Environmental Microbiology of Aquatic and Waste Systems

11.3 Nature of Municipal Solid Wastes 285

conserves resources for our children’s future, and
reduces the need for new landfills and combustors.
Recycling also helps reduce greenhouse gas emis-
sions that affect global climate. In 1996, recycling of
solid waste in the United States prevented the release
of 33 million tons of carbon into the air, roughly the
amount emitted annually by 25 million cars. The recy-
cling rate has been rising steadily in the US and in
2005 had reached about 32% (see Fig. 11.4).
A breakdown of the rates of recycling of various
items is given in Table 11.2, from where it will be seen
that the most widely recovered items were nonferrous
metals (69%), rubber and leather (11%), paper and
paper board (55%), steel (38%), plastics (7%) and
wood (10%).
Around the world, many of the waste items recycled
are paper, glass jars and bottles (especially from hotels
since most drinks are put in bottles), plastics and met-
als such as aluminum cans. There are usually three
bottle banks, one for each color of glass: clear, green,
and brown.
With regard to bottles in many parts of the world,
they are collected in bottle banks from where they are
sent for recycling.
Plastics make up a large amount of waste, since
they are available in numerous forms especially for
packaging. There are two main types of plastic: ther-
moplastics, which are the most common; and thermo-
setts. Thermoplastics melt when heated and can
therefore be remolded. This enables thermoplastics to
be recycled relatively easily. Plastic waste tends to be

sorted by hand, either at a materials recycling facility

or the householder can separate it.

The metals recycled come from iron and steel and

aluminum. Most of this waste comes from scrap vehi-

cles, cookers, fridges, and other kitchen appliances. It

is mainly made up of aluminum drinks cans and tin-

plated steel food cans. Aluminum is an expensive

metal and can therefore produce high incomes for

recycling schemes. Copper, zinc, and lead are also

recycled in the UK.

Many countries also rethread tires and reuse them.

For example, every year in the UK, between 25 and 30

million scrap tyres are generated. Approximately 21%

of these tyres are retreaded and reused. The old tread is

ground off the tyre and replaced with a new tread.

However, about half of all used tyres are dumped in

landfill sites throughout the country; other tyres may

be incinerated.

11.3.1.3 Incineration with Energy Recovery

Scrubbing the (flue) gas released during incineration

of pollutants

The incineration of wastes is the combustion of wastes.

Modern incinerators used in economically devel-

oped countries such as in the EU and the USA are

designed to avoid or minimize the short-comings of

the older incinerators.

Modern incinerators generally burn MSW and the heat

produced is used to boil water to produce steam

which in turn drives turbines for the production of

electricity. This, in the parlance of modern energy

0

1960

5.6 6.5 8.0 9.3

14.5

16.7

16.2%

26.0%

29.0%

31.7%

55.8

69.4

79.2

82.9

84.2

33.1%33.2%

30%

20%

10%

0%

40%

50%

33.2

9.6% 10.1%

6.4% 6.2% 6.6% 7.3%

1965 1970 1975

Total MSW recycling

Total MSW recycling (million tons)

Percent of generation recycled

Percent recycling

1980 1985 1990 1995 2000200520072008

10

20

30

40

50

60

70

80

90

Fig. 11.4 Recycling rates in the US, 1960–2008 (From municipal solid wastes in the US http://www.epa.gov/wastes/nonhaz/
municipal/pubs/msw2008rpt.pdf, Anonymous 2009a)