainable plastic waste management in Accra. This thesis work will also serve as a working document for policy makers and as an exemplarily way of turning garbage into wealth (money) and therefore providing jobs to the urban poor. It is hoped that this document shall be useful to other countries particularly in subSaharan Africa where plastic waste is engulfing all the major cities and causing all sorts of environmental problems. 1.3

1.4 Literature Sources. Literature information for this thesis was gathered from diverse sources. A lot of information was obtained or collected through the internet from different sources such as journals, technical reports on international research work on plastic waste recycling, press releases on recycling and findings of research centres and pilot projects. A lot of usual information was obtained from some Asian countries particularly India which is a developing country such as Ghana. Also, important information and recycling techniques were obtained from countries such as Turkey, Egypt and South Africa. These are countries with similar characteristics such as Ghana and therefore it is worthwhile learning from their experiences. Majority of the terminologies and techniques on recycling and the practical demonstrations were drawn from the research work of WASTE Consultants on Plastic Waste.

Scope of the Study

1.5 Limitation of the Study The major limitation is the fact that the author was unable to visit recycling companies for first hand experience. This was due to lack of funds for a trip to Ghana. Much information was obtained through online

Figure 1. The waste management hierarchy. Source: Porteus, A., 2005

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Sustainable Plastic Waste Management- A Case of Accra, Ghana correspondence and with the visit of my co- supervisor to Ghana. It was not also possible to assess the market base for reprocessed plastic products in Ghana though that to a large extend determines the viability and feasibility of recycling. Plastic recycling in Ghana is at a very infant stage and therefore not much information would be obtained and comparisons made. It was not also easy obtaining accurate data from the various departments concerned. Most of the statistics given are very doubtful due to the fact that most of the plastic wastes remain uncollected on the streets. 1.6 Justification of the Study Plastic waste all over the world has become problematic. Plastic bag waste has already become a serious environmental dilemma in Ghana in general and in Accra in particular. Concern has been expressed from many stakeholders including the current president, Mr. John Agyekum Kuffour, various government organizations, environmental NGOs and the public at large. Plastic waste manufactures and importers were challenged to provide alternative ways for disposing waste or face a temporarily ban on plastic manufacturing and importation. Mr. Stanley Adjiri Blankson, Chief Executive of the Accra Metropolitan Assembly, who gave the warning, explained that plastic manufacturers and importers had an alternative of using biodegradable materials that were environmentally friendly (GNA, 2005). There is therefore the need to find a solution to this problem to safe the environment and human health. This research is essentially meant to contribute to the ongoing endeavours in Africa and the world at large to bring about a pattern of sustainable consumption and production of plastic products and plastic bags in particular. It is the author’s hope that this thesis or research work will contribute to finding a sustainable way of handling the plastic waste menace in Accra and indeed the country and beyond. The earth’s natural resources are also fast dwindling and adopting recycling of plastic waste into new products will safe the available scarce resources from being depleted faster.

Figure 2. Map of Ghana (Ghanaweb.com)

which are non-existent. Currently the population of Accra is estimated to be around two million people. This has led to increased waste generation and consequently waste management problems. Urbanization also has its attended environmental implications. Figure 2. Shows a map of Ghana with the red mark indicating the study area. 2

P L A S T I C M A T E RI A L S A N D PRODUCTION

2.1 The History of Plastics From a historical viewpoint, the development of plastics can be regarded as one of the most important technical achievements of the twentieth century. In just 50 years plastics have permeated virtually every aspect of daily life, paving the way for new inventions, and replacing materials in existing products. The success of these materials has been based on their properties of resilience, resistance to moisture, chemicals and photoand biodegradation, their stability, and the fact that they can be moulded into any desired form (Lardinois and Van de Klundert, 1995). The original breakthrough for the first semi- synthetic plastics material, cellulose nitrate, occurred in the late 1850s and involved the modification of cellulose fibres with nitric acid. Cellulose nitrate had many false starts following its invention by a Briton, Alexander Parkes, who exhibited it as the world’s first plastics in 1862. The world’s first plastics were produced at the turn of the twentieth century, and were based mainly on natural raw materials. Only in 1930 were thermoplastics, made

1.7 Description of the Study Area Accra was founded as a fishing village by the Ga in the 16th century. But after being chosen by the British as the seat of their administration in the late 19th century, it began to grow very rapidly (Yankson and Gough, 1999). Accra is now the administrative capital of Ghana. It consists of five administrative districts namely Accra Metropolitan Assembly (AMA), Ga, Tema, Dangme West, and Dangme East. In recent decades, there has been influx of people from the rural areas in search of white colour jobs

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Figure 3. Development of plastics production worldwide

from the basic materials styrene, vinyl chlorine and ethylene, introduced onto the market. But the main growth of the plastics industry did not take place before the 1960s, reaching a peak in 1973, when production reached over 40 million tonnes per year (Saechtling, 1987). Following a temporary drop in production during the oil crises and the economic recession in the beginning of the 1980s, the world production of plastics continued to increase to approximately 77 million tonnes in 1986 (Saechtling, 1987), and 86 million tonnes in 1990(Schouten and Van der Vegt, 1991). Figure 3 shows the rapid development of plastics production worldwide which now far exceeds the combined production of non-ferrous metals such as aluminium, zinc, lead and copper.

ethylene, propylene, styrene and vinyl chloride are linked together to form a chain called a polymer. Polymers such as polyethylene (PE), polystyrene (PS) and polyvinyl chloride (PVC) are the end products of the process of polymerization, in which the monomers are joined together. In many cases only one type of monomer is used to make the material, sometimes two or more. A wide range of products can be made by melting the basic plastic material in the form of pellets or powder (Warmer Fact Sheet, 1992). Plastics can be either thermoplastics or thermosets. Materials that repeatedly soften on heating and harden on cooling are known as thermoplastics. They can be melted down and made into new plastic end products. Thermoplastics are similar to paraffin wax. They are dense and hard at room temperature, become soft and mouldable when heated, dense and hard again and retain new shapes when cooled (see Figure 4a for a schematic overview of the structure of thermoplastics). This process can be repeated numerous times and the chemical characteristics of the material do not change. In Europe, over 80% of the plastics produced are thermoplastics (Warmer Fact Sheet, 1992).

2.2 What are Plastics Plastics are man-made organic materials that are produced from oil and natural gas as raw materials. Plastics consist of large molecules (macromolecules), the building blocks of all materials. The molecular weights of plastics may vary from about 20, 000 to 100,000 mg/L. Plastics can be regarded as long chains of beads in which the so-called monomers such as

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Sustainable Plastic Waste Management- A Case of Accra, Ghana

Figure 4. The structure of (a) thermoplastic and (b) thermosets Source: Nijenhuis, 1988

2.3.1

Thermosets, on the other hand are not suitable for repeated heat treatments because of their complex molecular structures (see Figure 4b). The structure of thermosetting materials resembles a kind of thinly meshed network that is formed during the initial production phase. Such materials cannot be reprocessed into new products unlike thermoplastics. Thermosets are widely used in electronics and automotive products. The properties of plastics can be modified by a number of substances known as additives.

Polyethylene (PE)

The two main types of polyethylene are low-density polyethylene (LDPE) and high-density polyethylene (HDPE). LDPE is soft, flexible and easy to cut, with the feel of candle wax. When very thin it is transparent, when thick it is milky white, unless a pigment is added. LDPE is used in the manufacture of film bags, sacks and sheeting, blow-moulded bottles, food boxes, flexible piping and hosepipes, household articles such as buckets and bowls, toys, telephone cable sheaths, etc. HDPE is tougher and stiffer than LDPE, and is always milky white in colour, even when very thin. It used for bags and industrial wrappings, soft drinks bottles, detergents and cosmetics containers, toys, jerry cans, crates, dustbins, and other household articles.

