to read the full report - Ecolateral by Peter Jones

Evaluation of Opportunities for Converting Indigenous UK Wastes to Wastes and Energy
AEA/ED45551/Issue 1
The most common system being promoted for the treatment of MSW using MHT is based around a
thermal autoclave. The development for use with MSW started in the early 1990’s, and a number of
patents have been issued.
A second type of MHT system is a non-pressurised heat treatment process, where waste is mixed to a
slurry with water and then heated in a rotating kiln prior to mechanical separation. This process requires
a pre-treatment stage, which includes the use of a shredder.
Both the autoclave process and the non-pressurised treatment processes have the following effect on the
waste:
• Biodegradable materials, including paper and card, are broken down into a high biomass fibre for
which the main use is as a fuel product;
• Glass bottles and tins have their labels removed as the glue disintegrates under the action of the
heat;
• Plastics are softened, and labels are removed. Certain types of plastics are deformed by the heat,
but remain in a recognisable state, whereas other plastics soften completely forming hard balls of
dense plastic.
Both processes produce product streams (fibre product, plastic, etc) using well-established mechanical
processing techniques (screening, density separation, magnetic separation, eddy current separation).
The resulting outputs are relatively clean ‘hard’ recyclables (tins, glass and plastics with no labels and
most of the food waste removed) a fibrous material from the breakdown of paper, card and green/kitchen
waste constituents, and a reject fraction.
Both systems heat the waste to temperatures in the range of 120-170°C, which is sufficient to destroy
bacteria present in the waste. This has benefits in terms of storage, transport and handling of the outputs
as they are sanitised, and are free from the biological activity that may give rise to odour problems. There
is also a significant volume reduction in the waste.
Table 52 shows the typical specification of a mechanical heat treatment plant. About 15-20% of the input
waste is landfilled.
Table 52: Typical specification of a mechanical heat treatment process
Product stream Wt % of input MSW
Fibre product 60-65
Metal products 4-5
Mixed plastic product 7-12
Aggregate substitute product 4-8
Landfilled 180 15-20
Total 100
Other products from the process
Suppliers of MHT plants claim that these plants can recycle up to 20% of the input waste. However, whilst
established markets for the metal products are available, the total level of recycling achieved will depend
on the availability of markets for both the mixed plastic product and the aggregate substitute product. If
these markets are not available then these products will be landfilled.
Commercial status
Some demonstration projects are operating under the Defra Waste Innovation Programme, such as the
Huyton MHT site in Merseyside.
180 The landfilled reject stream is mainly other combustible materials.
101