The impact of waste management activities on a national scale can be assessed by comparing emissions from such processes with emissions from other activities.
Emissions of chemicals such as sulfur dioxide, nitrogen oxides, carbon dioxide, and methane are also of relevance to global issues such as acid rain, ozone depletion, and the greenhouse effect. In this context, waste disposal activities that discharge gases to atmosphere on a continuous basis are of importance, i.e.
combustion and landfilling.
It is generally acknowledged that, save perhaps for emissions of methane from landfills and of certain trace chemicals associated with waste incineration, waste management activities do not make a significant contribution to national inventories of emissions to atmosphere. For example in the UK, emissions of nitrogen oxides from two sources (motor vehicles and power stations) account for 90% of the national budget for these compounds in urban areas in episode conditions. Power stations alone account for 72% of sulfur dioxide emissions.
Combustion of coal, motor spirit, and gas accounted for 75% of the carbon dioxide emitted into the UK atmosphere.21 In Germany, carbon dioxide emitted from municipal solid waste (MSW) incinerators accounted for less than 1% of total emissions from all sources. Emissions of volatile organic compounds (VOCs) from waste incinerators contributed 0.1% to the total atmospheric burden in Germany, the main sources being road traffic (49%) and the use of solvents in industry (38%).22 An assessment of the contribution of MSW incineration to the US environment23 indicated that total acid gas and nitrogen oxide emissions amounted to less that 1% of these emissions from utility coal boilers. The carbon monoxide (CO) released from MSW incinerators amounted
16 US Environmental Protection Agency, ‘Estimating Exposure to Dioxin-like Compounds’, Review Draft EPA/600/6-88/005B, Office of Research and Development, Washington DC, 1992.
17 F. T. DePaul and J. W. Crowder, ‘Control of Emissions from Municipal Solid Waste Incinerators’, Noyes Data Corporation, New Jersey, 1989.
18 J. Brosseau and M. Heitz, Atmos. Environ., 1994, 28, 285.
19 P. J. Young and A. Parker, Waste Manage. Res., 1983, 1, 213.
20 P. J. Young and A. Parker, Chem. Ind., 1984, 9, 329.
21 Department of the Environment, ‘First Report of the Quality of Urban Air Review, Review Group’, London, 1993.
22 Ministry for Environmental Affairs, ‘Questions on Waste Incineration’, Document UM-8-89, Ministry for Environmental Affairs, Stuttgart, 1989.
23 R. S. Egdall, A. J. Licata, and L. A. Terracciano, in ‘Proceedings of the GRCDA/SWANA Sixth Annual Waste-to-Energy Symposium’, Arlington, Virginia, 1991.
Table 4 Average global emissions of trace elements in 1983 from a variety of industrial and waste-related activities (tonne y~1)27
As Cd Cr Cu Hg Ni Pb
Coal combustion 1980 530 11275 5185 2080 13 760 8160
Oil combustion 57 144 1410 1960 — 27 100 2420
Pyrometallurgy 12 255 5430 — 23 260 120 7990 46 535
Iron and steel 1420 160 15 620 1490 — 3570 7630
MSW incineration 270 730 540 1470 1120 260 2100
Sewage sludge 40 20 300 105 40 105 270
Wood combustion 180 120 — 900 180 1200 2100
Cement production 535 270 1335 — — 490 7130
Miscellaneous 2025 — — — — — 4500
Percentage contribution from incineration
1.7 10 2.8 4.5 33 0.7 3
*Non-availability of data is indicated by a (—) sign. The percentage contribution from incineration is therefore an upper limit.
to less than 0.05% of CO emissions from motor vehicles. A comparison was made of emissions of 15 carcinogenic organic compounds and 17 non-carcinogenic organic compounds emitted from hazardous waste incinerators in the US against releases reported by industry in the 1990 Toxic Release Inventory.24 The total mass emissions of all 32 organics from hazardous waste incinerators (100 tonnes) was less than 0.03% of the corresponding releases from industry (388 430 tonnes).
