Methane and carbon dioxide generation

Immediately after tipping the organic matter is in an aerobic state, i.e. air pockets exist in the soil or waste, so the initial decomposition of the organic matter is an aerobic biological process and the two main end products are carbon dioxide and water. This process also occurs during the composting of waste. 

Bacteria do not flourish in dry conditions, so biodegradation starts only when the soil or waste is moist. Once aerobic biodegradation has commenced oxygen is soon depleted and if no replenishment is available, the material becomes anaerobic. Anaerobic processes use oxygen present in the structure of the organic matter or from inorganic ions such as sulphate. The main products from anaerobic degradation are methane and carbon dioxide. A simple summary of the process is provided in Box 4.1. 

In very simple terms aerobic processes can be smnmarised as  

An example of the aerobic biochemical oxidation of acetic acid is thus CH3COOH + 20 -I- bacteria 2CO2 -I- H O 

An example of the anaerobic biochemical oxidation of acetic acid is  

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Minor components of landfill gas will be affected by similar factors such as methane and carbon dioxide generation as described in Section 4.1. The potential to generate component gases depends on the specific cocktail of the various sources deposited in the waste, the interaction of gases themselves and the chemical and biochemical interaction between soil, water and the waste. 

These tanks at a water treatment facility are used to generate a mixture of methane and carbon dioxide by anaerobic digestion of sewage. The methane produced provides much of the power needed to run the facility. 

The following describes the process of mannre and straw mixtnre digestion. For the first 3 days, the methane yield is almost 0% and carbon dioxide generation is roughly 100%. In this period, digestion occius as aerobic fermentation to carbon... 

Single pulse, shock tube decomposition of acetic acid in argon inv olves the same pair of homogeneous, molecular first-order reactions as thermolysis (19). Platinum on grapliite catalyzes the decomposition at 500—800 K at low pressures (20). Ketene, methane, carbon oxides, and a variety of minor products are obtained. Photochemical decomposition yields methane and carbon dioxide and a number of free radicals, wliich have complicated pathways (21). Electron impact and gamma rays appear to generate these same products (22). Electron cyclotron resonance plasma made from acetic acid deposits a diamond [7782-40-3] film on suitable surfaces (23). The film, having a polycrystalline stmcture, is a useful electrical insulator (24) and widespread industrial exploitation of diamond films appears to be on the horizon (25). 

The simulations presented are for the conversion of acetic acid to microorganisms, methane, and carbon dioxide. Yields will be diflFerent for other volatile acids, especially noticeable being the increased ratio of methane to carbon dioxide produced as the length of the carbon chain increases. For field digesters utilizing complex substrates it would also be necessary to include the carbon dioxide generated by the acid-producing bacteria. 

In a field digester these steady-state values would also be affected by the fact that most of the volatile acids and cations (NH4 ) would be generated internally with some carbon dioxide being produced by the acid-producing bacteria. Acids other than acetic are also formed, and it is of interest to compare the steady state conditions for other volatile acids with those of acetic. The formulas for converting propionic and butyric acids to microorganisms, methane, and carbon dioxide are given in Equations 32 and 33. 

Anaerobic digestion also occurs in the biogas that is generated in sewage or manure as well as the landfill gas produced by refuse. The resulting gas is a mixture consisting mainly of methane and carbon dioxide. 

The majority of natural methane and carbon dioxide in peat and alluvium has already been generated and is largely trapped in the soil. It therefore represents a finite source and the likely risk is related to the volume present in the ground and factors that could affect surface emissions such as variations... 

Box 4.2 Factors affecting generation of methane and carbon dioxide... 

Estimating the volume of methane and carbon dioxide that is generated... 

Variable mixture of gases generated by decaying organic matter within a landfill site. Principal components are methane and carbon dioxide but it can contain many other trace gases and vapours... 

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