Basics of Anaerobic Digestion Process
Basics of anaerobic digestion process is presented in this chapter. Principal reactions are Hydrolysis, Fermentation Acetogenesis/dehydrogenation, Methanogenesis. The critical step in the anaerobic digestion process is Methanogenesis.
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Basics of Anaerobic Digestion Process
Abstract Basics of anaerobic digestion process is presented in this chapter. Principal reactions are Hydrolysis, Fermentation Acetogenesis/dehydrogenation, Methanogenesis. The critical step in the anaerobic digestion process is Methanogenesis.
Keywords Anaerobic digestion process Hydrolysis Fermentation acetogenesis Dehydrogenation Methanogenesis Acetophilic Methane bacteria Hydrogenophilic
In the anaerobic digestion process the organic matter is broken down by a consortium of microorganisms in the absence of oxygen and lead to the formation of digestate and biogas which mainly consist of methane and carbon dioxide. This digestate which is the decomposed substrate resulting from biogas production can be used as a bio-fertilizer (Al Seadi 2001; Kelleher et al. 2000; Chen et al. 2008; Al Seadi et al. 2008). Figure 2.1 shows the anaerobic pathway. Originally, anaerobic digestion was perceived as a two stage process involving the sequential action of acid forming and methane forming bacteria. Now, it is known to be a complex fermentation process brought about by the symbiotic association of different types of bacteria (Allen and Liu 1998; Edmond-Jacques 1986; Speece 1983; Kosaric and Blaszczyk 1992). The products produced by one group of bacteria serve as the substrates for the next group. The principal reaction sequences can be classified into four major groups involving the following (Fig. 2.1; Table 2.1): • • • •
Hydrolysis Fermentation Acetogenesis/dehydrogenation Methanogenesis
In the first stage i.e. hydrolysis/liquefaction/solubilisation step, large organic polymers such as starches, cellulose, proteins and fats are broken down or © The Author(s) 2017 P. Bajpai, Anaerobic Technology in Pulp and Paper Industry, SpringerBriefs in Applied Sciences and Technology, DOI 10.1007/978-981-10-4130-3_2
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Fig. 2.1 Anaerobic pathway based on Wilson (2014)
Table 2.1 Steps involved in anaerobic oxidation of complex wastes
Hydrolysis C6H10O4 + 2H2O! C6H12O6 + H2 Acidogenesis C6H12O6 $ 2CH3CH2OH + 2CO2 C6H12O6 + 2H2 $ 2CH3CH2COOH + 2H2O C6H12O6 ! 3CH3COOH Acetogenesis CH3CH2COO− + 3H2O $ CH3COO− + H+ + HCO3− + 3H2 C6H12O6 + 2H2O $ 2CH3COOH + 2CO2 + 4H2 CH3CH2OH + 2H2O $ CH3COO− + 3H2 +H+ Methanogenesis CH3COOH ! CH4+ CO2 CO2+ 4H2 ! CH4 + 2H2O 2CH3CH2OH + CO2 ! CH4 + 2CH3COOH Zupančič and Grilc (2012), Biarnes (2013), Ostrem (2004), Bilitewski et al. (1997), Verma (2002), van Haandel and van der Lubbe (2007), EPA (2006)
depolymerized by acidogenic bacteria into sugars, amino acids, glycerol and long chain fatty acids by hydrolytic exo-enzymes (example, cellulase, amylase, protease, and lipase) excreted by fermentative microorganisms (EPA 2006; van Haandel and van der Lubbe 2007). The hydrolysis reaction is presented in Table 2.1.
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The fermentative microorganisms consist of both facultative and strict anaerobes (Broughton 2009). In the enzymatic hydrolysis step, the water-inso
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