2.3 Types of Plastics In industrialized countries, literally hundreds of plastic materials are available commercially. In economically less developed countries however, fewer types of plastics tend to be used. In both economically less developed and industrialized countries, the four types of plastics that are most commonly reprocessed or recycled are polyethylene (PE), polypropylene (PP), polystyrene (PS) and polyvinyl chloride (PVC). Each of these can be subdivided according to their density, the type of process involved in their manufacture, and the additives they contain. These four types are briefly described below. For a more extensive list of common recyclable plastics and their characteristics, see Table 2 in appendix.

2.3.2

Polypropylene (PP)

Polypropylene is more rigid than PE, and can be bent sharply without breaking. It is used for stools and chairs, high-quality home ware, strong mouldings such as car battery housings, domestic appliances, suitcases, wine barrels, crates, pipes, fittings, rope, woven sacking, carpet backing netting surgical instruments, nursing bottles, food containers, etc.

2.3.3

Polystyrene (PS)

In its unprocessed form, polystyrene is brittle and usually transparent. It is often blended (copolymerized) with other materials to obtain the desired properties. 5

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High-impact polystyrene (HIPS) is made by adding rubber. Polystyrene foam is often produced by incorporating a blowing agent during the polymerization process. PS is used for cheap, transparent kitchen ware, light fittings, bottles, toys, food containers, etc.

2.3.4

and cannot be scratched. Also, very thin material made of any polymer may seem flexible, very thick may seem rigid (Vogler, 1984). 3. Flotation test. This test can be used to disentangle larger quantities of mixed or shredded polymers, as well as to separate them from non-plastics. The test is also useful for making the complicated distinction between PP and HDPE, and between HDPE and LDPE. When placed in a tube of water and alcohol in certain proportions (this can be tested using a "hydrometer", with arrange of 0.9-1.0) the materials will separate according to their density; one material will sink and the other will float. For example, in a mixture with an exact density of 0.925, the PP will float and HDPE will sink; in one with a density of 0.93, LDPE will float and HDPE will sink. Note, however, that the flotation test is not exact enough to distinguish between PP and LDPE, since their densities can overlap as shown in Table 2 (see appendix). In this case the fingernail test and the visual appearance of the material may be more conclusive indicators. Another flotation test using pure water and salt can be used to distinguish between PS and PVC, both of which sink in pure water. When a specific amount of salt is added to the water, the PS will float to the surface, while the PVC and dirt will remain on the bottom of the container. The amount of salt need not be measured, but may be determined by experience. 4. Burning test. This test is carried out as follows (Vogler, 1984). Cut a 5 cm long sliver of the plastic material, 1 cm wide at one end, and tapering to a point at the other. Hold the sample over a sink or stone, and light the tapered end. The colour and smell of the flame can be used to tell the type of polymer. PVC can be confirmed by touching the sample with a red hot copper wire and returning the wire to the flame; it should burn with a green flame. Burn of all residues before repeating the test with the same wire. Caution: When conducting this test, be sure to hold the sample at a safe distance from the body and clothing, since the melted material may drip and burn if it falls directly from the flame. Do not breathe in the smoke, since it may contain dangerous substances. Figure 5 shows a demonstration of the burning test. In economically less developed countries, particularly in the informal sector, polymers are usually identified by manual/visual inspection, whereas in industrialized countries, mechanical separation techniques are used.

Polyvinyl chloride (PCV).

Polyvinyl chloride is a hard, rigid material, unless plasticizers are added. Common applications for PCV include bottles, thin sheeting, transparent packaging materials, water and irrigation pipes, gutters, window frames, building panels, etc. If plasticizers are added, the product is known as plasticized polyvinyl chloride (PPVC), which is soft, flexible and rather weak, and is used to make inflatable articles such as footballs, as well as hosepipes and cable coverings, shoes, flooring, raincoats, shower curtains, furniture coverings, automobile linings, bottles, etc. Other types of plastics include polycarbonate (PC), polyethylene terephthalate (PET), polyurethane (PU) and nylon or polyamide (PA). 2.4 Identifying the types of Plastics When recycling plastics it is essential that the materials are correctly identified. If not, this can create severe problems during reprocessing, leading to products with a poor appearance and impaired mechanical properties. It is usually difficult to tell exactly which type of plastic is present solely from the type of product. Many different types of plastics may look identical, or one type of plastic may appear to have several physical and chemical characteristics depending on the type of additive that has been used. Detailed chemical tests, such as infrared analysis, may be needed to make a definite identification of a polymer. However, experience in this field can be gained with practice, and in case of doubt, testing is the only option. Some simple tests using basic equipment can provide adequate information for identification. In Istanbul, for example, some reprocessors claim to be able to distinguish plastics by touch (Konings, 1989), but when in doubt, they apply the "burning test" or the "flotation test. 1. To make a general distinction between thermoplastics and thermosets, heat a piece of wire just below red hot and press it into the material. If it penetrates the material, it is a thermoplastic; if it does not, it is a thermoset. 2. The type of plastic can be identified by scratching it with a fingernail or from the flexibility of the material. However, these tests are not always reliable. For example, PE that has been exposed to all kinds of weather conditions may have become rigid and brittle,

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Figure 5. Demonstration of the burning test Source: WASTE consultants

Technology is also becoming available to sort plastics using instrumental analytical methods, such as infrared spectroscopy and thermal analysis. Even in Europe and North America, the recovery of household plastics is burdened with several problems, including the high costs of separation and the general low level of purity of the waste materials. The packaging industry, for example, uses more than 60 different kinds of plastics. These are often mixtures or combinations of plastics and other materials, which preclude the melting option, as uncontrolled mixing of different kinds of plastics leads to inferior properties of the resulting material. The plastics may also be contaminated with residues of the packaged product, particularly food, or other packaging material (paper, aluminium). Even elaborate sorting and cleaning procedures cannot resolve these problems satisfactorily. A number of measures have been proposed to reduce the number of different kinds of plastics, combined with the introduction of an effective system for coding such plastics during their manufacture. These measures, which would certainly make identification easier, are now gaining general acceptance and changes in the packaging of some products, for example using less material or only one type of material, are slowly becoming apparent (Halbekath, 1989). To facilitate identification, in the United States, the Society of Plastics Industry (SPI) has developed a model coding system (using numbers combined with the abbreviations PE, PP, etc.), which is now also being introduced in Europe (see Figure 6). This coding system is especially suitable for moulded products where the coding can be engraved onto the moulds. In this way, households will be able to identify and separate the various types of plastics before disposal.

Figure 6: American SPI coding system Source: APME and PWMI

which it originates. There are a lot of plastic manufacturing industries in Accra, Ghana. Also Accra is heavily populated due to rural-urban migration. There is heavy influx of people from the rural areas in search of “white colour jobs” which are non-existent. All these people finally settle in the city contributing immensely to the waste problem. Basically, there are two types of plastic waste that is generated in Accra, Ghana namely primary and secondary waste. A distinction between these is relevant for recycling/reprocessing. Primary waste plastics are generated within the plastics producing and goods manufacturing industries themselves. A characteristic of primary waste is that the quality of plastics recovered for reprocessing is almost as high as that of virgin plastics. The waste is pure and suitable for reprocessing with standard equipment into the same kind of products manufactured from virgin materials. The processing of primary waste into products with characteristics similar to those of the original products is called primary recycling (Ehrig, 1992). Primary plastic waste is usually homogeneous and therefore its recycling is comparatively economical and easier. The term “secondary waste” refers to waste plastics from sources other than the industrial ones. This type of plastic waste is enormous in Accra, Ghana due to the consumption and littering habits of the inhabitants. These plastic wastes are impure, i.e. they may be contaminated and often consist of mixtures of various types of plastics. The direct reprocessing of such mixed

2.5 Plastic Waste The quantity and composition of the solid waste generated by a society provide a mirror that reflects among others the cultural habits of the population. The amount of solid waste generated is also closely related to the overall economic level of the population from 7

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2.6.3

plastics/supplies is called secondary recycling and results in products with poor mechanical properties because of the different characteristics of the plastics they contain. The potential for marketing these materials is relatively low. 2.6 Sources of waste plastics As becomes clear from section 2.2, plastics can be used for many purposes, and thus, waste plastics are generated from a wide variety of sources. The main sources of plastic waste in Ghana can be classified as follows: industrial, commercial and municipal waste.

