1. Solid Wastes

2. Composition of solid waste in 1993 (% by weight)

3. 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.

4. 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

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

6. 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.

7. Incinerated 16% Landfilled 57% 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).

9. Degradable plastics (2) Photodegradable plastics: light sensitive functional groups such as carbonyl group (C=O) 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.

10. 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.

11. 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

12. 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.