MUNICIPAL SOLID WASTE MANAGEMENT: Options for … · 2018-02-12 · MUNICIPAL SOLID WASTE MANAGEMENT: Options for Developing Countries Paper Presented at IPAC Technical Meeting, Kaduna,

MUNICIPAL SOLID WASTE

MANAGEMENT:

Options for Developing Countries

Paper Presented at IPAC Technical Meeting, Kaduna, Nigeria April, 2006

Engr. Abubakar, M. Bello, MNSChE EEMS Limited 1b, Ribadu Road, Gidan Manema, Gamji Gate P.O. Box 1258, kaduna Nigeria +234 805 502 7755, 803 786 9508 [email protected] www.eemslimited.com

MSW Management Options for Developing Countries: EEMS Limited

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CONTENT

INTRODUCTION ................................................................................................................................................ 3

HUMAN ACTIVITY AND WASTE .......................................................................................................................... 3

WHICH WAY FOR NIGERIA? ............................................................................................................................... 4

LANDFILL ........................................................................................................................................................... 4

INCINERATION.................................................................................................................................................. 6

RECYCLING........................................................................................................................................................ 6

COMPOSTING .................................................................................................................................................... 7

ZERO WASTES ................................................................................................................................................... 8

WASTE-TO-ENERGY (WTE) ........................................................................................................................... 9

WTE AND THE ENVIRONMENT ......................................................................................................................... 10

CONVERSION TECHNOLOGIES.................................................................................................................. 11

PYROLYSIS ....................................................................................................................................................... 11

CONVENTIONAL GASIFICATION ........................................................................................................................ 12

COMBINED PYROLYSIS AND GASIFICATION ...................................................................................................... 13

PLASMA GASIFICATION .................................................................................................................................... 13

ENVIRONMENTAL BENEFITS ............................................................................................................................. 13

PREDICTING WASTE GENERATION TO ENSURE ACCURATE CAPACITY PLANNING.............. 14

HOW TO PREDICT FUTURE WASTE QUANTITIES ............................................................................................... 15

Waste Densities ........................................................................................................................................... 15

Rural-Urban Migration............................................................................................................................... 15

REFERENCES ................................................................................................................................................... 16

TABLES

TABLE 1. TYPICAL LEACHATE QUALITY DATA FROM LARGE LANDFILLS WORLDWIDE........................................... 5

TABLE 2. POLICIES THAT CAN BE USED TO PROMOTE RECYCLING.......................................................................... 7

TABLE 3. ANNUAL DIOXIN EMISSIONS IN GERMANY BY SOURCE. ........................................................................ 10

TABLE 4. ANNUAL EMISSIONS FROM WTE PLANTS IN GERMANY........................................................................ 10

FIGURES

FIGURE 1. BLOCK DIAGRAM OF A TYPICAL CONVERSION FACILITY........................................................................ 11

FIGURE 2. PYROLYSIS PROCESS FLOW DIAGRAM ................................................................................................... 12

FIGURE 3. GASIFICATION PROCESS FLOW DIAGRAM .............................................................................................. 13


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INTRODUCTION

Waste is generally defined as any material that is no longer needed or no longer useful to the owner. Wastes can be solid, liquid or gaseous, however, our focus is on solid wastes. Solid wastes typically may be classified as follows:

Garbage: decomposable wastes from food.

Rubbish: non-decomposable wastes, either combustible (such as paper, wood, and cloth) or non-combustible (such as metal, glass, and ceramics).

Ashes: residues of the combustion of solid fuels.

Large wastes: demolition and construction debris and trees.

Dead animals:

Sewage-treatment solids: material retained on sewage-treatment screens, settled solids, and biomass sludge.

Industrial wastes: such materials as chemicals, paints, and sand

Mining wastes: slag heaps and coal refuse piles.

Agricultural wastes: farm animal manure and crop residues.

Hazardous wastes: include toxic chemicals and flammable, radioactive, or biological substances. These wastes can be in the form of sludge, liquid, or gas, and solid. Radioactive substances are hazardous because prolonged exposure to ionizing radiation often results in damage to living organisms, and the substances may persist over long periods of time.

Human Activity and Waste

Human beings are the most powerful species on Earth. We use our favoured status endowed by the Almighty in demanding more goods, services, food, clothing and shelter. It is this penchant for demanding more every day that fuels our economic system. As a result, we live comfortable lives, especially in the urban centers. However, we discard more solid waste than ever before.

Human beings have always disposed of unwanted solid waste in their backyards. We have always believed that the capacity of the earth to absorb our waste is infinite, perhaps until recently. In search of good and easy life, we try to improve the packaging of materials we use every day. We would then discard these materials when they are of no more value to us. Many waste management methods have been practiced, perhaps from the beginning of human existence on earth. We sweep and take it to the back of the house, and when it becomes large, we burn it. However, with time other methods came into being. These include recycling, landfill, incineration, pyrolyses, composting, etc. our African ancestors have practiced all of these, depending on economics and convenience. The use of organic manure from solid waste dumps, recycling of gold and silver, dumping of wastes in borrow pits, burning of agricultural wastes to make potash, etc.