2.6.1

Industrial waste

Industrial waste and rejected material (so-called primary waste) can be obtained from large plastics processing, manufacturing and packaging industries. Most of this waste material has relatively good physical characteristics, i.e. it is sufficiently clean, since it is not mixed with other materials. It has been exposed to high temperatures during the manufacturing process which may have decreased its characteristics, but it has not been used in any product applications. Many industries discard polyethylene film wrapping that has been used to protect goods delivered to the factory. This is an excellent material for reprocessing, because it is usually relatively thick, free from impurities and in ample supply. Many industries may provide useful supplies of primary waste plastics: •

The automotive industries: spare-parts for cars, such as fan blades, seat coverings, battery containers and front grills.

•

Construction and demolition companies: e.g. PVC pipes and fittings, tiles and sheets.

2.7

2.7.1

Hazardous effects of Plastics

Polluting Substances

In terms of environmental and health effects it is important to differentiate between the various types of plastics. Most plastics are considered nontoxic (PVC is an important exception). Polyethylene (PE) and polypropylene (PP), for example, are inert materials (Mewis, 1983), but it should be realized that plastics are not completely stable. Under the influence of light, heat or mechanical pressure they can decompose and release hazardous substances. For example, the monomers from which polymers are made may be released and may affect human health. Both styrene (which is used to make polystyrene, PS) and vinyl chloride (used to make PVC) are known to be toxic, and ethylene and propylene may also cause problems (Beumer, 1991). The environmental effects of plastics also differ according to the type and quantity of additives that have been used. Some flame retardants may pollute the environment (e.g. bromine emissions) .Pigments or colorants may contain heavy metals that are highly toxic to humans, such as chromium (Cr), copper (Cu), cobalt (Co), selenium (Se), lead (Pb) and cadmium (Cd) are often used to produce brightly coloured plastics. Cadmium is used in red, yellow and orange pigments. In most industrialized countries these pigments have been banned by law. The additives used as heat stabilizers (i.e. chemical compounds that raise the temperature at which decomposition occurs), frequently contain heavy metals such as barium (Ba), tin (Sn), lead and cadmium, sometimes in combination (Nagelhout, 1989).

•

Electrical and electronics industries: e.g. switch boxes, cable sheaths, cassette boxes, TV screens, etc. This type of plastic waste is not common in Ghana. Plastics processing industries in Ghana sometimes recycle the waste they generate but this is relatively very low. Considerable amounts of waste plastics generated by many industries remain uncollected or end up at the municipal dump. Industries are often willing to cooperate with private collecting or reprocessing units.

2.6.2

Municipal waste

Waste plastics can be collected from residential areas (domestic or household waste), streets, parks, collection depots and waste dumps. In Ghana, considerable amounts of plastic waste can be found within the Municipal Solid Waste stream due to the littering habit of the population. The most common type of plastic waste within the municipal waste stream is the “sachet” water film bags that are discarded indiscriminately soon after consuming its contents. In Asian countries in particular, the collection of this type of waste is widespread. However, unless they are bought directly from households, before they have been mixed with other waste materials, such waste plastics are likely to be dirty and contaminated. Sometimes the plastics can be separated and cleaned quite easily, but contamination with hazardous waste is not always visible and may be more difficult to remove. Litter that has been waiting for collection for some time may have been degraded by sunlight. This is mainly a superficial effect, however, and does not always mean that the plastics cannot be reprocessed.

Commercial waste

Workshops, craftsmen, shops, supermarkets and wholesalers may be able to provide reasonable quantities of waste plastics for recovery. A great deal of such waste is likely to be in the form of packaging material made of PE, either clean or contaminated. Hotels and restaurants are often sources of contaminated PE material.

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Sustainable Plastic Waste Management- A Case of Accra, Ghana

Industrial t

commercial t

agricaltural t

Manicipal t

collection cleaning sorting Size reduction Sorted clean plastic pieces

Extrusion Strands/strings Pelleting Pellets

Extrusion

Injection moulding

Pipes,tubes

Miscellaneous

Blow moulding Bottles

Film blowing Bags, sheets

Figure 7: Flow chart of a typical waste plastics processing stream in a low-income country .Source: WASTE Consultants.

From the heavy metals mentioned, lead and cadmium are the most serious environmental pollutants, and have different effects on human health, depending on their concentrations. When present at or above specific concentrations, they interfere with processes in plant and animal tissues, and in the soil. Plastics such as PVC may also have serious impacts on the environment because they contain a number of hazardous substances. For example, PVC contains chlorine which can be released during heating as hydrochloric acid (HCl). Other potentially hazardous substances in PVC include the relatively large quantities of additives such as plasticizers (up to 60%) and heat stabilizers (sometimes up to 3%) (Nagelhou, 1989). In the opinion of some environmental and consumer organizations in Western Europe, the use of PVC and other plastics containing chlorine (or bromine), especially for packaging, should be halted entirely. Apart from the afore mentioned effects of waste plastics, the waste plastic water sachets are discarded randomly after usage. These then scatter around the city, choking drains, threatening small animals, damaging the soil and polluting beaches. Plastic waste has had a terrible impact on tourism, particularly on the

beaches east of Accra where rain water carries the waste. Almost all the major gutters in Accra are currently choked with plastic waste and this has resulted in floods, loss of property and in Ghana recording high rate of malaria and cholera even in the 21st century.

2.7.2

Air pollution

Taking into consideration the process of plastic recycling, the most important environmental problem caused by the (afore mentioned) polluting substances is air pollution, either within the reprocessing units or in the open air. During the extrusion process several substances such as additives, may be released. Since PE and PP do not contain large amounts of additives, potential problems with PE and PP are far less than with PVC. While extruding PVC additives may be released, but also vinyl chloride and HCl. It is very common to see plastic waste being burnt in Ghana. However, unless the combustion is complete, burning plastics release considerable quantities of polluting substances. The incomplete combustion of PE, PP, PS and PVC can cause further problems, as CO and smoke may be produced. As a result of incomplete combustion of PVC also dioxins and other hazardous substances may be formed. The burning of

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plastics releases CO2 which is a major contributor to the global warming problem. 3

14.8% in 2002 and 2003 respectively of total plastic waste recovered (APME, 2002-2003). This technique is also well suited for developing countries since it is less cost-intensive compared to the others. It is mechanical recycling that is currently being employed in Accra, Ghana to recycle about 50% of the plastic waste that is generated in the city daily.

PLASTICS WASTE RECYCLING PROCESSES

Plastics recycling or reprocessing is usually referred to as the process by which plastic waste material that would otherwise become solid waste are collected, separated, processed and returned to use (Lardinois and Van de Klundert, 1995). Figure 7 shows an online of the waste plastics reprocessing stream in Accra, Ghana and other economically less developed countries. Developing an efficient and cost-effective method for recycling waste plastics that have served their intended purpose, retrieving them from the waste stream and getting them back into the manufacturing process requires collection, sorting and cleaning and finally reclamation. For homogeneous plastic waste streams recycling by mechanical (or physical) methods is the economically preferred recovery option. Heterogeneous plastic waste streams however are more efficiently treated or handled by chemical and thermal processes, for recovery of basic chemicals and /or energy (GaikerIVL and KTH, 2005). These processes are briefly discussed below.

3.2 Feedstock or Chemical Recycling Chemical recycling or feedstock recycling means that a polymeric product could be broken down into its individual components (monomers for plastics or hydrocarbon feedstock – synthesis gas) and that these components could then be fed back as raw material to reproduce the original product or others. Feedstock recycling include chemical depolymerisation (glycolysis, methanolysis, hydrolysis, ammonolysis etc), gasification and partial oxidation, thermal degradation (thermal cracking, pyrolisis, steam cracking, etc), catalytic cracking and reforming, and hydrogenation. Besides conventional treatments (pyrolisis, gasification), new technological approaches for the degradation of plastics, such as conversion under supercritical conditions and co processing with coal are being tested (Aguado and Serrano, 1999). This technique of recycling is however not suitable for developing countries. This is because it requires a lot of expertise, capital intensive and is quite cumbersome. Even in the developed countries, it is still under development and is being practiced by only a few companies. A number of companies have successfully developed and demonstrated technologies many of which can process mixed plastics streams. There has been some renewed interest in other areas of feedstock recycling, such as the depolymerisation of PET or treatment of PVC to make chemicals which can then be used in the production of new plastics (APME, 20022003).

