Environmental Engineering Reference
In-Depth Information
Carbohydrates
Sugars
Carbonic
acids
Alcohols
Hydrogen
Carbon
dioxide
Acetic acid
Methane
Carbon
dioxide
Fats
Fatty acids
Hydrogen
Carbon
dioxide
Ammonia
Proteins
Amino acids
Hydrolysis
Acidogenesis
Acetogenesis
Methanogensis
FIGURE 5.11
Key stages of anaerobic digestion.
A simplified generic chemical equation for the overall processes outlined above
is as follows:
C
6
H
12
O
6
→ 3CO
2
+ 3CH
4
Biogas is the ultimate waste product of the bacteria feeding off the input biodegrad-
able feedstock and is composed primarily of methane and carbon dioxide, with a
small amount of hydrogen and trace hydrogen sulfide (see Table 5.11). Keep in mind
that the ultimate output from a wastewater digester is water; biogas (methane) is
more of an off-gas that can be used as an energy source. Wastewater digestion and
the production of biogas are discussed in the next section.
Anaerobic Digestion of Sewage Biosolids (Sludge)
Equipment used in anaerobic digestion typically includes a sealed digestion tank
with either a fixed or a floating cover or an inflatable gas bubble, heating and mix-
ing equipment, gas storage tanks, solids and supernatant withdrawal equipment, and
safety equipment (e.g., vacuum relief, pressure relief, flame traps, explosion-proof
electrical equipment).
Caution:
Biosolids are inherently dangerous as possible sources of explosive
gases, and biosolids sites should never be entered without following OSHA's con-
fined-space entry permit requirements. Only fully trained personnel should enter
permit-required confined spaces.
TABLE 5.11
Typical Contents of Biogas
Matter
Percentage (%)
Methane (CH
4
)
50-75
Carbon dioxide (CO
2
)
25-50
Nitrogen (N
2
)
0-10
Hydrogen (H
2
)
0-1
Hydrogen sulfide (H
2
S)
0-3
Oxygen (O
2
)
0-2
