National Atmospheric Emissions Inventory

Appendix 1

The Methodology of the National Atmospheric Emissions Inventory

2.10 Waste Treatment and Disposal

2.10.1 Landfill

Emissions are reported from landfills only, since open dumps are not a significant source in the UK. The estimation of methane emissions from landfill is uncertain because the amount of waste disposed of to landfill is not reliably known. The NAEI estimate is based on a recent study (Aitchison et al., 1996). The main features of this model are:

  • Two types of landfill site are considered - type I with gas collection and type II without.
  • The proportion of waste disposed of to the two types of landfill site was derived from data on landfill site flare sales from 1980 to 1995 and estimates of landfill gas utilization. Thus the proportion for each type varied from year to year.
  • The methane generation of current and historical waste was assumed to follows a first order model.
  • A proportion of methane is removed either by biological oxidation or combustion in flaring or energy utilization systems.
  • Recent data (Bellingham et al. 1994) with some updates and additions on waste arisings, sources and composition were extrapolated to 1970-1994 based on indexes appropriate to the respective sources (i.e. household waste - number of dwellings; construction and demolition - number of employees; industrial waste and sewage sludge - population)

The emissions of pollutants from the flare stacks were not estimated but those from electricity generation and heat generation were. Emissions from electricity generation are considered under Power Stations in Section 1.3.3. Emissions from heat generation are included in the base combustion module under Miscellaneous.

An estimate of NMVOC was made using an emission factor of 0.01 t NMVOC/ t methane produced which is equivalent to 5.65g NMVOC/ m3 landfill gas (Passant, 1993).

The NAEI no longer reports carbon dioxide emissions from the anaerobic decay of landfilled waste since this is considered to be part of the carbon cycle and is not a net source.

The estimates include the contribution of sewage disposed of to landfill.

2.10.2 Sewage Treatment

The NAEI estimate is based on the work of Hobson et al. (1996) who estimated emissions of methane for the years 1990-95. The 1990 estimate was scaled on the basis of population to estimate emissions for previous years and the 1995 estimate was extrapolated to 1996 in a similar manner. Sewage disposed of to landfill is included under landfill. Emissions from the combustion of sewage gas to generate electricity are reported under power stations, and for heat generation under public administration.

2.10.3 Waste Incineration

The NAEI estimates emissions from the categories Incineration: MSW and Incineration: Sewage. The emission factors used are shown in Table A28.

Table A28 Emission Factors for Waste Incineration (kg/t waste)

  C1 CH4 N2O NOx CO NMVOC SO2 BS PM10
MSW 75a 0.0008d 0.148b 1.8c 0.487e 0.0231d 0.647e 3.29f 0.647
MSW(old) 75a 0.0008d 0.148b 1.8c 0.709c 0.0231d 1.36c 15g 15
Sewage 0 0.39b 0.227b 2.5b 15.5b 0.84b 2.3b 7.5g 0.075
Factor estimated by scaling uncontrolled factor (Munday,1990) with total particulate data
1Emission factor as kg carbon/ t waste
aRoyal Commission on Environmental Pollution (1993)
bEMEP/CORINAIR(1996)
cClayton et al. (1991)
dEstimated from THC data in CRI(Environment Agency, 1997a) assuming 3.3% methane split given in EMEP/CORINAIR(1996)
eEstimated from emissions reported in CRI (Environment Agency, 1997a)
f given in CRI(Environment Agency, 1997a)
gMunday (1990)

The arisings of waste and their method of disposal are not known with any reliability. The estimates of municipal solid waste disposed of to incinerators is based on incinerator capacity (Patel et al., 1995). Under the UNECE /CORINAIR SNAP 94 reporting format, incineration refers only to plant that do not generate electricity. Only those emissions of carbon dioxide deriving from recently photosynthesised carbon are estimated. It was assumed that the proportion of recently photosynthesised carbon was 25% of the total carbon content of the waste (Brown, 1995) and this assumption is reflected in the factors in Table A28. From the end of 1996, waste incinerators have had to meet new standards. As a result, many incinerators have closed down, been renovated or new ones have been built. The emission factors in Table A28 are for 1996 and are largely based on CRI data. Those given for MSW(old) are used for years prior to 1995.