organic loading rate (OLR), HRT, and influent COD ranges. Typical data of some
anaerobic process is presented in Table 8.6.
As shown in Fig. 8.5, anaerobic digestion has four steps which are hydrolysis,
acidogenesis, acetogenesis, and methanogenesis [56]. The first step is the solubilization (hydrolysis) step in which hydrolytic microorganisms degrade insoluble
polymer-type organic material such as polysaccharides and proteins by extracellular
enzymes to monomers. This reduction results in no reduction of COD. Hydrolysis is
the rate-limiting step of overall anaerobic digestion because of the semi-rigid
Fig. 8.4 A schematic representation of a trickling filter system [51]
Table 8.6 Typical process and performance data of anaerobic wastewater treatment processes [9]
Process
Input COD
(mg/L)
HRT
(h)
OLR (kg COD/m
3
.
day)
COD removal
(%)
Anaerobic lagoons
N.D.
0.6–1
Anaerobic contact process 1,500–5,000
2–14
0.5–5.3
75–90
Fixed-bed reactor
10,000–70,000
24–48 1–15
75–85
UASB
5,000–15,000
4–12
2–12
75–85
Expanded bed reactor
5,000–10,000
5–10
5–30
80–85
Fluidized bed reactor
N.D.
N.D.
40–60
N.D.
Internal circulation
(IC) reactor
N.D.
N.D.
31
N.D.
N.D. No data
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