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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 1). 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 1 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
There are several possibilities to designanaerobic digestion systems. A system can be as simple and cheap as a single static cylindrical digestor or as complex and expensive as a multi digestor system with moving parts and intelligent sensors that support the operation of the plant. The efficiency of the plant is directly affectedby the type of system installed and the way it is managed. Simple plants are easy to design but require constant monitoring and are less efficient, while complex plants are designed to detect errors and warn operators, thus making them more efficient.
Although the microbial processes are the same for all anaerobic digestion processes, each plant is unique and should be designed according to its own input parameters and economic possibility. At the moment of designing a new plant, or making an old facility more efficient, several factors must be taken into consideration. Some of these factors are the capacity of the plant, the types of waste to be treated, the area available, the climate of the region, the demographics and the location of the plant.
Two factors have a dominant influence on the biogas production: the content of digestible matter of the waste treated and the transfer rate of this waste fraction into the digester.
Depending on the water content in the digester and the way of feeding the digester, the anaerobic digestion process can be classified into wet and dry fermentation and continuous and discontinuous fermentation.
Wet fermentation refers to total solid content in the Digestor with less than 12 percent of dry matter, while dry fermentation refers to digester feed with 30 percent of dry matter or more.