Biodegradation

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Biodegradation is the decomposition of organic material by microorganisms.

Contents

1 Introduction
2 Landfills
3 Standards
4 Plastics
5 List of biodedgradability times
6 See also

Introduction

The term biodegradation is often used in relation to sewage treatment, environmental remediation (bioremediation) and to plastic materials. Organic material can be degraded with oxygen (aerobically) or without oxygen using other electron acceptors such as nitrate, ferric iron or sulphate. A term closely related to biodegradation is mineralisation, in which organic matter is converted to into CO₂ , H₂O, (+ CH₄ in anaerobic conditions) 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. No, papers do not have any "metals" in them. Papers are made from wood which consists cellulose. Cellulose is hard to breakdown so paper will not decompose easily.

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.

Landfills

Modern landfills are designed to be anaerobic inhibit degradation so that toxic wastes do not seep into the surrounding soil and groundwater, so paper and other materials that normally degrade in a few years will not biodegrade over longer periods of time. 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 21 times the global warming potential of carbon dioxide. In modern landfills this biogas may be collected and used for power generation.

Standards

A difficulty is that the term compostable 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 compostable in respect of a time period of 6 months. In the case of EN13432 a material is deemed compostable 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 also imposes 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).

Plastics

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.

List of biodedgradability times

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: