The NAEI reports methane and NMVOC emissions managed landfills only, as open dumps and unmanaged landfills are considered insignificant in the UK.

The UK method conforms to IPCC good practice since a first order decay (Tier 2) methodology is used based on estimates of historical data on waste quantities, composition and disposal practices over several decades.

The UK method is based on equations 4 and 5 in the Revised 1996 IPCC guidelines (IPCC, 1997) (pp 6.10-6.11) which are compatible with equations 5.1 and 5.2 in the Good Practice Guidance (IPCC, 2000). A slightly different version of equation 5.1 is used, which takes into account the fact that the model uses a finite time interval (one year). The full derivation of the equations used is given in Appendix 2 of Brown et al (1999).

The UK method divides the waste stream into four categories of waste: rapidly degrading, moderately degrading, slowly degrading and inert waste. These categories each have a separate decay rate. The decay rates are based on data from the Netherlands and range from 0.05 (slowly degrading waste) to 0.185 (rapidly degrading waste), which lie within the range of 0.03 to 0.2 quoted in the Good Practice Guidance.

The model extends back to 1945, which gives a time period of around 4 half lives for the slowest of the three decay rates (0.05, half life 14 years). This lies within the range of 3 to 5 half lives recommended by the Good Practice Guidance.

The model distinguishes between four separate categories of landfill site with different degrees of gas collection control:

• closed sites;
• sites with no gas collection;
• sites with limited collection;
• sites with comprehensive collection.

Each site type has different gas collection and oxidation rates. As recommended, the model attempts to take into account changes in landfill practice over past decades by altering the proportion of waste disposed of to each of these categories of site in past years, and also by modifying the gas collection rate over time where appropriate. The model also simulates retrofitting of sites, i.e. upgrading from a site with no gas control or limited gas control to one with comprehensive gas control.

The estimates of historical waste disposal and composition data are based on various data sources, described fully in Brown et al 1999. As recommended in the Good Practice Guidance, estimates for municipal waste are based on population where data is absent.

As recommended in the Good Practice Guidance, the estimates of waste disposal quantities include commercial and industrial waste, demolition and construction waste and sewage sludge, as well as municipal waste. For industrial and commercial waste, the data are based on national estimates from a recent survey, although the survey was incomplete at the time of finalising the model estimates. The data were extrapolated to cover past years based on employment rates in the industries concerned.

All sites in the UK are managed, and therefore have a methane correction factor of 1.0. However, as described above, differences in oxidation rates have been simulated by the practice of dividing waste disposal sites into four types as described above.

Degradable organic carbon (DOC) was estimated based on a national study, as recommended in the Good Practice Guidance. However the figures used were based on expert opinion rather than measured data.

The fraction of degradable organic carbon dissimilated (DOC_F) was assumed to be 60%. At the time when the model was set up, the IPCC recommended default value was 77%, but there were indications that this could be an overestimate, so a lower figure was used. The new IPCC recommended range quoted in the Guidance is 50-60%.

The fraction of CH₄ in landfill gas is generally taken to be 50%, which is in line with the Guidance. For old shallow sites it is taken to be 30% to reflect a higher degree of oxidation.

The fraction of methane recovered was assumed to be 85% for sites with full gas control and 40% for sites with limited gas control. The estimates are not derived from metering data, as recommended by the Guidance, as such data were not readily available at the time of the study. A panel of UK industry experts selected the figures.

The oxidation factor is assumed to be 10% for all site types. Recovered methane is subtracted before applying the oxidation factor. This is in line with the Guidance.

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 2.3 and emissions from heat generation are included under Miscellaneous in the base combustion module.

The NAEI category Sewage reports an estimate of emissions from the public wastewater treatment system. The NAEI estimate is based on the work of Hobson et al (1996) who estimated emissions of methane for the years 1990-95. Subsequent years are extrapolated on the basis of population. Sewage disposed of to landfill is included in landfill emissions.

The methodology of the UK model differs in some respects from the IPCC default methodology. The main differences are that it considers wastewater and sewage together rather than separately. It also considers domestic, commercial and industrial wastewater together rather than separately. Emissions are based on empirical emission factors derived from the literature expressed in kg CH₄/tonne dry solids rather than the BOD default factors used by IPCC. The model however complies with the IPCC Good Practice Guidance as a national model (IPCC, 2000).

The basic activity data are the throughput of sewage sludge through the public system. The estimates are based on the UK population connected to the public sewers and estimates of the amount of sewage per head generated. From 1995 onwards the per capita production is a projection (Hobson et al, 1996). The main source of sewage activity data is the UK Sewage Survey (DOE, 1993). Emissions are calculated by disaggregating the throughput of sewage into 14 different routes. The routes consist of different treatment processes each with its own emission factor.

The model accounts for recovery of methane and its subsequent utilization and flaring by estimating the proportion of anaerobic digester emissions that are recovered.

Nitrous oxide emissions from the treatment of human sewage are based on the IPCC (1997c) default methodology. The average protein consumption per person is based on the National Food Survey (MAFF, 2000e). These range from 22.7 to 23.7 g protein/person/day. The food survey is based on household consumption of food and so may give a low estimate.

The NAEI estimates emissions from the categories MSW incineration and sewage incineration. Included in the inventory for the first time are the categories clinical incineration and cremation. However the coverage of these new sources is incomplete due to a lack of emission factor data. The emission factors used are shown in Table A39. The emission factors for N₂O have been revised in the new inventory based on IPCC (2000).

1 Emission factor as kg carbon/ t waste

a Royal Commission on Environmental Pollution (1993)

b EMEP/CORINAIR (1996).

c Clayton et al. (1991)

d Estimated from THC data in CRI (Environment Agency, 1997) assuming 3.3% methane split given in EMEP/CORINAIR (1996)

e EMEP/CORINAIR (1996). A factor of 14 kt/Mt is used prior to 1996.

f IPCC (2000)

g Emission factor for 1999, Environment Agency (2000)

h EMEP/CORINAIR (1999)

i Munday (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 are based on incinerator capacity (Patel et al., 2000). The amounts of sewage sludge incinerated are reported in DETR (2001). Data on cremations are published by the Cremation Society of Great Britain (CSGB, 2000). Under UNECE /CORINAIR SNAP 94 reporting format, incineration refers only to plant that do not generate electricity. From the end of 1996, MSW incinerators in the UK had to meet new standards. As a result, many incinerators closed down, were renovated or new ones built. From 1997 onwards all MSW incinerators generated electricity and are classified as power stations so no emissions are reported under Incineration: MSW. Emission factors for modern incinerators based on 1999 data are reported as MSW (new) for comparison with the emission factors used for old incinerators. The emission factors given for MSW (old) pertain to old incinerators prior to 1993. Emission factors for the years 1993-1999 were derived from the Pollution Inventory (Environment Agency, 2000). Emissions of carbon dioxide deriving from recently photosynthesised carbon are excluded. It was assumed that the proportion of recently photosynthesised carbon was 75% of the total carbon content of the waste (Brown, 1995) and this assumption is reflected in the factors in Table A39.