3.1 Mechanical Recycling Mechanical recycling is the material reprocessing of waste plastics by physical means into plastics products. The sorted plastics are cleaned and processed directly into end products or into flakes or pellets of consistent quality acceptable to manufactures. The steps taken to recycle post-consumer plastics may vary from operation to operation, but typically involve inspection for removal of contaminants or further sorting, grinding, washing and drying and conversion into either flakes or pellets. Pellets are made by melting down the dry plastic flakes and then extruding it into thin strands that are chopped into small, uniform pieces. The molten plastic is forced through a fine screen (filter) to remove any contaminants that may have eluded the washing cycle. The strands are cooled, chopped into pellets and stored for sale and shipment. Different plastics may also under different reforming conditions such as different processing temperatures, the use of vacuum stripping, or other procedures that could influence contaminant levels. During the grinding or melting phases, the reprocessed material may be blended with virgin polymer or compounded with additives. Mechanical recycling is the preferred recovery route for homogeneous and relatively clean plastics waste streams, provided end markets exist for the resultant recyclate. It is the second largest recovery technique after energy recovery in Europe representing 13.6% and

3.3 Energy Recovery Plastics are almost all derived from oil and plastic wastes is a waste with a high calorific value. Energy recovered from plastic waste can make a major contribution to energy production. Plastics can be coincinerated with other wastes or used as alternative fuel (e.g. coal) in several industry processes (cement kilns). The energy content of plastic waste can be recovered in other thermal and chemical processes such as pyrolisis. As plastic waste is continuously being recycled, they loss their physical and chemical properties at their endof-life cycle. Continuous recycling could lead to substandard and low quality products. Hence it would no longer be economically profitable to recycle any longer. Incineration with energy recovery would be the economically preferred option at this stage. Table 3(see appendix) shows the Lock – In Potential (LIP) rating

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Sustainable Plastic Waste Management- A Case of Accra, Ghana

Small reprocessing industries

Initial reprocessors/scrape dealers

Dumpsite contractors

Waste pickers

Small traders

Scrap dealers

Itinerant waste buyers

Municipal waste

Commercial waste

Industrial/Agricu ltural waste

Figure 8 Informal plastics recycling network in Istanbul (Source: Adapted from Konings, 1989)

energy recovery capabilities in countries across Western Europe (APME, 2002-2003). This technique of recycling if not developed to the highest level can result in emissions which will pollute the atmosphere and also contribute to the current global warming issue. There is an exemplary incineration plant in Austria that is worthy of emulation. It has been developed with such high technology that emission levels are very low and conforms to EU directives on climate change. Numerous other examples also abound in Sweden. Ghana as a developing country may not be able to adapt it now for lack of capital and technological knowhow.

for a range of common plastics compared with that of conventional fuels (Horrocks, 1996). It can be seen for a range of common plastics compared with that of conventional fuels (Horrocks, 1996). It can be seenfrom the above figure that the energy of 1 litre of diesel oil = LIP of 1 kgm of polyolefin. In 2003, 4,750,000 tonnes of post-user plastics waste collected in Western Europe was reclaimed through energy recovery. This represented 22.5% of total collectable waste plastics and means energy recovery remains the most common recovery route for post-user plastics waste in Western Europe. Capacity expansions and new incineration plants have led to an increase in

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Michael Mensah Wienaah

4

TRITA LWR Masters a paddle. If the waste is greasy, hot water with soap, detergent (e.g. from scrap detergent bottles) or caustic soda should be used (Lardinois and Van de Klundert, 1995). The waste plastics can also be washed mechanically. At Blowplast, the waste plastics are washed mechanically. In the mechanical washing installation, a water-filled basin is equipped with a motor that drives a set of paddles at low speed. The plastic materials are left to soak for several hours, while they are stirred continuously by the paddles. Dirt (mainly sand) settles out during the process, and the clean plastic material is removed with a drainer. All waste plastics need to be washed, except for some rejected materials from industrial and commercial sources. The washed and unwashed fractions should be kept separate from non-plastics and dirt.

INITIAL UPGRADING TECHNIQUES

4.1 Collection Waste plastics from municipal sources (i.e. refuse containers and waste dumps), are collected by hand and are roughly pre-selected by waste pickers or primary traders. This stage is labour-intensive and requires little capital investment. There are several points within a municipal solid waste system where waste can be retrieved for recovery: at source, i.e. directly from private homes; from waste bins; from refuse collection vehicles; and at municipal waste dumps (Cointreau, 1984). In general, the nearer to the source, the less mixed and dirty will be the materials. In Ghana, the only medium scale recycling company, Blowplast Limited has an organized network of about 100 people engaged in collecting plastic waste sachets. They supply the company’s 14 trucks that regularly pick up the waste in the various areas of Accra. The plastic wastes are transported to Tema where they are stored in a depot or warehouse that belongs to the company. The company collects between 7-8 tonnes of waste plastics sachet per day, but the capacity of the recycling plant is 24 tonnes. The company pays 2000 cedis per kilo of waste plastic sachet. One kilo contains about 200 plastic waste sachets. Some people can collect up to 200 kg per day. Hence some collectors have become so rich that they have also organized people below them to collect the plastic sachets. An organizational network such as the one shown below could be adapted if the recycling process is well developed. Figure 8 shows an informal plastics recycling network in Istanbul.

4.2.2

Washing and drying waste plastics are not separate activities but tend to be carried out within the same unit. As with washing, plastics waste can be dried either manually or mechanically. With the manual method the plastics are spread out in the sun to dry, and turned regularly. Plastic films can be hung on lines and thus require only half the area normally used when plastics are spread out to dry. At Blowplast, the drying process is carried out mechanically. A water drier, which in principle is a thermal drying machine at 700C, is used to dry the washed shredded plastic waste. 4.3 Sorting While plastics waste can be recycled in mixed form to make plastic lumber products, separated plastics have higher values and are preferred by most reclaimers.The degree of sorting of plastics waste varies considerably, depending on the demand and the special wishes of the manufacturers to whom it will be sold. The waste plastics may be sorted at any stage in the recycling process, according to colour, type of plastic, etc. The sorting stage is therefore crucial in plastic recycling and for that reason available sorting techniques are described below.

4.2 Cleaning The cleaning stage consists of washing and drying the plastic items. A number of these techniques are described in the following, together with some illustrative examples of the cleaning processes that are used in a number of cities.

4.2.1

Drying

Washing

4.3.1

It is important that the waste plastics are washed, because clean waste materials fetch better prices and they improve the quality of the end product. The plastics can be washed at various stages of reprocessing: before, after, or even during sorting. Films and rigid materials are usually cleaned before the size reduction stage. Foreign materials such as glued paper labels are also removed. Rigid plastics are often washed a second time after they are shredded. The plastic waste material can be washed manually or mechanically. Manual washing may be done in oil drums that have been cut in half, in bath tubs or in specially built basins, and the water may be stirred with

Manual sorting

Manual sorting of waste plastics is the identification of different materials by people with a “trained eye” while the materials pass by them on a moving conveyor (Scheirs, 1998). The materials are identified by the idcodes and by the different characteristics of the plastics that distinguishes it for visual identification. At Blowplast in Ghana, the sorting is done manually using a flat conveyor with about five boys on each side who try to look for metals, stones, etc. But sorting of materials is not necessary as Blowplast only recycles one specific grade of material – PE (HDPE+LDPE/LLDPE). There are also metal 12

Sustainable Plastic Waste Management- A Case of Accra, Ghana selectivity, achieved by high speed rotation, because the equipment can produce a centrifugal field of 1000-1500 times higher than the acceleration accomplished due to gravity.

detectors attached to the conveyor which rejects ferrous and non-ferrous metals. Manual sorting techniques can be used where the plastic components are large enough to justify the time and effort involved, since the method is very labour intensive, has bad working environment and is economically unviable. The possibility of human errors should not be neglected. The materials are used for low value applications (Scheirs, 1998).

