Environmental Engineering Reference
In-Depth Information
Fed
materials
Biodegradable organic polymers
(carbohydrates, proteins, lipids)
Non-biodegradable
Organics and inorganics
Hydrolysis
Organic monomers (sugars, amino
acids, long chain fatty acids)
Hydrolytic
fermentative bacteria
Acidogenesis
NH
4
+
Intermediates
(propionate, butyrate, etc.)
Hydrogen-producing
bacteria
Acidogenesis
Acetate
H
2
+CO
2
/formate
Homoacetogens
Methanogenic
archaea
Methanogenesis
CH
4
+CO
2
(Biogas)
Products
Digested sludge
Fig. 8.9
Schematic representation of the methane fermentation with the microorganisms respon-
sible for each step
metabolites. Furthermore, fermentation enables a controlled stabilization of the or-
ganic material, can reduce greenhouse gas emissions and contributes to the closing
of nutrient cycles. Indeed, the methane fermentation process can be applied for
treatment of various types of bio-waste in a more sustainable way than many al-
ternative processes (Niu et al.
2013
,
2014
; Qiao et al.
2013
). Bio-methane can also
be used in a combined heat and power plant (CHP) to generate heat and electricity,
increasing the efficiency of its use.
8.4.2
Methane Fermentation Conditions and the Functional
Microbial Communities
Methane fermentation is effected by various specialized groups of bacteria in four
successive steps, each step depending on the preceding one (Fig.
8.9
):
1. Hydrolysis: where by complex molecules (carbohydrates, lipids and proteins)
are depolymerized into soluble compounds by hydrolytic enzymes.
2. Acidogenesis: a biological reaction where simple monomers are converted into
volatile fatty acids (VFA).
3. Acetogenesis: a biological reaction where volatile fatty acids are converted
into acetic acid, carbon dioxide, and hydrogen. The first process is catalyzed
