Municipal solid waste (MSW) refers to the stream of garbage collected through community sanitation services. Such waste can consist of a variety of materials, including both renewable energy sources (such as food, paper, and wood) and non-renewable energy sources (such as glass, plastics, and tires). Obviously, as several sources have shown, from an environmental perspective the most sustainable option for MSW is to reduce the amount of waste. According to Schönning (2006), “The goal of waste management, in any country, should be to reduce the amount of garbage generated, while reusing as much of what remains.”
The stream of garbage collected through community sanitation services is referred to as Municipal Solid Waste (MSW). Such waste can consist of a variety of materials, including both renewable energy sources (such as food, paper, and wood) and non-renewable energy sources (such as glass, plastics, and tires). Obviously, as several sources have shown, from an environmental perspective the most sustainable option for MSW is to reduce the amount of waste (Schönning, 2006).
However, incineration of MSW streams allows for the production of electricity and/or heat. At waste incinerators, MSW is unloaded from collection trucks and shredded or processed to ease handling. Recyclable materials are separated out, and the remaining waste is fed into a combustion chamber to be burned. The heat produced by this process boils water and the resulting steam can be used directly in a heating system or a factory. Usually, however, the steam is used to turn a turbine-generator to produce electricity (see image above). Additional steps allow the recycling of metals from the incombustible residue.
Waste To Energy (WTE) is the pre-eminent method of waste disposal in Europe and Asia because of its ability to reduce the volume of waste, generation of valuable energy, and the reduction of GHG emissions. In the Netherlands, 2535 GWh electricity was produced in 2004 from MSW of which 78% has been delivered to the grid or to other installations. The remaining share has been used on-site by the waste incinerators themselves. Moreover, 8.8 PJ of heat has been delivered for external use, e.g., for industrial processes or district heating. As can be seen in Figure 1, an ever increasing share of waste generated in the Netherlands is incinerated to produce electricity. For instance, in 2001 the amount of MSW incinerated in the Netherlands was two and a half times as big as in the USA (37.8 vs 14.7%). In 2008, over 50% of Dutch MSW was incinerated (SenterNovem, 2009).
[media:image:1]However, advanced technology is needed in order to minimise emissions of other GHGs than CO2 and non-GHGs during the combustion process. For example, good combustion minimises the formation of CO and products of incomplete combustion. The formation of N2O can be reduced by spraying ammonia or urea into the hot flue gas (a technology called Selective Non-Catalytic Reduction, which converts N2O to harmless nitrogen and water) (IWSA, no date). Moreover, acid gases can be removed by a ‘(dry) scrubber’, which sprays a mixture of lime and water into the exhaust thereby neutralising acid gases. A side-benefit of deploying such devices is the entrapment of heavy metals and organics. In fact, waste to energy is the only solid waste management method that permanently removes significant quantities of mercury from the environment.
All these potential contaminants need to be removed. This can be done with electrostatic precipitator’s that use electrically charged plates to capture the small particles of fly ash. Alternatively, ‘bag houses’ can be used that work like giant vacuum cleaners with hundreds of fabric filter bags. A bag house is an emission control device that consists of an array of fabric filters through which flue gases pass. Particles are trapped and thus prevented from passing into the atmosphere. The flue gas that is eventually discharged from the stack is primarily CO2, nitrogen, oxygen and water. Burning waste at extremely high temperatures also destroys chemical compounds and disease-causing bacteria whereas residual ash can be landfilled. In fact, in the USA, about 10% of the total ash formed in the combustion process is used for beneficial use such as daily cover in landfills and road construction. 
Conventional WTE is an important means of waste disposal in many countries in continental Europe. In most Western European countries WTE accounts for 30 to 60% of MSW disposal. Total worldwide installed capacity is over 3 GWh of which about half is in Europe (Boyle, 2004: 125). Europe annually treats over 50 million tonnes of wastes at WTE plants, which, according to the Confederation of European Waste-to-Energy Plants (CEWEP), could generate electricity for 27 million people or heat for 13 million people (Stengler, 2006). Incentives for combusting MSW are a shortage of suitable landfill sites (as was the case in Denmark) and the high cost of transporting wastes to distant sites.
