(best examples and good practices) on household organic waste ...

39
conditions are achieved to prevent the formation of harmful products of
incomplete combustion. Such conditions may occur when high quantities of
wet garden waste come through the waste stream, especially in spring <strong>and</strong>
autumn.
Several material streams emerge from mass-burn incineration. The greatest of
these is the ash residue discharged from the combustion chamber, which may
represent between 20 – 30% of the mass of waste consumed. The ash may be
processed by stabilizing <strong>and</strong> grading to form a useful secondary construction
material that can be used for low-grade applications such as road or car-park
base layers. Re-use of incinerator ash varies from country to country. Most of
the existing incinerators in the UK <strong>and</strong> all plants in the Netherl<strong>and</strong>s have an
ash processing facility. Ash which cannot be re-used is l<strong>and</strong>filled. Metals can
also be recovered from the bottom ash <strong>and</strong> sold to preprocessors. In plants with
an ash-processing facility, nearly all of the ferrous metal can be recovered;
otherwise up to 90% can be recovered. Non-ferrous metal can also be recovered
in plants with ash processing.
Emissions st<strong>and</strong>ards for incinerators have recently been tightened through new
emission limits imposed under the new incineration directive <strong>and</strong> extensive
treatment of the flue gases is necessary to meet the new limits. Residue is
produced from the air pollution control system, representing about 2-4% by
weight of the incoming waste.
This material consists of salts <strong>and</strong> surplus alkali from acid gas neutralization;
although some plants using wet scrubber systems currently discharge the
scrubber residues to water as a salts solution. In addition, fly ash containing
dioxin <strong>and</strong> heavy metals is produced. This material requires disposal at
hazardous waste l<strong>and</strong>fills, usually after some form of stabilization or
immobilization in an inert medium such as cement has taken place. In
Germany, salt caverns are used for storage of such hazardous materials.
To be cost-effective, mass burn incinerators require a guaranteed supply of
waste within known limits of composition, available throughout the life of the
plant. Because of the large scale of operation, such facilities may effectively
‘lock-in’ supplies of waste that could otherwise go for recycling. In addition,
the requirement for bulk waste to be provided within a relatively narrow range
of calorific value means that removal of particular waste streams for recycling
could cause the remaining waste to fall outside the acceptable range.
For example, removal of paper <strong>and</strong> / or plastics for recycling would increase the
relative proportion of putrescible waste in the residue <strong>and</strong> lower its calorific
value. On the other h<strong>and</strong>, removal of putrescible wastes as well, for
composting, would help to keep the calorific value of the residue in the