The main process of the anaerobic digestion is consisted of two phases, and there are three stages in second phase. The schematic chart is provided below.
The first phase is referring to hydrolysis, which is producing the necessary acid for bacteria. Usually, only large organic polymers like carbohydrates, fats, cellulose, and proteins make up the organic fraction of the waste. The microorganisms cannot utilize these polymers directly until they are broken down. To initiate the first phase, extracellular enzymes are introduced to break down the high-molecular-weight polymeric components into soluble components for next stage. In details, the enzyme amylase, cellulase and cellobiase break down the carbohydrates and cellulose to glucose; protease breaks the proteins into amino acids; lipase breaks down the fats to fatty acids. In addition, bacteria expend more energy than is salvageable during the first phase. By the end of the first stage, all the large polymer should be break down for bacteria metabolic reactions.
When the acids are available after the first stage, the second phase should be initiated. Three steps are concluded in this phase, which are acidogenesis, acetogenesis and methanogenesis. In the process of acidogensis, the acids are caused into further breakdown by biological process. Acidogenic bacteria are used and digest the sugars, fatty acids and amino acids into carbon acids, alcohol and hydrogen carbon dioxide. And then at the stage of actogenesis, acetogens breakdown the products from the preceding process into simple soluble molecules such as acetic acid and carbon dioxide.
The production of the methane is occurred in the final step, which is called as methanogenesis. In this step, soluble materials are converted into methane, carbon dioxide and water. The equations of main reactions are listed below.
CH3COOH ------> CH4 + CO2 (acetic acids)
4CH3CH2COOH + 2H2O ------> 7CH4 + 5 CO2 (propionic acid)
2CH3CH2CH2COOH + 2H2O ------> 5CH4+ 3 CO2 (butyric acid)
2CH3CH2OH ------> 3CH4 + CO2 (ethanol)
CH3COCH3+H2O------>2CH4+CO2 (acetone)
It can be seen that all the fermentations of different acids, ethanol and acetone has the same products including methane and carbon dioxide. Acetic acid is the most prevalent organic acid intermediate found in the methane fermentation of fats, carbohydrates, and proteins and about 70 percent of the methane produced in the subsequent anaerobic fermentation results from its degradation.
According to Van Niel, the organic compounds fermented by methane bacteria are completely oxidized to carbon dioxide, and this oxidation process is followed by a reduction process in which some or all of the carbon dioxide is reduced to methane (Van Niel – carbon reduction theory – 1938) Relevant equations is shown below.
Oxidation CH2COOH + 2H2O----à2CO2+8H
Reduction 8H+CO2----àCH4+2H2O
Net CH3COOH----àCH4+CO2)
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