These methods have changed significantly in the developed world over the years. During the 1960s, the world was awash with burning open dumps that resulted in degradation of local and global environments. The US, Canada, Western Europe, Japan, Australia and New Zealand – all rich countries moved to gradually eliminate open dumps, replacing them with modern, regional landfills. Landfill gas is captured at most large landfills for energy recovery, thereby reducing their potential for worsening global warming. In contrast, Japan and the European Union have embarked on a different approach to reduce the materials that require landfilling. They use various conversion technologies such as composting and thermal treatment.


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What about the rest of the world? Open dumps – controlled or uncontrolled continue to receive solid waste every day. They constitute the most serious threat to global environmental quality. China, south Asia, Africa and South America are regions where open dumps continue to prevail. In Nigeria today the open dumps are neither even organised nor controlled. People can just dump anything anywhere.

More than 8m tons of solid waste is produced daily in developing couintries1. Over 95% of this is deposed of in landfills, open dumps, dranaiges, riverbanks and the sea or simply combusted on-site due to insufficient waste collection and final disposal systems. The many environmental and health risks associated with open dumping put landfilling at the bottom of the waste management hierarchy in the industrialised world. What are feasible options for developing countries?

Asia is already moving forward; a seven year research programme on landfilling and waste management in Asia is currently being implemented to improve the protection of human health and the environment. This initiative included five universities from China, India, Sri Lanka and Thailand and supported by Swedish International Co-operation Agency. The research programme is seen as an important step towards replacing open dumps with sanitary landfills1.

Which Way for Nigeria?

In order to really improve our environmental quality – at least in the near term (20 years), we need to develop efficient integrated waste management facilities across the country, which shall included: transfer stations and modern regional landfills as well as implement policies that will encourage recycling and waste-to-energy (WTE) to serve the ever growing urban regions of rapidly growing population. Such landfills could also augment electrical energy supply using the methane gas that they generate. Solid waste fractions could be separated for material recovery.

This paper attempts to review waste management options practiced in various parts of the globe, and tried to evaluate how it applies to Nigeria. Finally, it highlighted some of the problems militating against effective MSW management in Nigeria, with particular reference to projection of future MSW generation and planning.

LANDFILL Sanitary landfill is the cheapest satisfactory means of disposal, but only if suitable land is within economic range of the source of the wastes; typically, collection and transport account for 75-80% of the total cost of solid waste management. In a modern landfill, refuse is spread in thin layers, each of which is compacted by a bulldozer before the next is spread. When about 3 m of refuse has been laid down, it is covered by a thin layer of clean

earth, which also is compacted. Pollution of surface and groundwater is minimized by lining and contouring the fill, compacting and planting the cover, selecting proper soil, diverting upland drainage, and placing wastes in sites not subject to flooding or high groundwater levels. Gases are generated in landfills through anaerobic decomposition of organic solid waste. If a significant amount of methane is present, it may be explosive; proper venting eliminates this problem.

Landfills appear to be best option for developing countries, including Nigeria at the moment because of it is relatively simpler in technology. However, many non-governmental organizations

1 Asian Regional Research Programme on Environmental Technology (www.serd.ait.ac.th/sidaSWM)


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oppose landfills in these regions and support other conversion technologies instead of landfills. Their reasons border mainly about landfill stability concerns. In 2000 a landfill site in the Philippines failed and caused a major landslide, killing more than 200 people with hundreds more missing. And more recently in 2003 another lanfill failure was recorded in Athens, Greece. Kolsch, F and Ziehmann, G (2004).

Heterogeneous waste composition, obstacles in determining waste strength parameters, and a lack of knowledge about the principles of waste mechanics resulted in considerable uncertainties in stability calculations. Other problems that landfill have to tackle; especially in Nigeria is the issue of leachates, health and safety, keeping out scavengers at dumpsites.

Despite EU policy to divert biodegradable waste from landfill, landfilling remains the dominant method used in Europe – approximately 50% of the 243 million tonnes of municipal solid waste (MSW) generated in the 25 Member States of the EU (EU-25) each year is still landfilled. Stengler, E. (2005).

Leachates can be classed as acetogenic or methanogenic depending on the state of degradation of the waste materials in the landfill. Leachate from ‘young’ wastes is characterized by high chemical oxygen demand (COD) and biological oxygen demand (BOD) values, and by high ratios of BOD to COD. Methanogenic leachates are derived from older landfills where extensive degradation of organic materials in the wastes has occurred. End products of methanogenesis in the landfill are methane and carbon dioxide. Leachate is categorized by lower COD values, very much lower BOD values, and lower BOD to COD ratios.