[media:image:2]The US case is interesting though because it shows that the large availability of land and consequently low opportunity costs for constructing landfills works as an important disincentive for constructing MSW-fired plants. Nowadays, in the USA, there are 89 operational MSW-fired power generation plants, which generate approximately 2,500 MW, or about only 0.3% of total national power generation. In general, MSW becomes more attractive if land becomes scarce and fuel and electricity prices increase. Moreover, the continuing development of cleaner and more efficient WTE conversion technologies could make MSW combustion for energy production more attractive.
The situation in developing countries is expected to be quite different, although a clear picture is difficult to describe since not much information is available about MSW handling in developing countries. First, garbage or MSW collection programmes are not very well developed in developing countries which results in low garbage collection rates. Second, collected waste is usually dumped at uncontrolled landfills. Only recently, there has been growing awareness of the need to collect and properly treat MSW. Hence, several improvements can be made in many developing countries in terms of waste collection programmes and the useful application of collected waste, either through disposal of waste in well-managed landfills or through incineration of waste for electricity and/or heat generation.With respect to industrialised countries MSW combustion technology, it has been estimated that waste could supply up to 15% of the UK’s electricity demand; a figure that may apply to other developed countries as well (Wheeler, 2006).
An increasing wealth generally results in increasing amounts of waste (Bartelings, 2003), which can be observed in both industrialised and developing countries. Although less waste is produced per capita in developing countries, in many countries waste collection services are often poorly developed or do not exist and the awareness of people of the technology is generally poor. Based on the experience in a number of industrialised countries, it is important that developing countries develop centrally planned collection services on the basis of which a preferred method of waste handling can be chosen, either through disposal of MSW in landfills or for electricity and/or heat generation.
Social acceptability of WTE facilities has proven a big hurdle in the further development of this technology, in particular fear of environmental contamination, most notably dioxin emissions, prevented development of WTE power plants. In order to address this concern, in Europe specific EU Directives have put barriers to landfilling; in Germany, since 1 June 2005, untreated waste is no longer landfilled (Federal Ministry for the Environment, Nature Conservation and Nuclear Safety of Germany, 2005). As a result, in 2005, dioxin emissions from all 66 waste incineration plants in operation in Germany dropped from 400 grams to less than 0.5 grams as a result of the mandatory installation of filter units. In Sweden, fifteen years ago, eighteen waste incineration plants emitted a total of about 100 grams of dioxins every year. Nowadays, the number of operational waste incineration plants has grown to 29, but taken together they emit 0.7 gram only.Note that these plants more than doubled the amount of energy produced in 1985. Despite these improvements, still the perception that MSW incinerators are old and contaminating remains persistent in some cases.
Through MSW combustion, the waste volume can be reduced significantly in a controlled burning process; up to 90% in volume and 75% in weight (US EPA, no date). Material (e.g. metals, cans, glass, etc.) can be removed during the process as well for recycling purposes. Hence, it is important to note that waste incineration does not necessarily compete with recycling programmes. It has been found that increases in incineration of MSW with energy recovery do not correlate with low recycling rates (Kiser, 2003; Jackson, 2006; Stengler, 2006).
MSW combustion reduces the need for new landfills; only the ashes generated in the incineration process are disposed off at landfill sites but these are much smaller in volume and thus require less space. Such ashes may be toxic, however, which requires regular testing to prevent toxic substances from migrating into groundwater supplies. In the EU, waste incineration is preferred over landfilling and the landfilling of biodegradable waste is limited by law.
Other sustainable development benefits are quite similar to those of landfill gas (LFG) capture technologies.
Combustion of MSW could contribute to reducing GHG emission when the heat and electricity produced replace fossil fuel-based capacity. Depending on the prevailing mix of primary fuels used for electricity generation or heat production and the composition of collected waste (which may very much differ from country to country, or from region to region, see Figure 3 and Figure 4, which show the composition of MSW in the UK and Vietnam, respectively), CO2 emissions could potentially be reduced by 0.6 to 1.2 tonne/MWh electricity and some 0.25-0.6 tonne/MWh could potentially be reduced in heat production.
[media:image:3][media:image:4]Nonetheless, MSW combustion also causes CO2 emissions. As far as the biomass part of the MSW is concerned, this is considered CO2 neutral, but the burning of other fossil fuel-derived material such as plastics cause CO2 emissions and should be accounted for in calculating emission reductions (in the US case, these emissions make up about one-third of all CO2 emissions from MSW combustion). Again, this very much depends on the composition of the waste (see Figure 3 and Figure 4).
