Waste Treatment and Disposal

Landfill

The NAEI category Landfill maps directly on to 6A1 Landfills for methane emissions. Emissions are reported from landfills only, as open dumps are considered insignificant 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 study by Brown et al (1997). The main features of this model are:

• The methane generation of current and historical waste was assumed to follows a first order model.
• Landfill sites were classified into four types each with a different methane generation and collection characteristics.

1. Those with no gas control
2. Those with limited gas control
3. Those with comprehensive gas control
4. Old sites which closed before 1980

• Waste arisings, and waste composition data were compiled using the latest UK national statistics, together with an analysis of past trends in waste generation and disposal.
• Data on methane generation, oxidation and collection has also been collected, from literature studies and expert opinion, together with an analysis of past trends in landfill design and management specific to the UK
• Waste can be divided into three fractions which decay at different rates - these are classified as rapidly degradable, moderately degradable and slowly degradable organic fractions.
• A full review of baseline data for input to the model was undertaken. Data on the division of waste between sites with different degrees of methane control was based on the Environment Agency’s landfill GIS datbase. Waste arisings and composition were based on national statistics supplemented by new analysis. Parameters governing methane generation, such as the carbon content and decay rates of the waste, were based on literature studies supplemented by expert opinion.

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. 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).

Neither the GHGI nor the NAEI 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.

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 1998 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.

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.

Waste Incineration

The NAEI estimates emissions from the categories Incineration: MSW and Incineration: Sewage. The emission factors used are shown in Table A39.

Table A39 Emission Factors for Waste Incineration (kg/t waste)

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 Factor estimated by extrapolating 1998 PM10 emission with total 1994 PM data in Pollution Inventory, (Environment Agency, 1999)

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

g 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., 1999). Under the UNECE /CORINAIR SNAP 94 reporting format, incineration refers only to plant that do not generate electricity. From the end of 1996, waste incinerators have had to meet new standards. As a result, many incinerators have closed down, have been renovated or new ones have been 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 are reported in Table A15. The emission factors given in Table A35 pertain to old incinerators prior to 1993. Emission factors for the years 1993-1996 were derived from the Pollution Inventory (Environment Agency, 1999). 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 A35.