TABLE 1. Typical leachate quality data from large landfills worldwide

Determinant Units Hong Kong South-east Asia South Africa UK

pH - 7.4-8.6 6.0-8.4 7.5-8.3 7.4-8.5

COD mg/l 650-2800 1600-13,000 1400-6000 2600-8500

BOD5 mg/l 45-400 - 300-700 90-3000

Total organic carbon (TOC) mg/l - 400-10,000 - 400-3400

Ammoniacal-N mg/l 1200-3000 1200-3000 900-3000 1100-2500

Chloride mg/l 500-3000 2500-6300 1200-4000 1700-5200

Alkalinity (as CaCO3) mg/l 3000-12,000 8000-40,000 3000-12,000 7000-17,000

Conductivity µS/cm 14,000-30,000 14,000-42,000 10,000-30,000 14,000- 30,000

Nitrate-N mg/l < 0.1-22 < 1-12 < 0.1-1.3 < 0.3-2

Nitrite-N mg/l < 0.1-3.6 < 1 - < 2 < 0.1-1.4 < 0.1-2

Sulphate (as SO4) mg/l - < 5-1200 < 0.6-400 < 5-150

Phosphate (as P) mg/l 3-125 2-50 1-25 4-20

Sodium mg/l 200-2100 1500-3000 800-2500 1700-4000

Magnesium mg/l 18-50 60-500 75-400 17-150

Potassium mg/l 375-1200 1000-3000 550-1600 750-1700

Calcium mg/l 20-45 50-3000 50-200 40-420

Chromium µg/l - 200-2500 80-300 40-2200

Manganese µg/l - 250-17,500 12-900 70-2200

Iron µg/l 5000-9000 1000-20,000 2500-20,000 3000-72,000

Nickel µg/l - 400-1500 80-120 150-3000

Copper µg/l - < 50-400 < 50 20-80

Zinc µg/l 200-2200 150-1500 30-200 30-330

Cadmium µg/l - < 50 < 1 - < 10 < 10-20

Lead µg/l - < 300 - <1000 < 4-56 < 40-600

Arsenic µg/l - - 7.5-62 2-24

Mercury µg/l - - < 50 < 0.1

Source: ISWA directory (2005)


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INCINERATION

In incinerators of conventional design, refuse is burned on moving grates in refractory-lined chambers; combustible gases and the solids they carry are burned in secondary chambers. Combustion is 85 to 90 per cent complete for the combustible materials. In addition to heat, the products of incineration include the normal primary products of combustion—carbon dioxide and water—as well as oxides of sulphur and nitrogen and other gaseous pollutants; nongaseous products are fly ash and unburned solid residue. Emissions of fly ash and other particles are often controlled by wet scrubbers, electrostatic precipitators, and bag filters.

RECYCLING

The practice of recycling solid waste is an ancient one. Metal implements were melted down and recast in prehistoric times. Today, recyclable materials are recovered from municipal refuse by a number of methods, including shredding, magnetic separation of metals, air classification that separates light and heavy fractions, screening, and washing. Another method of recovery is the wet pulping process: Incoming refuse is mixed with water and ground into slurry in the wet pulper, which resembles a

large kitchen disposal unit. Large pieces of metal and other nonpulpable materials are pulled out by a magnetic device before the slurry from the pulper is loaded into a centrifuge called a liquid cyclone. Here the heavier non-combustibles, such as glass, metals, and ceramics, are separated out and sent on to a glass- and metal-recovery system; other, lighter materials go to a paper-fibre-recovery system. The final residue is either incinerated or is used as landfill.

Public interest in recycling has increased dramatically over the last 15 years throughout the industrialized world, and is presently gaining ground in developing countries. This interest has been driven in the developed economies by a variety of factors, including concerns about increasing waste generation and dwindling landfill capacity, air pollution from incineration, and a general appreciation of the need for environmental protection. In response, a wide array of policies, regulations and programmes have been implemented. These include changing the requirements for recycling in households and businesses, banning recyclables from being landfilled, creating deposit-refund programmes, and creating financial incentives for source separation and waste reduction. Other policies have been designed to stimulate the demand for recycled materials. These include guidelines for buying recycled products, requirements for a minimum recycled content and tax incentives for products with recycled content.

In some countries, comprehensive extended producer responsibility (EPR) frameworks have been introduced to target both supply and demand. EPR policies shift the responsibility for meeting government-specified recycling targets to the industries that produce the recyclables. Governments are also increasingly encouraging industries to adopt environmental management systems (EMS). These holistically address waste generation through source reduction, reuse and recycling. Table 2 provides a list of policies in each of these categories implemented in 14 countries: Germany, Belgium, the Netherlands, Luxembourg, Italy, Spain, the UK, Finland, the US, Japan, Hong Kong, Singapore, Australia and Brazil (Morton, A.B. et al (2005).


