METHANOGENS AND BIOGAS

Methanogen An anaerobic microorganism that grows in the presence of carbon dioxide and produces methane gas. Methanogens are found in the stomach of cows, in swamp mud, and in other environments in which oxygen is not present.

Methanogens Methanogens are bacteria that produce methane gas They can be used as sources of biogas gas This would be a renewable source of energy

Methanogenesis Methanogenesis- The microbial formation of methane, which is confined to anaerobic habitats where occurs the production of hydrogen, carbon dioxide, formic acid, methanol, methylamines, or acetate—the major substrates used by methanogenic microbes. In fresh-water or marine sediments, in the intestinal tracts of animals, or in habitats engineered by humans such as sewage sludge or biomass digesters, these substrates are the products of anaerobic bacterial metabolism. Methanogens are terminal organisms in the anaerobic microbial food chain—the final product, methane, being poorly soluble, anaerobically inert, and not in equilibrium with the reaction which produces it.

Methanogenesis Two highly specialized digestive organs, the rumen and the cecum, have been evolved by herbivores to delay the passage of cellulose fibers so that microbial fermentation may be complete. In these organs, large quantities of methane are produced from hydrogen and carbon dioxide or formic acid by methanogens. From the rumen, an average cow may belch 26 gallons (100 liters) of methane per day.

What is anaerobic digestion ? A high yield biological process used to process organic pollutants.

What are the advantages of AD ? Rapid Can degrade concentrated substrates Can degrade difficult substrates Energy can be recovered Produces very few sludges

Ecology Methanogens require anaerobic conditions In the digestive systems of herbivores, marshes or lake bottoms. Many require warm conditions to work best. They are associated with a source of organic matter (e.g. plant remains or sewage) and with heterotrophic bacteria The heterotrophs break down this organic matter to release compounds such as ethanoic acid (aka acetic acid or vinegar) and hydrogen The ethanoate ions are a substrate for the methanogens

Classification Methanogens belong to the Archaea group of the Prokaryotes Examples Methanococcus jannaschii Methanobacterium thermoautotrophicum The prokaryote kingdom has been split into several further kingdoms (e.g. Archaea and Eubacteria) based on the genetics and biochemistry of microbes.

Genome sequence of the Methanococcus jannaschii confirms that there is a third main branch of life on Earth. tRNA and synthetase taken from Methanococcus jannaschii, a methane-producing archeabacterium that lives in extremely hot hydrothermal vents in the ocean. Here the microbe Methanococcus jannaschii, lives at crushing pressures 245 times greater than at sea level and at scalding temperatures just a few degrees below the boiling point of water.

Biochemistry Methanogens are chemoautotrophs Methanogens use a number of different ways to produce methane using ethanoate (acetate) that may be derived from the decomposition of cellulose: CH 3 COO + + H -  CH 4 + CO kJ mol -1 Or using hydrogen and carbon dioxide produced by the decomposers: 4 H 2 + CO 2  CH H 2 O kJ mol -1

Biogas production From the decomposition of wastes in farming sewage treatment A bi-product of the cleaning up of waste water Biogas consists of about 40% CO 2 and 60% CH 4 BEA Dithmarschen

Requirement a fermenter, which is supplied with an innoculum of bacteria (methanogens and decomposers) anaerobic conditions an optimum temperature of 35°C an optimum pH of 6.5 to 8 This needs to be monitored as the decomposers produce acids and they work faster than the methanogens consume the acids organic waste (biomass) e.g. sewage, wood pulp

Methanogens and the greenhouse effect Half of the methane produced by methanogens is used up as an energy source by other bacteria Half is lost to the atmosphere (600 M tonnes y -1 ) where it acts as an important greenhouse gas As more land is converted to rice paddy fields and pasture for grazing animals more methane will be produced

Warming up the brew As global warming progresses the permafrost with thaw in the regions covered by tundra Tundra contains extensive reserves of frozen peat As the peat warms and melts, it will provide a source of material for methanogens

The knock on effect The amount of methane release into the atmosphere will accelerate. This in turn will drive global warming even further Peatlands form 20-30% of organic C in N. Hemisphere UNEP