4.3.2

4.3.3

Sorting by selective dissolution

Sorting by selective dissolution is based on batch dissolution of mixed plastics using solvents. A complete separation of the plastics can be obtained by careful control of temperature and selection of solvent. The same solvent can be used for separation of PS, LDPE, HDPE, PP and PVC, because these plastics dissolve at different temperatures. When the plastic mix is added to the solvent tank, PS dissolves almost immediately. The PS solution is drained and another hotter batch (750C) of solvent is added dissolving LDPE. The solution is drained again and a batch of 1200C warm solvent is added and HDPE dissolves and so on. If PVC and PET are to be separated, a mixture of solvents is used in which PVC dissolves at a lower temperature than PET (Gaiker-IVL and KTH, 2005). The advantages of this method are that: individual plastics can be separated from complex mixtures, contaminations such as dirt or soil or food residues do not cause any problems, labour requirement is minimal and the recycled plastics are chemically and functionally equivalent to the virgin plastics. The disadvantage of this technique is the amount of solvent used, even though most of the solvents are recycled within the process. It is also important to control the levels of residual solvent in the recycled plastic and to restabilise the separated material since additives are extracted during dissolution (Baldisimo, 1985). Other sorting techniques are: Optical sorting, Spectroscopic-based sorting, Electrostatic sorting and sorting by differential melting temperature.

Density-based sorting methods

Sorting by density technique is carried out in a floatsink tank or hydrocyclone. This method however is poor for polyolefins as these have very similar densities. It is also impossible to separate PVC and PET, since their specific gravities overlap. The density can be altered by different fillers in the materials, which makes it difficult to have a complete separation (Tall, 2002). In the float-sink separation, the plastics are placed in a fluid that has a density in-between the materials making it possible for less dense materials to float and the heavier to sink. Common fluids used are: water for the separation of polyolefins from other plastics. Water/methanol mixtures for separation of plastics with lower specific gravities, NaCl solutions and ZnCl2 solutions for plastics with higher specific gravities (Scheirs,1998). Float baths can be arranged in a series, with each bath set at a desired specific gravity to sort the materials. Pumps provide circulation and direct the flow. The problems with this method are that the separation can be slow, difficult to control and give low-purity products. To achieve good separation, long retention times are required to allow the flakes to settle. Since the method uses gravity for separation, it is essential that the sizes of the material flakes are equal throughout the mix. One of the advantages of this separation is that before the plastic mixture is introduced to the separation fluid, the collected materials are exposed to wet grinding, where the paper labels and dirt particles are removed (Scheirs, 1998). The hydrocyclone uses the principle of centrifugal acceleration to separate plastic mixtures. The mixed plastics waste is separated first from polyolefins, then polystyrene and finally PVC and other materials. Plastics that are hard to separate with float-sink method, such as PE from PP or PS from a mixture of PVC and nylon, can be sorted by using an appropriate medium in the centrifuge. Dirt and paper labels are pulled off in the process. The technique can selectively separate, wash and dewater plastic flakes from a mixture of plastics waste materials. The apparatus used is a double-cone, solid bowl screw centrifuge. This achieves efficiency of over 99.5% purity. The separation is fast and has a high

5

SIZE REDUCTION TECHNIQUES

Size reduction techniques such as cutting, shredding and agglomeration of the waste plastic articles serve to increase the density of the material. This "densification" helps to reduce transport costs, and the smaller pieces can be more easily fed into further reprocessing machines. There are many variations of the techniques and procedures that can be applied. These are described below with some illustrative examples from economically less developed countries. 5.1 Cutting The first step in the process of plastics waste transformation involves cutting up the waste plastic materials into smaller pieces. This is needed for items such as jerry cans, plastic bottles and buckets which are too large to fit into the hopper of the shredder. These

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Figure 10. Shredder with a horizontal axis in Istanbul Source: Waste consultants

shows a schematic overview of the interior of a shredder, which can have either a horizontal or a vertical axis. Figure 12 shows the inside of a shredder equipped with two rotating cutting blades, one of which is visible on top of the rotor. On the right, attached by three bolts, the adjustable blade can be seen. At the bottom of the drum is a grid with holes that determine the size of the final pieces. The cutting continues until the pieces are small enough to fall through the grid. The end products of shredding are irregularly shaped pieces of plastics that can then be sold to reprocessing industries and workshops. Figure 13 shows shredded PE produced from plastic water sachets, detergent bottles, jerry cans, and other waste plastic containers. The materials have been sorted according to colour, in this case white. If the waste plastics have not already been washed, the shredded pieces may then be washed at this stage to remove any dirt or dust. Depending on the quality and type of raw material, and the desired quality of the end product, different types of plastic waste may be mixed to a certain extent.

Figure 9. Cutting of plastics with bandsaw Source: WASTE Consultants

items can be cut first with a circular saw or with a bandsaw, as shown in Figure 9. The cut pieces either fall to the floor to be collected later, or are thrown directly into a washbasin before being fed into the shredder. Uncut soft plastics tend to stick in the spiral screw of an extruder and therefore usually not fed directly into this machine. In Manila, soft plastics such as films and sheets are cut into 5cm strips with ordinary scissors to prevent damage to the palletizing machine (CAPS, 1992). In Cairo, the sorted and washed plastics are cut into small pieces with special scissors fixed on a wooden base. It is estimated that three labourers, each using a pair of scissors can cut up one tonne, of sorted plastics per day (EQI, 1991).

5.3 Agglomeration Agglomeration is the coalescing of small particles into a clump. It is not advisable to feed soft plastic waste, such as bags and sheet plastics, directly into a shredder or extruder. Preferably, an agglomerator should be used to cut, pre-heat (or pre-plasticize) and dry these plastics. Agglomeration improves the quality of the final product. Also, it will increase the density of the material, which results in a more continuous flow of material in the extruder and thus, in an increase of efficiency. The agglomerator is filled through the lid at the top and its contents are emptied into bags via the valve below. The plastic waste fed into the agglomerator should be clean, since all foreign objects will be processed together with the plastics, and will be evident in the partially plasticized materials. They can only be removed during the extrusion process. Figure 14 shows an agglomerator in use in Istanbul. In such a machine, mechanical energy produced by the rotation of the cutting blade at high speed is transformed into heat through friction.

5.2 Shredding The shredding process consists of feeding the cut plastic pieces into a shredder. The clean pieces selected according to product form, plastic type and colour are fed into the hopper on top of the shredder as shown in Figure 10 below. In Ghana, the rotating cutting blades of the shredder then shred the plastic sachets into pieces of 50 mm in diameter. When the pieces are small enough, they fall through a grid into a tray. In the shredder shown above, the rotating blades are driven by an electric motor located behind the machine; the belt transmission is visible on the left. A bag or piece of cloth covers the hopper to prevent pieces of plastic being thrown back by the rotating blades. The shredded material is scooped into bags from the tray to be stored, or is fed directly into an extruder. Figure 11 14

Sustainable Plastic Waste Management- A Case of Accra, Ghana

Figure. 11 Shows the interior of a Shredder with a horizontal axis Source: Vogler, 1984

Figure 12: The rotor and cutting blades of a shredder. Source: WASTE Consultants.

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Figure 13: Shredded PE Source: WASTE Consultants

Figure 14: An agglomerator in Istanbul Source: WASTE consultants

Figure 15: Inside of an agglomerator showing the cutting blade and remains of fine- cut film. Source: WASTE Consultants.