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TABLE 2. Policies that can be used to promote recycling

Policies that promote the recyclable supply

• Implement kerbside collection or improve its efficiency

• Implement use-based pricing for waste disposal

• Improve availability of drop-off collection in public places

• Set ambitious, yet achievable, recycling and diversion targets

• Introduce regulatory requirements for source separation

• Require municipalities to operate recycling programmes

• Landfill bans for recyclables

• Landfill taxes

• Adopt deposit-refund programmes

• Charge consumers advanced disposal fees at the time of purchase

• Increase public education

• Provide funding to municipalities

• Promote long-term contracts to eliminate the risk of price fluctuation for recyclables

Policies that promote recyclable demand

• Require minimum recycled content in products

• Implement procurement guidelines for government purchases

• Fund research and development

• Provide tax credits and/or low-interest loans

• Educate the public on the quality of products with recycled content

• Tax virgin materials

Policies that promote recyclable supply and demand

• Implement extended producer responsibility

• Encourage companies to adopt comprehensive environmental management systems

Source: Environmental Research and Education Foundation, Alexandria, Virginia, US.

Which of these policies can be adaptable to the Nigerian environment? How do we make it work; in a situation where infrastructural facilities are either in bad shape or non-existent; where unemployment is high; where low capacity utilisation bedevils the manufacturing sector; where responsibility for waste management is wrongly though constitutionally given to local councils; etc.

Since all the materials we use cannot be recycled indefinitely, a portion of the recycled waste material becomes waste again and is dumped on land. Following this logic, recycling could be viewed as a time-delay mechanism instead of a waste reduction programme.

COMPOSTING

Composting operations of solid wastes include preparing refuse and degrading organic matter by aerobic micro-organisms. Refuse or garbage is pre-sorted, to remove non-compostible materials, and is ground up to improve the efficiency of the decomposition process. The refuse is placed in long piles on the ground or deposited in mechanical systems, where it is degraded biologically to humus with a total nitrogen, phosphorus, and potassium content of 1 to 3 per cent, depending


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on the material being composted. After about three weeks, the product is ready for curing, blending with additives, bagging, and marketing.

Compared with animal manure, nutrients in the organic matter of compost are released more steadily over a prolonged period of time. Compost produced from organic waste can also help reduce the consumption of peat by acting as a substitute in, for example, potting compost.

Composting process technology has improved over the years with the introduction of biocel process and bio-stimulators. Compost can be enriched with high concentrations of plant-stimulating organisms that improve the biological balance in the soil and that build in capacity to fight plant disease in agriculture. Bio-stimulators are composts with a high concentration of micro-organisms that are claimed to enhance biodiversity and soil stabilization. The combination of compost with bio-stimulators scores much better compared than a combination of mineral fertilizers and chemical pesticides and/or herbicides.

BIOCEL PROCESS: Consists of multiple anaerobic fermentation cells. Front-end loaders pick up the unsorted waste for loading into the cells. Organic waste inputs are not limited to solid and semi-solid materials, but also include liquids. The oxygen in the cell is 'driven out' by pumping exhaust gases through the waste until the correct oxygen-free conditions for the anaerobic digestion process are achieved. Eize de Vries (2005).

The unsorted waste ranges from household kitchen waste to tree cuttings and includes supermarket waste such as 'expired' instant meals, plastic and paper wrappings, and bread, cheese or sausages still wrapped in their packaging. In addition, there are plastic-lined paper cartons, empty beer and soft drink cans, bread and cakes, and citrus fruit skins, etc.

Materials such as plastics, inert materials and garden waste in the untreated waste provide the structure necessary to allow leachate, which is spread continuously over the waste mass during the process, to permeate it during the digestion process. The lack of machinery in the cells and tunnels means that packaging materials, metals and inert materials can form part of the waste fractions; the product is sorted at the end of the process not before, as the case in conventional composting.

Anaerobic fermentation itself is a relatively cheap process that can be controlled well, provided material handling is kept simple and robust. The biogas generated is used to power one or both gas engines, depending on methane production. Actual gas production depends mainly on the type of waste material fed into the cells.

ZERO WASTES

Advocates promote Zero Waste as an alternative disposal technology, in direct competition with landfill and incineration. But whereas the latter are single and often large-scale technologies, Zero Waste is a 'brand' for change and a diverse, flexible range of polices, technologies and actions, starting with 'Design For Disassembly' at the beginning of the supply chain and embracing Industrial Ecology, Cleaner Production, Extended Producer Responsibility (EPR) responsible consumption, public education, and local economic development, right through to waste minimization and resource recovery at the end of the pipe.

Zero Waste is a whole-system approach to redesigning the way resources and materials flow through society. This is its key competitive advantage. The thinking behind landfills and incineration accepts that waste exists and must be got rid of; Zero Waste says that it shouldn't exist and that waste is a signal of design failure, representing inefficiency, or as business puts it, 'lost profit'. It says that existing technologies are born of the thinking that caused the problem and, as such, will never solve it.

