Solid Wastes

1
Solid Wastes
2
Composition of solid waste in 1993 ( by weight)
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Solid Wastes in Hong Kong

• An average of 2x107 kg solid waste daily in 1993.
• About 3.5 kg per head of population.
• Plastic, paper and metals occupied about 40 by
  weight of the domestic waste.

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Disposal of solid waste

• Two common disposal methods Land filling and
  Incineration
• Incineration
• Advantages
• reducing the volume by 85
• No risk of underground water pollution
• Disadvantages
• polluting the atmosphere with toxic chemicals

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Disposal of solid waste

• Incineration
• Disadvantages
• Potential environmental problems are
• (1) Generation of landfill gas, mainly methane.
• (2) Polluting the underground water

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Pollution problem associated with the disposal of
plastics

• Plastic waste in the sea pose direct danger to
  fishes.
• Plastic waste can last for a long time, they do
  not undergo decomposition easily in the landfill
  site.
• Incineration produces air pollutants such as
  hydrogen chloride from PVC and other chlorine
  containing polymers.

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Disposal methods of solid waste in Hong Kong
Commercial
6
Landfilled
73
21
Domestic
Industrial
8
Degradable plastics

• Slow degradation because enzymes in
  micro-organisms attack only at the ends of the
  polymer chains.
• Several types of degradable plastics
• (1) Biopolymers polymers made by living
  organisms. Poly(hydroxybutyrate), PHB, a natural
  polyester made by bacteria. Degradation completed
  within 9 months. 15 times more expensive than
  poly(ethene).

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Degradable plastics

• (2) Photodegradable plastics light sensitive
  functional groups such as carbonyl group (CO)
  can in incorporated into the polymer chains. The
  long chains can be broken down under the action
  of sunlight into shorter fragments which can then
  be biodegradable.
• (3) Synthetic biodegradable plastics made by
  incorporating starch or cellulose into the
  polymer during production.

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Recycling of plastics

• Direct recycling applies only to
  thermoplastics. The plastics are separated,
  cleaned, pulverized, and remoulded into plastic
  items.
• The regenerated plastics usually have
  deteriorated properties due to repeated thermal
  and mechanical processing.

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Coding symbols for sorting waste plastics
Low density polyethene
Polyethylene terephthalate
4
1
LDPE
PETE
High density polyethene
Polypropene
5
2
PP
HDPE
Polystyrene
6
Polyvinyl chloride
3
PS
V
All other resins
7
OTHER
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Recycling of plastics

• Recycling of energy energy values of plastics
  are quite high. The energy obtained from burning
  plastic waste can be used for heating or
  generation of electricity.
• Recycling of chemicals plastic waste is
  pyrolysed (decomposed at high temp. in the
  absence of air), the products are fractionally
  distilled to give useful chemicals.