Biodegradation

Biodegradation is the process by which organic substances are broken down by 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 into minerals.

Biodegradable matter is generally organic material such as plant and animal matter and other substances originating from living organisms, or artificial materials that are similar enough to plant and animal matter to be put to use by microorganisms. Some microorganisms have the astonishing, naturally occurring, microbial catabolic diversity to degrade, transform or accumulate a huge range of compounds including hydrocarbons (e.g. oil), polychlorinated biphenyls (PCBs), polyaromatic hydrocarbons (PAHs), pharmaceutical substances, radionuclides and metals. Major methodological breakthroughs in microbial biodegradation have enabled detailed genomic, metagenomic, proteomic, bioinformatic and other high-throughput analyses of environmentally relevant microorganisms providing unprecedented insights into key biodegradative pathways and the ability of microorganisms to adapt to changing environmental conditions.^[1]

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Anaerobic biodegradation in landfill

Biodegradable waste in landfill degrades in the absence of oxygen through the process of anaerobic digestion. The byproducts 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.

Methods of measuring biodegradation

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 can be measured by anaerobic microbes and the amount of methane or alloy that they may be able to 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 dte 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.^[2] 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 be monitored as a measure of the biodegradability of the test material and converted into oxygen consumption units.

Measurement of anaerobic decomposition

BMP100 test, 100 day biogenic methane potential test, is a test method that determines the potential biodegradability of biodegradable wastes under anaerobic conditions by measuring the production of biogas. The test has not been peer-reviewed by the international community and no known official publication exists for it. Other published tests that accomplish this in shorter time are the GB21 protocol (DIN 38414).

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; for all practical purposes most organic materials reach the majority of decomposition in less than 45 days. By being run so long, however, the BMP100 test therefore measures the complete degradation of the waste.

Plastics

Biodegradable plastics made with plastarch material (PSM), and polylactide (PLA) will compost in an industrial compost facility. There are other plastic materials that claim biodegradability, but are more often (and possibly more accurately) described as 'degradable' or oxi-degradable; It is claimed that this process causes more rapid breakdown of the plastic materials into CO₂ and H₂O.

Indicative lengths of degradation

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: (from http://www.worldwise.com/biodegradable.html)

• Banana peel, 2 – 10 days
• Cotton rags, 1 – 5 months
• Sugarcane Pulp Products, 30 - 60 days
• Paper, 2 – 5 months
• Rope, 3 – 14 months
• Orange peels, 6 months
• Wool socks, 1 – 5 years
• Cigarette filters, 1 – 12 years
• Styrofoam cup, non-biodegradeable
• Tetrapaks (plastic composite milk cartons), 5 years
• Plastic bags, 10 – 20 years
• Leather shoes, 25 – 40 years
• Nylon fabric, 30 – 40 years
• Plastic six-pack holder rings, 450 years
• Diapers and sanitary pads 500 – 800 years
• Tin cans 50 - 100 years
• Aluminium cans 80 - 100 years

See also

• Anaerobic digestion
• Bioplastic - biodegradable plastics
• Biodegradable polythene film
• Bioremediation
• Landfill gas monitoring
• List of environment topics
• Microbial biodegradation

References

1. ^ Diaz E (editor). (2008). Microbial Biodegradation: Genomics and Molecular Biology, 1st ed., Caister Academic Press. ISBN 978-1-904455-17-2. 
2. ^ Biodegradability Testing Environment Agency (2005) Guidance on monitoring MBT and other pre-treatment processes for the landfill allowances scheme (England and Wales)