In many senses, the battle between community and corporate ownership of the discard supply is the battle between two completely different world views. Small local community organizations


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and recycling companies on the one hand, who want to create jobs and business opportunities from waste, pitched against big national and international companies who see it as a straight business proposition to increase shareholder returns.

In 1996, the Australian city of Canberra was the first in the world to set a target of 'No Waste'; it is currently diverting 64% of its waste from landfill. Canberra's ideas were copied and popularized in New Zealand, where over half of all local authorities now have Zero Waste polices. The New Zealand Government has since adopted Zero Waste as the national vision for their new waste strategy. Warren S. (2003). The Nigerian Government also established a Zero Waste Research Initiative (ZERI) at Abubakar Tafawa Balewa University (ATBU), a collaborative center between Federal Ministry of Environment and ATBU Bauchi, to promote zero waste approach. However, due to lack of coherent policy, funding and other problems the center is yet to make any appreciable impact.

Zero waste proponents claim that we, as a civilized society, should be able to segregate and recycle practically all the materials discarded from our homes. While this sounds good on paper and on television, the global market capacity for absorbing the segregated materials for remanufacturing does not yet exist. Proponents of zero waste also believe that we can reduce the quantity of household waste discarded, and that residents could be trained to efficiently segregate their solid waste.

Human beings are not efficient at segregating materials, particularly in a developing country like Nigeria. A study showed that if 70% of households participate and they are 70% efficient in segregating materials, the diversion efficiency would result in less than 50% of the materials being recovered. Vasuki, N.C. (2005). The proponents of zero waste also ignore the constantly changing mix of packaging materials that are introduced each year to improve the shelf life and material safety of the products we use. In effect, zero waste initiatives actually retard programmes for efficient collection and disposal of municipal solid waste.

WASTE-TO-ENERGY (WTE)

This is a thermal process where wastes are combusted in conventional incinerators to produce steam. A few local authorities produce steam in incinerators in which the walls of the combustion chamber are lined with boiler tubes; the water circulated through the tubes absorbs heat generated in the combustion chamber and produces steam. The steam may then be used drive turbines that generates electricity or used directly for heating.

Europe currently treats 50 million tonnes of wastes at WTE plants each year, generating an amount of energy that can supply electricity for 27 million people or heat for 13 million people. Forthcoming changes to EU legislation will have a profound impact on how much further the technology will help achieve environmental protection goals. Stengler E. (2005).

Waste-to-energy has the potential to have a significant impact on carbon dioxide (CO2) emissions by supplying energy that would otherwise be supplied by conventional power plants using fossil fuels. Each year in Europe, 7,778 million cubic metres of natural gas or 7,428 million litres of oil are substituted by treating 50 million tonnes of waste in WTE plants.

At this level, some 27 million MWh of electricity or 63 million MWh of heat can be generated, supplying 27 million inhabitants with electricity or 13 million inhabitants with heat. This is equivalent to supplying the entire population of the Netherlands, Denmark and Finland with electricity, or nearly the entire population of Austria, Ireland and Estonia with heat throughout the year.

The calorific value of MSW is about 10 GJ/tonne – about a third of that for coal. When burnt in a modern, well controlled mass-burn incinerator, such systems give about 20% efficiency for


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conversion to electricity. Much higher conversion efficiencies are possible with improved recycling and refining through processes such as mechanical-biological treatment (MBT) to create an energy-rich and consistent material that is better suited as a fuel. It is possible to achieve electrical conversion efficiencies of as high as 40%.

WTE and the Environment

The major environmental concern of WTE is air emissions and fly ash, disposal of the residual ash and how to handle the non-combustibles. However, due to tighter emission regulations and application of technology incineration plants in the developed world have significantly reduced emissions of dioxins, dust, heavy metals, etc. For instance, in September 2005, a report2 by the German Environment Ministry (BMU) stated:

‘...because of stringent regulations waste incineration plants are no longer significant in terms of emissions of dioxins, dust and heavy metals. And this still applies even though waste incineration capacity has almost doubled since 1985.’ ‘Total dioxin emissions from all 66 waste incineration plants in Germany has dropped to approx. one thousandth as a consequence of the installation of filter units stipulated by statutory law: from 400 grams to less than 0.5 grams’.

Referring to other industries, the report says:

‘The decline, however, has nowhere been as drastic as in the incineration of household waste. The consequence is that whereas in 1990 one third of all dioxin emissions in Germany came from waste incineration plants, for the year 2000 the figure was less than 1%’.

These comments and the data in Tables 3 and 4 reflect the improvements that have occurred in reducing emissions in Germany as a result of strict national legislation and significant investment in efficient flue gas cleaning systems. The flue gas cleaning system referred to hear is a well known technology, which can applied in Nigeria.

