Methane anaerobic production

It is estimated that about 500 million tons of methane are being added to the air each year (Craig and Chou, 1982), largely by anaerobic production in rice paddies and wetlands as well as from the metabolism of ruminant domestic animals and, possibly, African termites (Rasmussen and Khalil, 1981 Zimmerman et d., 1982). This gas is slowly oxidized by reactions with Hydroxyl free radical. Its atmospheric content is around 5 gigatons, indicating that the residence time in the atmosphere is about 10 years. As Figure 12 shows, since 1965 the atmospheric concentration of methane has increased by about 3096. If this rate continues, the methane concentration will have doubled early in the 21st century. 

Emission of CH4 from soils to the atmosphere is a balance between methane oxidation, production, and transport within the soil systems (Chan and Parkin, 2000 Bradford et al., 2001). Methane is released from anaerobic wetland soils to the atmosphere through diffusion of dissolved methane, through ebullition of gas bubbles, and through wetland plants that develop aerenchyma tissue (Figure 16.1). Large portions of methane formed in an anaerobic soil remain trapped in the flooded soil. Entrapped methane can be oxidized to carbon dioxide when the floodwater is drained or when the soil dries. Entrapped methane can escape to the atmosphere immediately after the floodwater is removed or recedes. 

Table 37. Potential Methane Production from Commercial U.S. Anaerobic Digestion Systems...

Fermentation is an anaerobic cataboHc process that uses organics as electron receptors. Since fermentation produces organic products that have lower free energy than their precursors, it is usefijl in remediation. The lowest free energy form of carbon produced is methane [74-82-8]. 

Anaerobic treatment is usually employed for high strength wastewaters. In anaerobic treatment, complex organics are broken down through a sequence of reactions to end products of methane gas, CH, and carbon dioxide, CO2 ... 

Recovery of Riologieal Conversion Products Biological conversion produces that can be derived from solid wastes include compost, methane, various proteins and alcohols, and a variety of other intermediate organic compounds. The principal processes that have been used are reported in Table 25-64. Composting and anaerobic digestion, the two most highly developed processes, are considered further. The recovery of gas from landfills is discussed in the portion of this sec tion dealing with ultimate disposal. 

The rate of decomposition in unmanaged landfills, as measured by gas production, reaches a peak within the first 2 years and then slowly tapers off, continuing in many cases for periods up to 25 years or more. The total volume of the gases released during anaerobic decomposition can be estimated in a number of ways. If all the organic constituents in the wastes (with the exception of plastics, rubber, and leather) are represented with a generahzed formula of the form QH O N, the total volume of gas can be estimated by using Eq. (25-27) with the assumption of completed conversion to carbon dioxide and methane. 

A number of biological processes result in the marine production of gases that have a greenhouse role, similar to water vapour and COj. In low oxygen environments, of the sort discussed in the next section, methane is produced by anaerobic bacterial decay ... 

The production of reduced products like methane and ammonia by the gut microbial flora has important environmental consequences, as such compounds contribute to the chemical and biological oxygen demand. The detection of dissolved oxygen in the gut of piglets led to tests to show whether methane and other reduced products could be oxidized in the pig gut. The production of C-labelledCOj from C-labelledmethane has been demonstrated however, it is calculated that at most only a very small proportion of methane produced is likely to be oxidized using O 2 as electron acceptor. Methane may also be oxidized anaerobically, but only an extremely small amount of methane is likely to... 

Biogas is composed primarily of methane (GH ) and carbon dioxide. Biogas is a by-product from anaerobic bacteria breaking down organic material. Large amounts of biogas can be released from areas such as... 

It is less conunonly known that methane was one of the original atmospheric gases and is a normal product of the microbial decomposition of organic matter under anaerobic conditions. Bacteria involved in production of methane are unique in their metabolism and other properties. The balanced... 

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