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5.1.2 Anaerobic Digestion Process
AD is a biological process that degrades organic material by the concerted actions
of a wide variety of microbial communities in the absence of oxygen. In a simplified
description, AD is divided into four phases (Fig. 5.1): hydrolysis, acidogenesis
(acid-producing), acetogenesis (acetic acid-producing), and methanogenesis
(methane-producing).
5.1.2.1 Stage 1: Hydrolysis
Insoluble organic compounds, such as cellulose, protein, fat, and some insoluble
forms of organic compounds, are decomposed by enzymes (produced by bacteria)
and anaerobic bacteria. Small soluble organic molecules, which are produced in this
stage, are the raw material for the bacteria in the next stage. Hydrolysis of carbohydrates can occur within a few hours, while protein and fat hydrolysis may take several days. However, lignocellulose and lignin substances decompose slowly and
incompletely (Deublein and Steinhauser 2011). Facultative anaerobes consume dissolved oxygen in water, which results in reduction of redox potential that is favorable to the AD process. In this stage, carbohydrates are broken down into simple
Fig. 5.1 Four stages of methane production. (Adapted from Zinder (1993))
5 Anaerobic Digestion of Rice Straw for Biogas Production
5.1.2 Anaerobic Digestion Process
AD is a biological process that degrades organic material by the concerted actions
of a wide variety of microbial communities in the absence of oxygen. In a simplified
description, AD is divided into four phases (Fig. 5.1): hydrolysis, acidogenesis
(acid-producing), acetogenesis (acetic acid-producing), and methanogenesis
(methane-producing).
5.1.2.1 Stage 1: Hydrolysis
Insoluble organic compounds, such as cellulose, protein, fat, and some insoluble
forms of organic compounds, are decomposed by enzymes (produced by bacteria)
and anaerobic bacteria. Small soluble organic molecules, which are produced in this
stage, are the raw material for the bacteria in the next stage. Hydrolysis of carbohydrates can occur within a few hours, while protein and fat hydrolysis may take several days. However, lignocellulose and lignin substances decompose slowly and
incompletely (Deublein and Steinhauser 2011). Facultative anaerobes consume dissolved oxygen in water, which results in reduction of redox potential that is favorable to the AD process. In this stage, carbohydrates are broken down into simple
Fig. 5.1 Four stages of methane production. (Adapted from Zinder (1993))
5 Anaerobic Digestion of Rice Straw for Biogas Production