TABLE 3. Annual dioxin emissions in Germany by source.

Emissions per year in gram per toxicity unit (g TU)

1990 1994 2000 a

Metal extraction and processing 740 220 40

Waste incineration 400 32 0.5

Power stations 5 3 3

Industrial incineration plants 20 15 < 10

Domestic firing installations 20 15 < 10

Traffic 10 4 < 1

Crematoria 4 2 < 2

Total emissions, air 1200 330 << 70 a Data for the year 2000 are estimates by the Federal Environment Agency SOURCE: BMU1

TABLE 4. Annual emissions from WTE plants in Germany.

1990 2001

Lead 57,900 kg 130.5 kg (= 0.2% of initial emissions)

Mercury 347 kg 4.5 kg (= 1.3% of initial emissions)

Dust (particulate matter) 25,000 tonnes (= a maximum of 30 mg/m3 of exhaust air)

< 3000 tonnes

SOURCE: BMU1

2 Federal Ministry for the Environment, Nature Conservation and Nuclear Safety. Waste incineration – a potential danger? Bidding farewell to dioxin spouting. September 2005,

translated from the original German version, published in July 2005. http://www.bmu.de/%20english/waste_management/downloads/doc/35950.php.


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CONVERSION TECHNOLOGIES

Conversion technologies have been used to treat a wide range of feedstocks (including coal, petroleum coke and wood waste) for over 100 years. Most of these feedstocks were fairly homogeneous. However, commercial-scale application of conversion technologies to municipal solid waste (MSW) did not occur until the 1980s and 1990s. Most of these facilities were installed in Europe and Japan in response to regulations to reduce MSW disposal to landfills. In North America, many cities and counties have evaluated, or are currently evaluating, conversion technologies as part of MSW management planning (ISWA directory, 2004).

A major advantage of conversion technologies is the production of an intermediate product - synthesis gas or syngas - which is used in a wide range of energy generation and chemical manufacturing processes. This distinguishes conversion technologies from WTE, which directly combusts the MSW. With no intermediate product, WTE is limited to the production of steam and/or electricity.

A typical conversion technology facility contains three subsystems (Figure 1). The first is a pre-processing subsystem for removing additional recyclables and, often, for size reduction and drying; for many conversion technologies, the more homogeneous the feedstock, the better. The processed MSW then enters the conversion unit, where it is thermally converted - not combusted - to syngas. This subsystem may include syngas cooling and cleaning, as well as removal of additional by-products and residuals. The syngas then enters the energy production subsystem, which may be for power generation or further processing to produce chemicals. This final subsystem may include flue gas emission controls.

SOURCE: MSW Technologies and treatment

Figure 1. Block diagram of a typical conversion facility

The most common thermal conversion technologies are:

pyrolysis

conventional gasification (fixed or fluidized bed)

combined pyrolysis and gasification

plasma gasification.

These four technologies are outlined below.

Pyrolysis

Figure 2 shows a process flow diagram for a pyrolysis facility. In pyrolysis, carbon-based materials are thermally degraded using an indirect, external source of heat, at temperatures of 400°-900°C in the absence of free oxygen. The volatile portion of the feedstock is thermally decomposed, producing syngas composed primarily of hydrogen and carbon monoxide.


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Figure 2. Pyrolysis process flow diagram

If the syngas is cleaned of contaminants, it can be combusted in a reciprocating engine, producing electricity. Otherwise, it can be combusted in a boiler, producing steam for power generation, with the flue gas treated in an emission control system.

Non-volatile organic materials are left behind as char, typically disposed of with the ash. Pyrolysis has been in use for hundreds of years, primarily for producing charcoal. MSW pyrolysis, with power generation, has been in successful operation in Europe for almost 20 years.

Conventional Gasification

Figure 3 shows a process flow diagram for a gasification facility. Gasification converts carbon based materials in the presence of direct heat at temperatures of 760°-1650°C and with a limited supply of oxygen to a syngas composed primarily of hydrogen and carbon monoxide. This is a chemical process, not combustion. As with pyrolysis, if the syngas is cleaned of contaminants, it can be combusted in a reciprocating engine, producing electricity. Otherwise, the syngas can be combusted in a boiler, producing steam for power generation.

The char that would have been left behind in pyrolysis is converted to additional syngas. Inorganic materials end up as either bottom ash (low-temperature gasification) or as an inert, glassy slag (high-temperature gasification).

Gasification has been in use for more than 200 years. It has been used worldwide to gasify coal to produce 'town gas' for heating, cooking and lighting. MSW gasification has been implemented worldwide in the past few years, with some facilities producing steam and others electricity.


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Figure 3. Gasification process flow diagram

Combined Pyrolysis and Gasification

This process essentially follows the pyrolysis reactor shown in Figure 2 with a separate gasification reactor as shown in Figure 3. At the end of the pyrolysis section, the syngas exits the chamber and the carbon char left over from pyrolysis is fed into the adjacent gasification chamber, producing more syngas. Together, this integrated process results in high conversion to syngas.

