Bacteria, methane and biodegradable plastics
Bacteria are now being used to convert methane (CH4) (See Figure on right) from landfill sites, or from anaerobic digestion biogas, into biodegradable plastics, at the same time preventing this important greenhouse gas from entering the atmosphere.
The importance of methane in global warming
Methane is a potent greenhouse gas, second only to carbon dioxide in its capacity to trap heat in the Earth’s atmosphere. The gas can originate from lakes and swamps, natural-gas pipelines, deep-sea vents, livestock, and is produced as a component of biogas by anaerobic digestion facilities.
The global levels of methane, along with the levels of other greenhouses gases, such as carbon dioxide and nitrous oxide, have been rapidly increasing over the last century. (See Figure on right).
Problems with non-biodegradable plastics
Most plastics, such as polyethylene, and polypropylene, are not biodegradable. This means that they accumulate in the environment causing many problems with the pollution of land and the oceans.
These combined problems of global warming, and the need to replace non-biodegradable plastics, make the technologies capable of converting methane into biodegradable plastics very attractive options. One such method is to use bacteria to convert methane into a biodegradable polymer - PHA.
Making biodegradable plastics from methane using bacteria
Polyhydroxyalkanoates (PHA) are a group of biodegradable biopolymers produced naturally as minute globules within some types of bacteria under nutritional stress - excess carbon and reduced nitrogen (See Figure on right).
The globules can represent up to 90% of the bacteria's dry weight. The mechanical properties of PHAs are useful, in that they can be modified, they are biodegradable, and their production does not depend on petroleum-based feedstocks. The main type of PHA produced by bacteria is polyhydroxybutyrate (PHB). Over 300 strains of bacteria have been identified as being able to produce PHB.
Typically, the bacteria producing PHAs are grown with sugars as the carbon source. This can represent up to 40% of the production costs. To reduce these costs, several waste streams have been trialled as a carbon source, including whey waste, sugar industry waste, agricultural crop waste and glycerol. Such systems normally require an expensive sterilisation stage.
Methane, from landfills, natural gas, and as a component of biogas produced by anaerobic digestion facilities, has been demonstrated to be an energy-rich feedstock and a much cheaper alternative to the above materials.