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Sustainable Plastic Waste Management- A Case of Accra, Ghana

6.1.1

Figure 16: Agglomerated PE film Source: WASTE Consults

The bulk density of the raw materials in the agglomerator increases through shrinkage and partial plasticization. When the material is cooled rapidly at this stage, it solidifies as it is being cut, resulting in coarse, irregularly shaped grains, often called crumbs. Figure 15 shows the inside of an agglomerator, a cutting blade with the remains of fine-cut film and Figure 16 shows the resulting product, in this case agglomerated PE film 6

The Pelletizing Process

Shredded rigid plastic objects or agglomerated films are subjected to the process of extrusion and pelletizing to produce plastic pellets. These can then be used as the input materials for various moulding processes. Besides plastics, the process is also used to produce such diverse materials as pasta (spaghetti) and some metals. Figure 17 shows an example of an extruder in India. The main parts of the extrusion phase are: compounding (mixing) the various substances, homogenization, compression, degassing, plasticization and melt filtration. The pieces of plastic raw materials (compounded with any desired additives, such as pigments) are fed into the hopper of the extruder. Figure 18 gives a schematic overview of the pelletizing process. The materials are picked up from the hopper by a rotating screw, and are forced down the barrel to the extrusion die head. Heat from friction and the heating elements fitted around the barrel cause plasticization and the special geometry of the screw compresses the material. Electric heaters, water or air coolers are fitted around the barrel to control the temperature. Just before the materials reach the extrusion die head, they are forced through a filter screen to remove any solid particles. The spaghetti-like plastic strings that emerge from the extrusion die head are then cooled by passing them through a basin of water or a ventilator. The strings supported by rollers placed at the end of the water basin, are then drawn by a mechanical system into the pelletizer. Figure 19 shows the strings being extruded from a machine in Turkey. The die head showed here produces 24 strings to be cut into pellets. It is tilted in such a way that a large part of each string is under water. Figure 20 shows an example of a pelletizer.The pelletizer chops the strings into short, uniform, cylindrical pellets that are ready for use in manufacturing processes. The plastic waste generated by this process can be extruded again. The production capacity of the pelletizing process depends on the size of the extruder that is used.

FURTHER REPROCESSING TECHNIQUES

Pelletizing and product manufacturing are the final steps in the plastics recycling process. These processes require that the waste plastics have first been sorted according to plastic type, and that they have been cut into small, relatively uniformly sized pieces. Shredded and agglomerated materials can be used directly for product manufacturing processes (except the pelletizing stage), although this is not usually done. Normally, the shredded and agglomerated waste plastics are pelletized first. This way, the quality of the moulded end products can be improved. The use of pellets also increases the efficiency of the product manufacturing process, due to the lower bulk density of shredded and agglomerated waste plastics compared to pellets. The most common moulding processes in lowincome or less economically developed countries are extrusion, injection moulding and blow moulding. Film blowing, the last technique described, is used in the manufacture of plastic bags.

6.1.2

Quality improvement

The quality of the pellets and thus of the final manufactured products can be improved by adding the following steps:

6.1 Pelletizing Pelletizing is the process of melting and extruding small, clean pieces of plastics into small regularly shaped pellets.

17

•

Virgin plastic pellets may be added at a ratio depending on the desired quality of the end product. The higher the percentage of virgin material, the higher will be the quality.

•

If shredded rigid plastics are pre-heated in a drying installation, the resulting pellets will be higher in quality.

Michael Mensah Wienaah

TRITA LWR Masters

Figure 17: Extruder in Bombay, India. Source: WASTE Consultants

•

posts, 100% waste plastics can be utilized. With other finer items, such as fishing nets, only a minimal amount of waste plastics can be used in the form of shredded or agglomerated waste plastics as well as pellets (CAPS, 1992). At Blowplast in Ghana however, film blowing is the only product manufacturing process that is under taken. The pellets obtained from recycling the plastic waste are consumed or used at the plastic bag making factory.

If the quality of the pellets is not high enough for the manufacture of consumer articles, the pellets may be extruded a second time through a finer filter screen. This reduces the moisture content of reprocessed pellets that have been cooled in a basin of water and therefore increases its quality.

•

An extruder equipped with a ventilator to release humid hot air, reduces the porosity of the pellets and thus improves their quality. In Manila, a classification system is used to denote the purity and quality of plastic pellets: Triple A (the highest quality), double A, single A, B and C. The prices of the pellets differ accordingly (CAPS, 1992).

6.2.1

6.2 Product Manufacturing A number of mechanical manufacturing processes are used by recycling enterprises to produce particular final products, including: •

Extrusion (piping and tubing);

•

Injection moulding (miscellaneous products);

•

Blow moulding (bottles); and

Extrusion

Extrusion is the process of forming continuous shapes by forcing molten plastic through a shaped die by means of pressure. Extrusion moulding is similar to the extrusion process preceding the pelletizing process described in Section 6.1.1, except that the end product is a continuous, parallel stream of plastic such as tubing. This is made by a special die - a steel plate pierced with a hole that determines the shape of the product. The extruded material is cooled and solidified in air, in a water bath, or on a chilled drum, before being wound onto a reel or cut into straight lengths. The principle of the technique is shown in Figure 21. Figures 22 and 23 shows the process of manufacturing soft PVC tubing which has two extrusion cycles. In the first cycle, shredded PVC scrap is used to produce a string of undefined shape. During this cycle, the moisture content of the material is reduced and the material is filtered and compounded with additives or pigments. The second cycle uses the shredded strings

• Film blowing (plastic bags). Usually the type of product and the demands on physical properties will determine the ratio of reprocessed to virgin plastics that can be used. For massive final products, such as furniture and (fence)

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Sustainable Plastic Waste Management- A Case of Accra, Ghana

Figure 18: Extruder with a pelletizer Source: Vogler, 1984

Figure 19 Extruder used in Turkey. Source: WASTE Consultants.

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Figure 20: A pelletizer Source: WASTE Consultants.

Figure 21: The principle of extrusion moulding Source: Vogler, 1984.

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Sustainable Plastic Waste Management- A Case of Accra, Ghana

Figure 22: Feeding extruded soft PVC into a shredding machine (first cycle). Source: WASTE Consultants

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TRITA LWR Masters resulting from the first cycle to produce tubing. Again the moisture content is reduced, the material is filtered and a die is put on the extruder to form the end product

6.2.2

Injection Moulding

The injection moulding process is similar to that of extrusion, except that the molten plastic is forced from the barrel through a nozzle into a strong, split steel mould, as shown in Figure 24.The rotating screw conveys the plastic pellets or powder forward and the heating elements plasticize it. The screw then stops moving, allowing the melt to accumulate in the front part of the barrel. When an adequate amount has

Figure 23: Soft PVC tubing being extruded (second cycle). Source: WASTE Consultants.

Figure 24: The principle of injection moulding Source: Vogler, 1984.

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Sustainable Plastic Waste Management- A Case of Accra, Ghana

6.2.4

Film Blowing

Film blowing is technically the most complicated of the product manufacturing processes. Various techniques are used. The process of making garbage bags for example shown in Figure 27 is as follows. After extrusion from a tubular die, the product, in this case a thin tube, moves upward to a film tower that contains a collapsing frame, guide rolls and motor-driven pull rolls. Compressed air is passed through the centre of the die and inflates the tube. The outside surface is cooled by air from an air ring mounted above the die. When the tube has passed through the pull rolls, it is sealed and cut to form the bag. For this process only high-quality pellets can be used as the raw material. 7

I M P O R T A N C E A N D B O T T L E NE C K S O F P L A S T I C R E C Y C LI N G

7.1 Importance The benefits of recycling can be categorized into environmental, economic and social. These are briefly explained below.

7.1.1

Figure 25: Shoe soles being removed from the mould. Source: WASTE Consultants

accumulated, the screw moves forward again, pushing the melt into a closed steel mould. The mould is kept cool so that the material quickly solidifies. The mould is then opened, the product is removed, and the mould is then made ready for the next amount of melt. The shape of the mould determines the type of product produced. Figure 25 shows a mould used for the production of flexible PVC shoe soles.