Plasma Gasification

In plasma gasification, a plasma arc is used to create a high-temperature stream of ions (plasma) using air, oxygen, nitrogen, steam or other gases at temperatures up to 5500ºC. The plasma is used to heat the MSW to a gasification temperature of 1100º-1650ºC, producing syngas and melting the inorganic components into slag. Metals can be segregated in molten form below the slag layer and reclaimed in fairly pure form. Particulate matter removed downstream can be re-injected into the reactor to produce slag.

Plasma gasification has been in use for years in steelmaking and is being used to melt WTE ash to meet limits on dioxin/furan content. It has been installed on a commercial scale in Japan for treating MSW and auto shredder residue (ASR). Plasma gasification has the potential to be more efficient in terms of electricity production than either conventional gasification or pyrolysis.

Environmental Benefits Conversion technologies have a number of environmental benefits.

Conversion technologies often incorporate pre-processing subsystems to produce a more homogeneous feedstock. This provides the opportunity to recover chlorine-containing plastic (as a recyclable), which could otherwise contribute to the formation of organic compounds or trace contaminants.

Syngas produced by thermal conversion technologies is a much more homogeneous and cleaner burning fuel than MSW.

Conversion technology processes occur in a reducing environment, so that formation of unwanted organic compounds or trace contaminants is precluded or minimized.

Conversion technologies are closed pressurized systems such that there are no direct air emission points. Contaminants are removed from the syngas and/or from the flue gases before being exhausted from a stack.


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The volume of syngas produced in the conversion of the feedstock is considerably lower than the volume of flue gases formed by WTE facilities. Smaller gas volumes are easier and less costly to treat.

Pre-cleaning of syngas is possible, thus reducing the potential for corrosion in power generation equipment and reducing overall air emissions. Sulphur compounds can be removed by commercially available equipment and recovered as marketable sulphur or gypsum.

Syngas produced by thermal conversion technologies is a much more homogeneous and cleaner-burning fuel than MSW.

Methane emissions from landfills are significant even with energy recovery. Using a conversion technology to convert the carbon content of the MSW to combustible syngas, instead of allowing it to degrade in a landfill to methane, eliminates this environmental impact.

The inert, glassy slag recovered from high-temperature gasification is similar to that produced from steel mills and coal-fired power plants. It can be used for making roofing tiles and as sandblasting grit or asphalt filler.

PREDICTING WASTE GENERATION TO ENSURE ACCURATE

CAPACITY PLANNING

Capacity planning of waste-processing facilities and infrastructure requires an idea of the future demand. An accurate estimate of the quantity of future MSW streams is required as a first step in the planning process. Neglecting this first step in the planning process is likely to lead to the wrong answers to questions such as ‘Do we need another new, enlarged waste incineration plant, landfill, recycling facility or composting plant?’ A growth forecast that is out by only 1% can lead to a deviation of more than 10% of the total waste generated over a planning period of 10 years. Under- or over-estimate thus has significant consequences in terms of additional investment and operating costs.

Waste-related data as well as demographic and socio-economic indicators are required for this estimates and shall include:-

Waste generated (kg/person/year)

Waste densities

Moisture content

Composition of waste (organic, plastic and composite, glass, paper and paperboard, metallic, etc)

Types of wastes (household routine, industrial hazardous, industrial non-hazardous, office, construction/demolition, etc.

Population

Population density

Population age structure (0–14; 15–25; 26-45; 45-60; > 60 years)

Sectoral employment (agriculture, industry, services, etc)

Gross domestic product/regional product

Infant mortality rate

Life expectancy at birth

Average household size

Unemployment rate

Overnight stays (tourism)


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How to Predict Future Waste Quantities

It is always difficult to decide the best way to deal with uncertainties. In planning practice, underlying assumptions or expectations concerning forecasts are, of necessity, not well defined and are, therefore, not transparent and are frequently overlooked. Although exact waste forecasts cannot be achieved, improvements in forecast accuracy are possible. However, they are often limited by:

Non-availability of accurate data

Failure to consider social and economic trends related to the region

Lack of reliable forecasts with regard to these trends.

The proper approach shall attempt to mitigate these problems. First, the relationship between the set of indicators shall be assessed statistically on the basis of an investigated period covered. The data generated over the period can be subjected to statistical analyses such as correlation and regression analyses or cluster analyses to develop the generation model. Secondly, the model shall be based on prioritized parameters represented by factors with a high inertia such as health or demographic indicators. This means that they are easier to predict than highly volatile indicators such as GDP. Forecasts of this economic measure have an early expiry date, whereas forecasts for health or demographic indicators (such as those updated regularly by the United Nations) are available at a national level up to the year 2050. Beigl, P et al (2005).