In addition, combusting MSW avoids the production of methane emissions from the decaying MSW. As per July 2010, two projects were registered as Clean Development Mechanism  (CDM) projects. Both projects are located in India
For the methane reduction effect, the GHG accounting methodology "Avoidance of methane production from decay of biomass through controlled combustion, gasification or mechanical/thermal treatment - version 16 " (AMS-III.E.) approved by the CDM executive board can be used. For larger installations "Avoided emissions from organic waste through alternative waste treatment processes- version 11"  (AM0025) can be used.
For the substitution of fossil fuels on the grid through the production of electricity by MSW combustion the GHG accounting methodology "Grid connected renewable electricity generation - version 16 " (AMS-1.D) can be used or the "Consolidated methodology grid-connected electricity generation from renewable sources - version 11"  (ACM0002) can be used. Which methodology is appropriate depends on the size of the installation. Both are approved by the CDM executive board.This methodologies help to determine a baseline for GHG emissions in the absence of the project (i.e. business-as-usual circumstances), how emission reductions below this baseline can be calculated, and how these reductions can be monitored.
General information about how to apply CDM methodologies for GHG accounting, as well as how to calculate GHG emission reductions from transportation or industrial use projects, can be found at: http://cdm.unfccc.int/methodologies/PAmethodologies/approved.html 
In Sweden, the investment costs of a plant that incinerates about 460 ktonnes of waste per year for both heat (for the nearby residential areas) and electricity amount to USD 286 million. Its annual revenues vary from USD 36 million to USD 70 million, so that the initial investment cost will be earned back within ten years (Schönning, 2006). MBT technology is economically less attractive when compared to conventional WTE systems. However, according to Wheeler (2006), local political, legislative or structural circumstances may favour the dedicated MBT combustion process over conventional technologies.
Depositing of untreated waste in landfills will no longer be permitted in Europe, which provides an incentive to offer household waste to power stations and waste incinerators. Moreover, as CO2 emissions can be reduced, this option may potentially benefit from emissions trading.
Based on the RES Electricity Directive, the biodegradable fraction of waste is considered biomass and thus a renewable energy source. Depending the composition of the waste (see Figure 3 and Figure 4), generally speaking, this fraction is over 50%. The EU Landfill Directive (1999/31/EC) stipulates that the amount of biodegradable waste going to landfills has to be reduced to 35% of the total amount (i.e. by weight, compared with the amount in 1995) by the year 2016.
Finally, the CDM may provide an interesting opportunity to reduce emissions from landfills in developing countries. However, at present, the focus within the CDM is very much on landfill management and recuperation of methane emitted from landfill sites (see also: Methane capture from landfills for electricity and heat), which is simpler in terms of GHG accounting (relatively straightforward methodology) and which generally result in larger GHG emission reductions (given that LFG capture projects reduce emissions of methane which global warming potential is 21 times stronger than that of CO2).
Boyle, G., 2004. Renewable Energy Power for a Sustainable Future, Oxford University Press, Oxford, United Kingdom.
Federal Ministry for the Environment, Nature Conservation and Nuclear Safety of Germany, 2005. Waste Incineration – a potential danger? Bidding Farewell to Dioxin Spouting, Bonn, Germany. Available at: http://www.bmu.de/english/waste_management/downloads/doc/35950.php 
Hanoi’s People Committee, 2003. Hanoi Urban Environment Company.
IWSA, no date. Available at: http://www.wte.org 
Jackson, C., 2006. Britian’s Waste: The Lessons We Can Learn from Europe, Conservatives in the European Parliament, London, United Kingdom.
Kiser, J.V.L., 2003. Recycling and Waste-to-Energy: The Ongoing Compatibility Success Story, MSW Management. Available at: http://www.mswmanagement.com/may-june-2003/recycling-and-waste.aspx 
Schönning, M., 2006. Integrated Waste Management in Sweden, Toronto Star, 2006.
SenterNovem, 2009. Afvalverwerking in Nederland: Gegevens 2008, Werkgroep Afvalregistratie.
Stengler, E., 2006. The Efficiency Question: The X factor for waste-to-energy in Europe, Waste Management World, Thermal Treatment and WTE Special.
US EPA, no date. International Activities – Landfill Methane Outreach Programme. Available at: http://www.epa.gov/lmop/international.htm 
Wheeler, P., 2006. Future Conditional: The role of MBT in recovering energy from waste, Waste Management World, Thermal Treatment and WTE Special.