6.2.3

Environmental

There is only one environment and it must be treated with the respect it deserves. If raw materials have already been extracted then it makes sense to use them again if possible. This means that reserves last longer into the future. Moreover, recycling of plastic waste conserves natural resources, particularly raw materials such as oil and energy. The more that is recycled, the longer will natural resources be available for future generations. It means that there is less environmental impact due to mining, quarrying, oil and gas drilling, deforestation and the likes. If there are fewer of these operations, the environment will be safe from continuous destruction and degradation. Another positive effect of recycling on the environment is that it may reduce emissions of substances such as carbon dioxide (CO2) into the atmosphere. From ‘lifecycle’ analysis of reprocessed plastics and virgin plastics, it is known that the emissions of CO2, SO2, NOX (NO and NO2) are much smaller for recycled plastics compared to that for virgin materials (Lardinois and Van de Klundert, 1995). Hence the environment will be better safe from air pollution and global warming if recycling is adopted on large scales. Recycling of plastic wastes will also safe both ground and surface waters from pollution. This is because if discarded randomly, they choke gutters and even find their way into water bodies that serve as sources of drinking water for communities and towns. They also help to breed leachate that can seep into the ground thereby contaminating groundwater bodies as well.

Blow moulding

The term "blow moulding" is used to describe the process of producing hollow articles such as bottles, where the tops or bottoms are narrower than the body itself. The process is similar to the one used in blowing glass objects. The principle of the process, which takes place in two stages, is shown in Figure 26. First, a piece of plastic tube or "parison" is extruded, and is then transferred to a split mould with the shape of the final product. The mould is then closed around the parison. Compressed air is blown into the open end to expand the parison to the shape of the mould. The formed shape is allowed to cool until the finished object solidifies, which is then ejected from the mould and the cycle is repeated. The production capacities of the blow moulding machines used in Cairo, for example, vary between 100 and 200 kg of final products per day, depending on the power of the motor, which can range from 10 to 15 HP (EQI, 1991).

23

Michael Mensah Wienaah

TRITA LWR Masters

Figure 26: The principle of blow moulding. Source: Vogler, 1984.

Figure 27: The production of garbage bags. Source: WASTE Consultants

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Sustainable Plastic Waste Management- A Case of Accra, Ghana

7.1.2

Economic

presence of impurities require an extensive cleaning and separation pre-treatments prior to reprocessing. Some studies have been done in order to evaluate to what degree separation brings value to waste plastics as raw materials for new products (Gaiker-IVL and KTH). There is also still no known technology for the recycling of mixed plastic wastes. The plastic waste may contain a large number of hazardous compounds that makes difficult the recycling processes, such as degradation products of additives, brominated flame retardants, etc. It is therefore necessary to develop techniques to identify, quantify and extract these products from plastic prior to recycling them. Some efforts have been done in this direction (Gaiker-IVL and KTH). The optimal solution to the plastic waste management problems will be provided by the conjunction of the different treatment alternatives available (i.e. mechanical recycling, feedstock recycling and energy recovery). Therefore, it is necessary to achieve a good integration between all the aspects of the recycling activity (collection and separation of the waste streams, characterization methods for recycled plastics, identification of the optimal treatment technique, etc)

Resource recovery reduces the quantity of raw materials needed in production processes. The reuse of plastics may therefore help to reduce the dependence on imported raw materials and to save foreign currency. Due to increasing cost of virgin plastics as a result of dwindling oil reserves, the use of reprocessed pellets for product manufacture will save recycling companies from folding up as a result of high cost of importation of virgin pellets. According to Mr. Manoj Lakhiani, the managing director of Blowplast Ltd, a plastic manufacturing company in Ghana, that is one of his main reasons for establishing a recycling company. The funds that accrue as a result can then be channeled into other areas that could lead to higher profits. The University of Nottingham in the UK has recently conducted ‘life-cycle’ analyses of materials consumption, energy use and emissions for both virgin and recycled low-density polyethylene (LDPE), a type of plastic. From this study it can be concluded that the use of reprocessed pellets in the production of plastic bags saves around 70% in energy use and 90% in water use, compared to the use of pellets made of virgin material (Lardinois and Van de Klundert, 1995). The low energy and water consumption will save recycling companies from paying huge bills that could otherwise have adverse effect on their operations.

7.1.3

7.2.2

Social

Recycling of plastic wastes helps to keep the environment clean. Therefore diseases associated with filth will be prevented and this will save foreign exchange in the importation of drugs to fight cholera and malaria that may result from the rubbish heaps. Recycling will also create a healthy environment for tourists attraction Recycling is a source of job creation. Through recycling, numerous poor people will get employed particularly at the collection stage and hence be able to earn their living. This will help raise social standards and to eliminate vices in society. 7.2 Bottlenecks Plastic waste recycling has increased the world over and has been largely successful. Nevertheless, much more effort must be done in order to reach the waste management requirements that our society needs in terms of sustainable development. There are still some difficulties that the plastic recycling industry must overcome regarding technological bottlenecks and those of demand from end-markets for the recycled materials.

7.2.1

Market

The potential demand for plastic recyclates by end market is determined by two factors. These are market acceptance, i.e. whether consumers are willing to accept recycled products based on the ‘image’ of recyclates or on health and safety requirements and technical acceptance, i.e. the need to assess the desired performance of products and suitability for manufacturing processes. Regarding these factors, the following general bottlenecks may be considered. Quality assessment is a matter of significant importance in order to guarantee a suitable usage of recycled plastic materials in further applications, since the properties of the recyclates must be specified and guaranteed within narrow tolerances by the manufactures according to the needs of their customers. Some researchers have cited three key properties in order to guarantee the quality of the recycled materials. These are degree of mixing (composition), degree of degradation and the presence of low-molecular weight compounds (degradation products, contaminants, etc). There are no standardized methods for measuring the critical properties of recycled plastic materials. This goes a long way to affect the quality of recycled plastic products and hence their marketability. Characterization, test methods and specifications of plastic families should be made putting the stress on the identification and quantification of significant properties and their critical values for specific recycled groups depending on their source and their foreseen end applications (Gaiker-IVL and KTH). There is also a mismatch between the availability of potential plastic waste material and demand of recycled plastic products

Technological

Development of automatic and effective separation methods for mixed plastic waste streams continues to be a problem. The complex nature of the plastic waste streams in terms of polymer composition and the

25

Michael Mensah Wienaah

TRITA LWR Masters

due to a probable mistrust of the consumers for the properties of the recyclates. This inconvenience is related to the above mentioned requirements of methods for quality assessment and standardization of the properties of the recycled materials and to the lack of information and appropriate marketing for the use of post-consumer products. 8

efficient waste management however, there should be an integrated approach as depicted in the waste management hierarchy. 8.2 Recommendations. For recycling to achieve its intended purpose of contributing to waste management in Accra, the following recommendations might be helpful. Environmental consciousness is certainly of paramount importance. This is where well designed and continued public awareness campaigns and education will be useful. People must be educated on the need to protect and preserve the environment. Environmental by-laws on improper littering and illegal dumping must be strictly enforced. Offenders must be reprimanded to serve as detriment to others. In this vain, the AMA should endeavour to provide litter bins at vantage points for people to drop in their waste. The plastic manufacturing companies in Accra should be made to contribute some finances towards the plastic waste management in the city. A levy can be placed on each water sachet bag or polyethylene bag produced. All such monies can be channeled to the AMA waste management department who can then employ people to clean up the waste plastics on the streets. These can then also be sold to the recycling companies for use in their production processes. Redemption points can be established for presentation and collection of the used plastic sachet bags and polyethylene bags for a token. In this case, the waste plastic sachets become money and people will be willing to deposit this for some cash. Thus a thrown away sachet bag or polyethylene bag becomes money and could be collected by anybody and redeemed for cash. With this strategy, more people would be engaged in plastic waste collection thereby cleaning up the environment. Mechanical recycling is a major consumer of energy but Ghana is currently experiencing energy crisis resulting in load shedding or power rationing. Hence for mechanical recycling to be successful, the current energy situation must be addressed and recyclers given subsidies to enable them stay in business. In this light, I recommend future studies of energy recovery from plastic waste to determine which one is the economically viable option for plastic waste management in Accra.