The MSW generation model developed can then be used to develop the final forecasting model. An interpolation method may be necessary to consider long-term changes between prosperity levels. Testing the accuracy of forecasts is crucial, because the real error of a forecast over 10 years cannot be tested for practical reasons. Beigl, P. (2006).

Waste Densities

One of the key areas of change has been the density of refuse (domestic waste). In the older days, when householders' lives revolved around a coal fire or stove and shopping was done on a daily basis, the density of waste remained stable. As soon as central heating (and smokeless fuels) became widespread, and the concept of the weekly visit to the supermarket started to replace the local corner store, the density of household material started to change dramatically. Plastic and paper packaging material increased and ash content decreased. Until recently, this was the premise on which new refuse collection vehicles (RCVs) were designed - that refuse density would continue to fall as packaging percentages grew. But the removal of paper and plastics from the waste stream via recyclables collection is having a dramatic impact on the contents of the normal household waste container. In addition, 'pay-by-weight' collection looks set to further increase the speed of that change.

Rural-Urban Migration

Another vital strand to our story concerns the aspirations of citizens to move from central, well established residential districts to new suburbs. Again, it's hardly a new phenomenon. However, a continuing increase in car ownership brings with it parked cars blocking residential streets at the very time the waste collection vehicle needs access. The result? The trend towards ever larger trucks has been questioned as local resident groups complain about the environmental damage such vehicles can cause. That's understandable, as the waste collection truck is one of the largest commercial vehicles many people see.


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REFERENCES

1. Beigl, P. (2006). Predicting waste generation to ensure accurate capacity planning. In: Waste Management World; Markets Policy Finance of 02/01/06. James and James (Science Publishers) Ltd, London. 2006.

2. Beigl, P., Gamarra, P. and Linzner, R. (2005). Waste forecasts without ‘rule of thumb’: improving decision support for waste generation estimations. SARDINIA 2005 – Tenth International Waste Management and Landfill Symposium, Paper No. 251 (in press). CISA Environmental Sanitary Engineering Centre, Cagliari. 2005.

3. Beigl, P., Wassermann, G., Schneider, F. and Salhofer, S. (2004). Forecasting municipal solid waste generation in major European cities. In: Pahl-Wostl C., Schmidt S., Jakeman, T. (editors). iEMSs 2004 Int. Congress: Complexity and Integrated Resources Management, Osnabrück, Germany. www.iemss.org/iemss2004/pdf/regional/beigfore.pdf

4. Bolton, Neal. The Handbook of Landfill Operations. Blue Ridge Services, Inc. Atascadero, California, US. 1995. pp. 228-232.

5. Conversion technology evaluation report. Prepared for the County of Los Angeles Department of Public Works and the Los Angeles County Solid Waste Management Committee/Integrated Waste Management Task Force's Alternative Technology Advisory Subcommittee by URS Corporation, August, 2005. Available on-line at http://ladpw.org/epd/tf/subs.cfm

6. Displacing carbon dioxide is a business opportunity: Report; Refocus weekly international renewable energy magazine, 2005, Elsevier Ltd, BOSTON, Massachusetts, US, March 29, 2006

7. Eize de Vries (2005). Organic Innovations. In: Waste Management World; Technologies and Treatment of 02/05/05. James and James (Science Publishers) Ltd, London.

8. Eric, M. et al (2005). Renewables 2005 Global Status Report; Presented at the Beijing International Renewable Energy Conference 2005 (BIREC 2005), November 7–8, 2005; Sponsored by the REN21 Renewable Energy Policy Network and the German government.

9. Federal Ministry for the Environment, Nature Conservation and Nuclear Safety. Waste incineration – a potential danger? Bidding farewell to dioxin spouting. September 2005, translated from the original German version, published July 2005. http://www.bmu.de/%20english/waste_management/downloads/doc/35950.php.

10. International Solid Waste Association (ISWA). IWSA Directory of waste-to-energy plants. 2004.

11. Kőlsch, F. and Ziehmann, G. (2004). Landfill Stability- Risks and Challenges. Waste Management World (May-Jun 3004);P55-59; James and James (Science Publishers) Ltd, London.

12. Morton A.B. and Daniel, H.L. (2005). Policies to strengthen markets for recyclables. In: Waste Management World; Markets Policy Finance of 02/07/05. James and James (Science Publishers) Ltd, London.

13. Stegler, E. (2005). Where is waste-to-energy, and where is it going? The European Position. In: Waste Management World; Markets Policy Finance of 02/07/05. James and James (Science Publishers) Ltd, London.

14. Vasuki, N.C. (2005). Zero waste and 100% recycling cannot yet be feasibly implemented worldwide.. In: Waste Management World; Markets Policy Finance of 02/09/05. James and James (Science Publishers) Ltd, London.

15. Warren Snow (2003). Zero Waste: Does it Mean an End to Waste?; environmental and sustainability planning group, Envision New Zealand Ltd.

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