Environmental Engineering Reference
In-Depth Information
Wastewater
biosolids
(sewage)
Farm
slurries
Landfill
Anaerobic digestion
Biogas
(methane)
FIGURE 5.10
Production of biogas (methane, CH
4
).
Cautionary Note:
Allowing air to enter an anaerobic digester should be prevented
because the mixture of air and gas produced in the digester can be explosive.
Stages of Anaerobic Digestion
Anaerobic digestion (see
Fig u re 5.11
)
has four key biological and chemical stages
(Spellman, 2009; USEPA, 1979, 2006):
1.
Hydrolysis
—Proteins, cellulose, lipids, and other complex organics are
broken down into smaller molecules and become soluble by utilizing water
to split the chemical bonds of the substances.
2.
Acidogenesis
—The products of hydrolysis are converted into organic acids
(where monomers are converted to fatty acids).
3.
Acetogenesis
—The fatty acids are converted to acetic acid, carbon dioxide,
and hydrogen.
4.
Methanogenesis
—Organic acids produced during the fermentation step are
converted to methane and carbon dioxide.
The efficiency of each phase is influenced by the temperature and the amount
of time the process is allowed to react. For example, the organisms that perform
hydrolysis and volatile acid fermentation (often called
acidogenic bacteria
) are
fast-growing microorganisms that prefer a slightly acidic environment and higher
temperatures than the organisms that perform the methane formation step (
metha-
nogenic bacteria
).