CONCLUSION AND RECOMMENDATIONS

8.1 Conclusion From the viewpoint of environmental health management, the collection and disposal of waste is usually considered to be the responsibility of government or municipal institutions. However, municipalities in many low-income countries are often unable to cope with the ever-growing quantities of waste because of inadequate public funds, increasing populations, the lack of equipment and spare parts, and often poorly trained staff. It is against this background that informal resource recovery (recycling) needs to be supported in order to improve existing practices and to integrate it within municipal solid waste management systems. The various recycling possibilities enumerated in this thesis work particularly mechanical recycling, should be incorporated at both the implementation and policy levels of municipal waste management. For recycling to achieve its intended purpose, government authorities must play an important role in the promotion and viability of plastics reprocessing activities not only by their approaches to local waste management but also by the economic policies they adopt. For example, import regulations on virgin pellets may determine the viability and feasibility of recycling. Mechanical recycling is a major consumer of energy and giving subsides to recycling companies on their energy consumption, will boost their profit margins and enable them stay in business for a longer time. The AMA which is the body responsible for waste management in the city of Accra also needs to provide public reclamation facilities at vantage points for residence to drop their waste in. In this way, source separation could be encouraged or enhanced and that would go a long way to boost recycling. It is my candid hope that if the recycling processes outlined in this thesis work are implemented, the plastic waste menace and its environmental impacts in Accra, Ghana will be minimized if not eliminated. For an effective and

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Sustainable Plastic Waste Management- A Case of Accra, Ghana

R E F E R E NC E Aguado, J. and Serrano, D. (1999). Feedstock Recycling of Plastic wastes. RSC Clean Technology Monographs. Royal Society of Chemistry, Cambridge. Association of Plastics Manufacturers in Europe (APME) (2002-2003). Plastics in Europe-An analysis of plastics consumption and recovery in Europe. Amankwah, A. (2005). Plastic Waste Wahala. http:www.ghanaweb.com/public_agenda/arcticle (access on 08 June, 2006). Baldisimo, J.M.(1985). Recycling Potential of Solid Wastes at source and disposal sites in Manila, The Philippines. Beumer, P.F.M. et al (1991). Arbeidsomstandigheden in de chemische industrie: Overzicht van de voornaamste knelpunten wat betreft de blootstelling aan geluid, trillingen, gassen, dampen en stof. Directoraat-Generaal van de Arbeid Van het Ministerie Van Sociale Zaken en Werkgelegenheid, Den Haag. CAPS (1992). Recycling Activities in Metro Manila. WAREN project WASTE CONSULTANTS, The Netherlands Cointreau, S. et al (1984). Recycling from municipal refuse: A state-of-the-art review and annoted bibliography. Integrated Resource Recovery. Technical Paper Number 30 World Bank, Washington, DC. Ehrig, R.J. (1992). Plastics Recycling: Products and Processes. Hanser Publishers, Munich. Environment and Plastics Industry Council (EPIC). Plastic Recycling Overview. Summary Report by Environment and Plastics Industry Council (EPIC). – A Council of the Canadian Plastic Industry Association. Available in www.plastic.ca/epic. EQI (1991). The recycling of solid waste in Cairo, Egypt. WAREN project, WASTE Consultants, The Netherlands. Gaiker-IVL and KTH (2005). Technological Reference Paper on Recycling Plastics. GNA (2005). AMA to temporarily ban use of plastics. http://www.ghanaweb.com/ghanahomepage/newsarch ive/article(accesson12 June, 2006) Halbekath, J. (1989). The hazards of recycling plastics.

Horrocks, R. A. (1996). Recycling Textile and Plastic Waste. Woodhead publishing Limited, Cambridge, England. IRIN (2006). Government declares recycling war on plastic waste http://www.irinnews.org (access on 08 June, 2006) IRIN (2004). Ghana: No to water in plastic bags. http://www.scienceinafrica.co.za/ghanaplastic.htm (08 June, 2006) Konings, P. (1989). Small-scale industrial reprocessing of plastics: A field–study in Istanbul, Turkey. Waste Consultants/Delft University of Technology, Netherlands. Lardinois, I. and Klundert, A. (1995). Plastic Waste: Options for Small Scale Recovery. The Netherlands: Waste Consultants. Mewis, J. (1983). Gevaarlijke stoffen. Monografieen leefmilienu. De Nederlandse boekhandel, Antwerp/Amsterdam. Nagelhout, D. (1989). Information document waste plastics. Report number 738902005. RIVM, Bilthoren. Scheirs, J. (1998). Polymer Recycling. Wiley, Chiches, UK. Saechtling, H. (1987). International Plastics Handbook for the Technologist, Engineer and User, 2nd edition. Hanser Publishes, Munich. Schouten, A.E. and Van der Vegt, A.K. (1991). Plastics, 9th edition. Delta Press BV, Amerongen. Tall, S. (2002). Recycling of Mixed Plastic Waste – Is Separation Worthwhile? PhD Thesis, Department of Polymer Technology, Royal Institute of Technology, Stockholm-Sweden. TNO (1993). Emissiefactoren Kunststof-en rubberverwerkende industrie. Publikatiereeks Emissieregistratie Nr.11. Ministry of Volkshuisvesting, Ruimtelijke Ordening en Milieubeheer, The Hague, Netherlands. Vogler, J. (1984). Small-Scale Recycling of Plastics. Intermediate Technology Publications, UK. Warmer Fact Sheet (1992). Plastic Recycling, number 32. Yankson, P.W. and Gough, K.V. (1999). The environmental impact of rapid urbanization in the peri-urban area of Accra, Ghana.

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Michael Mensah Wienaah

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TRITA LWR Masters

APPENDIX

Table 1 Households by mode of solid waste disposal and district Solid waste disposal

AMA

Ga

Tema

Dangme West

Dabgme East

All Districts

Collected

20.9

12.0

29.5

2.1

0.4

19.5

Burned by household

6.9

24.5

11.6

21.0

33.7

12.2

Public dump

62.7

33.9

37.9

37.8

32.0

51.4

Dumped elsewhere

5.8

21.6

13.7

32.8

26.2

11.5

Buried by household

3.1

7.7

5.9

5.3

7.4

4.6

Others

0.7

0.3

1.4

0.9

0.3

0.7

Total

100.0

100.0

100.0

100.0

100.0

100.0

Population

364,841

119,316

105,520

18,641

17,426

625,744

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Sustainable Plastic Waste Management- A Case of Accra, Ghana

Table 2. Characteristics of Recyclable Plastics Resin type

Low-density Polyethylene (LDPE)

Density 3 (g/cm )

0.910-0.925

High-density polyethylene (HDPE)

0.94-0.96

Polypropylene (PP)

0.90

Polystyrene (PS)

1.04-1.10

Polyvinyl chloride (PVC)

1.30-1.35

Softening or melting range (0C) 102-112

125-135

160-165

70-115

150-200

Source: Hegberg, B.A et al (1992)

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Properties/applications

Largest volume resin for packaging; moisture proof transparent film. Tough, flexible and translucent material, used primarily in packaging; product examples include milk and detergent bottles, heavy-duty films, wire and cable insulation Stiff, heat and chemical resistant, used in furniture and furnishings, packaging; product examples include drinking straws, finishing nets, food containers, and vehicle bumpers. Brittle, transparent, rigid, easy to process, used in packaging and consumer products; product examples include foam take-away containers insulation board, cassette and compact disc cases.

Hard, brittle and difficult to process, but processed easily using additives; a wide variety of properties and manufacturing techniques are possible using different copolymers and additives; product examples include sheet bottles, house siding, cable insulation.

Michael Mensah Wienaah

TRITA LWR Masters

Table 3. Locked-in potential (LIP) expressed as calorific value for a range of material Product

Energy

Polymer

LIP (MJ/kg)

Prolypropylene

46

Polyethylene

46

Polystrene

41

Polyurethane

24-31

Polyester

19-30

PVC

20

Conventional Fuels

Energy (MJ/kg)

Diesel Oil

46

Naphtha

42-46

Carbon

21-33

Wood

16-21

Paper

16-19

Source: Horrocks, R., 1996

30