Emissions of specific trace metals and organics from waste incineration have been assessed against total releases to atmosphere. In Sweden, MSW incineration was identified as contributing over 50% of total emissions of cadmium and mercury in the mid-1980s, but with progressively tightening incinerator emission standards coupled with a campaign to remove these metals from MSW, this contribution is expected to fall to about 2% by the mid-1990s.25 In the UK, cadmium and mercury emissions from waste incinerators are believed to represent 15% and 60%, respectively, of the total emissions of these metals to atmosphere.26 This study is currently being updated by the Department of the Environment.
A global inventory of trace metals emitted to atmosphere in 1983 has been attempted.27 Table 4 summarizes the average contribution from each source type, for a selection of metals. Incineration (which includes both MSW and sewage sludge incineration) generally accounts for a small percentage of metal emissions to atmosphere save for mercury.
A number of countries have compiled inventories of PCDD and PCDF emissions to atmosphere.28—31 Waste (in particular, MSW) incineration had,
24 C. R. Dempsey, J. Air Waste. Manage. Assoc., 1993, 43, 1374.
25 C. Porter, Warmer Bull., 1990, 25.
26 P. K. Leech, ‘UK Atmospheric Emissions of Metals and Halides’, Report LR 923, Warren Spring Laboratory, Stevenage, 1993.
27 J. O. Nriagu and J. M. Pacyna, Nature (London), 1988, 333, 134.
28 J. Schaum, D. Cleverly, M. Lorber, L. Phillips, and G. Schweer, Organohalogen Comp., 1993, 14, 319.
29 J. de Koning, A. A. Sein, L. M. Troost, and H. J. Bremmer, Organohalogen Comp., 1993, 14, 315.
30 H. Fiedler and O. Hutzinger, Chemosphere, 1992, 25, 1487.
31 S. J. Harrad and K. C. Jones, Sci. Total Environ., 1992, 126, 89.
Table 5 National inventories of PCDD and PCDF emissions to atmosphere (g y~1)28—31
US Netherlands W. Germany UK
MSW 60—200 382 5—432 11
Clinical 500—5100 2.1 5.4 1.7
Chemical 2.6—8.4 16 0.5—72 ]
Sewage sludge 1—26 0.3 0.1—1.13
Coal 3.7 2.99 12.8
Oil 1.0 1.29
Wood 70—1600 12
Forest fires 300—3000
Traffic \8—870 7 0.1—0.4 0.7
Sintering processes 230—310 26 38—380
*g y~1 of PCDDs and PCDFs expressed as International Toxic Equivalents, I-TEQ.
]‘a few grammes of TCDD’.
9Domestic heating only.
until the mid-1980s, been the focus of attention as the principal emission source, but more recent studies have identified operations such as secondary metal smelting as significant contributors to national budgets of PCDDs and PCDFs.
Whereas in the past MSW incineration may well have contributed dispropor-tionately to the national atmospheric burden of PCDDs and PCDFs, the imposition of increasingly stringent emission limits has resulted in a 100-fold reduction in emissions from this source, and similar emission limits or guidelines applied to other waste incineration processes should further reduce the significance of waste-related thermal processes relative to other sources of emissions. Table 5 summarizes information on annual releases in grams of PCDDs and PCDFs to atmosphere, expressed in terms of International Toxic Equivalents, I-TEQs. Inter-country comparisons are not valid, since the national inventories were conducted at different times and the more recent inventories tend to identify new sources not previously suspected of being significant emitters of PCDDs and PCDFs. In the UK, chemical, MSW, and clinical waste incinerators are believed to contribute approximately 40% of the national atmospheric burden of PCDDs and PCDFs31 but a broader examination of industry could well identify new sources.
