Thermal Treatment

Household waste has up to half the energy potential of coal. Recovering energy from the non-recyclable portion of household waste makes economic and environmental sense. The 230 million tonnes of municipal solid waste (MSW) created in Europe each year could meet 5% of Europe's energy needs. The main virtues of energy recovery are:

• waste volume reduction

• rendering waste inert

• recovering value from waste

• biodegradable waste diversion

• a practical method to manage increased waste arisings

There are four main reasons for public hostility to Energy from Waste (EfW) projects:

• reluctance to host new large-scale developments

• fears about the health impacts of emissions

• concern that EfW erodes enthusiasm to recycle, while still producing ash which must be landfilled

• anxiety of the scale of the plant - larger units imply a wider waste catchment area which can affect local networks

However, there are many examples of EfW plants blending in, usually because they have been there for many years and the communities are familiar with them. There are also examples of new facilities which quickly become part of the community. The unit in Dundee, Scotland signed a Good Neighbour Charter, which commits incinerator company Dundee Energy Recycling to standards above those required by law towards their neighbours in the community.

Dioxins. Dioxins are a family of chemicals, some of which are carcinogenic, which are present in the environment. Dioxins are also formed during manufacturing processes such as that for paper and in the metallurgical industries, as well as when organic materials such as coal and wood are burned.

A national study by the Swiss Environment Agency concluded that the largest source of dioxins in Switzerland is now the burning of household waste at home or in the garden. Controlled incineration of MSW results in the annual emission of 16 g dioxins, while domestic rubbish burning emits 27-30 g pa.

A materials flow analysis of dioxins in Denmark16 suggested that total formation of dioxins between 1989 and 1999 is around 90-830 g pa. The most important contributing activity is waste treatment and disposal, and municipal waste incineration is the dominant source for dioxin generation.

The National Society for Clean Air & Environmental Protection17 cites a survey of UK emissions of air pollutants which yields the data shown in Table 5. This suggests that MSW incineration contributed some 4% of UK dioxin emissions in 1998, a decline from 60% in 1990. The main factor in this fall was the closure of many older technology waste incinerators as a result of the 1989 EU municipal waste incineration directive.

Material versus energy. There is concern over whether materials recycling and energy recovery should be considered equally valuable. Most people agree that materials recycling should generally be considered before energy recovery, but that energy recovery is a valuable option within an integrated approach to resource and waste management. Because many recyclable materials, such as glass, aluminium and steel, are not combustible, their removal improves EfW plant efficiencies. Removal of wet organic wastes for composting also helps plant operation. Proper analyses of waste streams and volumes prior to plant construction can reduce conflict between material and energy recycling.

Ash management. Incinerator ash can be a source of pollution, but there is

16 E. Hansen, Substance flow analysis for dioxins in Denmark, Environmental project 570 for the Danish EPA, 2000.

17 P. Coleman, Dioxin measurement, Clean air & environmental protection Journal of the NSC A, Spring 2001, pp. 18-24.

Table 5 Sources of dioxins j ggf) j ggg

Source I

[g I-TEQ pa)

(g I-TEQ pa)

(%)

Power stations (coal and oil)

35

18

6

Coal combustion (industrial + domestic)

38

17

5

Wood combustion

26

26

8

(industrial + domestic)

Coke production

3

1

0

Sinter plant

42

43

13

Electric arc furnaces (iron and steel)

12

8

3

Non-ferrous metal production

27

22

7

Chemical industry

12

14

4

MSW incineration

602

14

4

Incineration - chemical waste

6

4

1

Incineration - clinical waste

51

24

7

Incineration - sewage sludge

5

3

1

Road transport

28

11

3

Accidental fires and open burning (farms)

121

64

20

Other sources

68

56

18

Total

1078

325

100

growing evidence that some of the material can be usefully recovered.18 In America, there has been growing interest in marine applications such as inshore erosion prevention and artificial reef construction. In Germany, half the incinerator bottom ash (IBA, residual material from the furnace) is used as road base material and for sound barrier construction. The Netherlands aim to use 80% of all MSW incinerator residues - 40% of incinerator fly ash (materials trapped in incinerator air pollution control equipment) is used as aggregate in asphalt. Around 60% of IBA (more than two million tonnes pa) has been used in road base, embankments, and as an aggregate in concrete. In Denmark, bottom ash has been used since 1974. Almost three-quarters (72%) is used as sub-base in car parks, cycle ways and roads.

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