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As the name anaerobic refers, the anaerobic digestion is carried out by microorganisms that can only live in an oxygen free environment. The decomposition of biowaste occurs in four stages: hydrolysis, acidogenesis, acetogenesis and methanogenesis (Figure 2.23). Woody waste, in spite of being biodegradable, has a high lignin content which slows down the hydrolysis phase, and for that reason woody waste shall not be used as an input material in anaerobic treatment plants (United Tech, 2003), that type of waste should be preferable recycled as raw material for particle board production or thermally treated.
Figure 1: Degradation steps of anaerobic digestion process
During hydrolysis, the first stage, bacteria transform the particulate organic substrate into liquefied monomers and polymers i.e. proteins, carbohydrates and fats are transformed to amino acids, monosaccharides and fatty acids respectively. Equation 2.13 shows an example of a hydrolysis reaction where organic waste is broken down into a simple sugar, in this case, glucose (Ostrem, 2004).
Equation 1: C6H10O4 + 2H2O → C6H12O6 + 2H2
In the second stage, acidogenic bacteria transform the products of the first reaction into short chain volatile acids, ketones, alcohols, hydrogen and carbon dioxide. The principal acidogenesis stage products are propionic acid (CH3CH2COOH), butyric acid (CH3CH2CH2COOH), acetic acid (CH3COOH), formic acid (HCOOH), lactic acid (C3H6O3), ethanol (C2H5OH) and methanol (CH3OH), among other. From these products, the hydrogen, carbon dioxide and acetic acid will skip the third stage, acetogenesis, and be utilized directly by the methanogenic bacteria in the final stage (Figure 2). Equations 2, 3 (Ostrem, 2004) and 4 (Bilitewski et al., 1997) represent three typical acidogenesis reactions where glucose is converted to ethanol, propionate and acetic acid, respectively.
Equation 2: C6H12O6 ↔ 2CH3CH2OH + 2CO2
Equation 3: C6H12O6 + 2H2 ↔ 2CH3CH2COOH + 2H2O
Equation 4: C6H12O6 → 3CH3COOH
Figure 2: Schematic representation of the course of anaerobic methane generation from complex organic substances showing scanning electron micrographs of individual microorganisms involved
In the third stage, known as acetogenesis, the rest of the acidogenesis products, i.e. the propionic acid, butyric acid and alcohols are transformed by acetogenic bacteria into hydrogen, carbon dioxide and acetic acid (Figure 2). Hydrogen plays an important intermediary role in this process, as the reaction will only occur if the hydrogen partial pressure is low enough to thermodynamically allow the conversion of all the acids. Such lowering of the partial pressure is carried out by hydrogen scavenging bacteria, thus the hydrogen concentration of a digester is an indicator of its health (Mata-Alvarez, 2003). Equation 5 represents the conversion of propionate to acetate, only achievable at low hydrogen pressure. Glucose (Equation 6) and ethanol (Equation 7) among others are also converted to acetate during the third stage of anaerobic fermentation (Ostrem, 2004).
Equation 5: CH3CH2COO- + 3H2O ↔ CH3COO- + H+ + HCO3- + 3H2
Equation 6: C6H12O6 + 2H2O ↔ 2CH3COOH + 2CO2 + 4H2
Equation 7: CH3CH2OH + 2H2O ↔ CH3COO- + 2H2 +H+
The fourth and final stage is called methanogenesis. During this stage, microorganisms convert the hydrogen and acetic acid formed by the acid formers to methane gas and carbon dioxide (Equations 2.20, 2.21 and 2.22) (Verma, 2002). The bacteria responsible for this conversion are called methanogens and are strict anaerobes. Waste stabilization is accomplished when methane gas and carbon dioxide are produced.
Equation 8: CO2 + 4H2 → CH4 + 2H2O
Equation 9: 2C2H5OH + CO2 → CH4 + 2CH3COOH
Equation 10: CH3COOH → CH4 + CO2