Biodegradation
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Biodegradation is the decomposition of organic material by microorganisms. It is often used in relation to sewage treatment, environmental remediation (bioremediation) and to plastic materials although biodegradation is perhaps better regarded as the closing of the loop commencing with photosynthesis. Whereas photosynthesis is the process of creating growing matter through the conversion of carbon dioxide (CO2) and water (H2O) into plant material through the action of sunlight, biodegradation is the process of converting organic materials back into CO2 and H2O through microbial action.
Biodegradable matter is material that will biodegrade. For example, most plastic bags are not biodegradable, but paper bags are in a general sense. However, paper is usually not biodegradable according to the principal world standards because paper generally contains too many heavy metals to comply.
In managing waste disposal, degradability can make an important difference, because plastic bags stay around forever if left as litter, whereas paper bags will eventually decompose. There is a third category of materials, biodegradable plastics, such as MaterBi, which will biodegrade, while others such as polylactide (PLA) will compost. However, in anaerobic landfills, whether a material is biodegradable makes little difference; biodegradable matter usually does not decay, because of the lack of oxygen required by the microorganisms. However, if biodegradation does occur it is likely to be through an anaerobic process, thus giving rise to methane production as distinct from carbon dioxide production. Methane has approximately 10 times the global warming potential of carbon dioxide. In modern landfills this biogas will be collected and used for power generation.
A difficulty is that the term biodegradable has both a general meaning and a precise meaning given by different standards bodies. The European Norm EN13432, Japanese Greenpla Standard and the American Society for Testing and Materials (ASTM International) D6400-99 standards all define biodegradability in respect of a time period of 6 months. In the case of EN13432 a material is deemed biodegradable if it will break down to the extent of at least 90% to H2O and CO2 and biomass within a period of 6 months. ASTM D6400-99 and ASTM D6400-99 also impose a time period of 6 months. Each of the named standards sets limits for the amounts of heavy metals that the material may contain. (Note that DIN V49000 sets the strictest standards, that is, it permits the lowest value of heavy metal presence).
There are plastic materials that claim biodegradability, but are more often (and possibly more accurately) described as 'degradable' or oxy-degradable; these usually utilize one or more heavy metals to promote oxidation and thus disintegration of the polyethylene. It is claimed that this process causes more rapid breakdown of the plastic materials into CO2 and H2O although there appears to be no scientific evidence to support this claim. The US-based Biodegradable Products Institute has found alarming levels of heavy metals in some such products.
The following table should be read with the above comments in mind, and care should be taken before accepting claims of biodegradability in view of the (dubious) claims being made. This is how long it takes for some commonly used products to biodegrade, when they are scattered about as litter:
| Product | Time to biodegrade |
|---|---|
| Cotton rags | 1-5 months |
| Paper | 2-5 months |
| Rope | 3-14 months |
| Orange peels | 6 months |
| Wool socks | 1 to 5 years |
| Cigarette filters | 1 to 12 years |
| Plastic coated paper milk cartons | 5 years |
| Leather shoes | 25 to 40 years |
| Nylon fabric | 30 to 40 years |
| Tin cans | 50 to 100 years |
| Aluminum cans | 80 to 100 years |
| Plastic bags | 450 years |
| Plastic 6-pack holder rings | 450 years |
| Glass bottles | 1 million years |
| Plastic bottles | Forever |