The relative merits of incineration versus landfilling have been examined in terms of emissions of greenhouse gases. It is estimated that between 17% to 58%
of the UK’s annual emissions of 5.3 million tonnes of methane derive from landfills, followed by agricultural sources (30%) and coal mining (14%).32 One tonne of biodegradable waste generates approximately 6 m3 of landfill gas annum~1, the main constituents being methane (55%) and carbon dioxide (45%).
Methane is estimated to be 25—30 times more potent than carbon dioxide as a
32 Anon, Environ. Data Services, 1993, 217, 7.
Table 6 Comparison of 42 MW Office from conventional 45 MW utility building landfill sources incinerator
Particulates 93 20 0 113 56
CO2 equivalent 368000 26000 550 000 944 000 575 000
CO 30 104 5 139 143
SO2 1256 73 4 1333 195
NOx 867 55 41 963 842
HCl 59 2 0 61 93
Hydrocarbons 4 9 52 65 5
greenhouse gas. The greenhouse effect of landfilling one tonne of MSW has been calculated as being equivalent to about 4.8 tonnes of carbon dioxide, as opposed to 0.8 tonnes of carbon dioxide released through incineration, a difference in potency of a factor of six.1
Comparisons have also been made between the release of carbon dioxide and other chemicals from waste-to-energy plants (i.e. power plants using waste materials as fuel) and from conventional power stations. For example, it has been estimated33 that the offset in carbon dioxide emissions arising from the utilization of waste-to-energy as a replacement for coal is approximately 430 kg tonne~1 of MSW, assuming that one tonne of MSW generates 480 kWh and the emissions of carbon dioxide from coal combustion is 1 kg (kWh)~1. This is equivalent to a saving of approximately 50% of the carbon dioxide produced by coal-firing for similar amounts of electricity generation. One estimate for the UK equates the energy value of the 30 million tonnes of MSW generated annually to 10 million tonnes of coal equivalent.34 A similar comparison, but on a more localized scale, has been provided for a proposed waste-to-energy plant in Bridgeport, Connecticut, to replace 10 MW and 35 MW of energy generated from oil and coal, respectively.23 The waste-to-energy option resulted in a net decrease of 85% in sulfur dioxide emissions, 44% in particulate matter, and 10% in nitrogen oxides. Due to the magnitude of these decreases, they were considered to more than offset increases in the quantities of carbon monoxide (by a factor of 3.5) and hydrogen chloride (by 48%) emissions.
Another interesting comparison has been made between options for development of the Bridgeport site and associated power sources.23 The options were development of the site as a 12-storey office building, or as a 1500 tonne day~1, 45 MW waste-to-energy plant to replace existing oil and coal fired power stations.
In the former case, 42 MW of power was supplied by an oil and coal fired utility, in addition, the office building had an on-site boiler for providing space heating and other services. Further, 1500 tonnes of MSW would need to be landfilled in the vicinity, resulting in the release of landfill gas, offset by the recovery of a proportion of the gas to supply 3 MW of energy. Table 6 summarizes the net
33 H. F. Taylor, in ‘Proceedings of the USEPA/AWMA Second Annual International Conference on MSW Combustion’, Tampa, Florida, 1991.
34 A. Porteous and R. S. Barrett, ‘Proceedings of ‘Incineration an Environmentally Acceptable Means of Waste Disposal?’’, Institution of Mechanical Engineers, London, December 1993.
emission impact from the above options. The emission budget for incineration does not take into account employee travel (included in the office building scenario) and transport of waste to and from the facility. An overall reduction of about 60% in emissions of carbon dioxide and about 50% for the total trace pollutants was computed when the incineration option was adopted.
Such comparisons are increasingly being undertaken on a regional or sub-regional scale in order to place waste management options within the wider context of other emission sources or other development options. Inevitably, this will necessitate a study of the interaction between waste management options (for example, between recycling, landfilling, and incineration) and between waste and non-waste related activities, and will form an important aid to regional and national planning.