Biodegradation
Biodegradation is the
decomposition of
organic material by
microorganisms.
Biodegradation is the process by which organic substances are broken down by other living organisms. The term is often used in relation to
ecology,
waste management,
environmental
remediation (
bioremediation) and to
plastic materials, due to their long life span. Organic material can be degraded
aerobically, with
oxygen, or
anaerobically, without oxygen. A term related to biodegradation is biomineralisation, in which organic matter is converted to into minerals.
Biodegradable matter is generally organic material such as plant & animal matter and other substances originating from living organisms.
Biodegradable waste in
landfills degrades
anaerobically in the absence of oxygen through the process of
anaerobic digestion. The biproducts of this anaerobic biodegradation are
biogas and lignin and cellulose fibres which cannot be broken down by
anaerobes (anaerobic microbes)
Engineered landfills are designed with liners to prevent toxic
leachate seeping into the surrounding soil and groundwater. Paper and other materials that normally degrade in a few years degrade more slowly over longer periods of time.
Biogas contains methane which has approximately 21 times the global warming potential of
carbon dioxide. In modern landfills this biogas can be collected and used for
power generation.
Biodegradation can be measured in a number of ways. The activity of aerobic microbes can be measured by the amount of oxygen they consume or the amount of carbon dioxide they produce. Biodegradation by anaerobic microbes can be measured by the amount of methane they produce.
Measurement of aerobic decomposition
The
DR4 test or 4-day
dynamic respiration index test is a test to measure the biodegradability of a substance over 4 days. The substance is aerated by passing air through it. This definition is used to differentiate the method from those where aeration is by diffusion of air into and out of the test material which is referred to as the SRI or
static respiration index test
[Biodegradability Testing Environment Agency (2005) Guidance on monitoring MBT and other pre-treatment processes for the landfill allowances scheme (England and Wales)]. Microbes are introduced to the test material whilst incubating it under aerobic conditions by aerating the mixture in a vessel through which air is blown. The microbes degrade the material producing CO2 as the product of biodegradation. This CO2 production can by monitored as a measure of the biodegradability of the test material and converted into oxygen consumption units.
Measurement of anaerobic decompostion
BMP100 test, 100 day
biogenic methane potential test, is a test method that determines the biodegrability of
biodegradable wastes under anaerobic conditions by measuring the production of
biogas.
Under
anaerobic methanogenic conditions the decomposition of organic carbon proceeds by producing biogas (containing methane and carbon dioxide)from the organic carbon. The amount of biogas production therefore measures directly the carbon which is mineralised.The test is set up in a small vessel containing the test substrate, a mineral aqueous medium and an
inoculum of methanogenic
bacteria taken from an active
anaerobic digester. The test is monitored by collecting and measuring the biogas produced. The test is incubated for an extended period until gas production ceases which may be up to 100 days or more. The test therefore measures the complete degradation of the waste.
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, kissit biodegradable plastics such as
Bio-Batch made plastics or
plastarch material (PSM), which will biodegrade, while others such as
polylactide (PLA) will
compost. There are plastic materials that claim biodegradability, but are more often (and possibly more accurately) described as 'degradable' or oxy-degradable; It is claimed that this process causes more rapid breakdown of the plastic materials into CO
2 and H
2O.
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:
*
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
*
Tetrapaks, (plastics composite) milk cartons, 5 years
*
Leather shoes, 25 to 40 years
*
Nylon fabric, 30 to 40 years
*
Plastic bags, 10-20 years
*
Plastic 6-pack holder rings, 450 years
*
Styrofoam cup,1-100 years
*
Banana peel, 2-10 days
*
Anaerobic digestion*
Bioplastic - biodegradable plastics
*
Biodegradable waste*
Biodegradability prediction*
Biodegradable/degradable polythene film*
Bioremediation*
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