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Anaerobic digestion of various feedstocks for second-generation biofuel production 169

between the bacteria and their food is better and results in a higher gas production
rate [63].

6.7.3 Single-stage and multistage system

In a single-stage system, the anaerobic digestion stage is completed in a single sealed
reactor. The main problem in the single-stage system is during the acid formation
stage; the acidogenic microorganism reduces the pH of the digester, which affects
the methane formation due to dissimilarity in the growth of acidogenic and
methanogenic bacterial communities. To avoid this problem in a two-stage or multi-
stage system, separate digester vessels are engaged for both acid and methane to bring
ultimate control over bacteria during acid and methane gas formation. In the first
digester vessel, the sequence of stages such as hydrolysis, acidogenesis, and
acetogenesis take place. The product of the first vessel is taken to the second vessel
for biogas formation. Based on the feedstock need, either thermophilic or mesophilic
temperature is maintained. Single-stage systems are preferred for dry batch systems or
continuous wet systems. Continuous and wet processes are ideal for the two-stage
system [64].

6.8 Types of anaerobic digesters/reactors

There are so many digesters available for biogas production from various organic
feedstocks. The most common types are the floating drum type digester, the fixed
dome digester, the low-cost polyethylene tube digester, balloon plants, and horizontal
plants, which are discussed here.

6.8.1 Floating drum type digester
A floating-drum type digester comprises a cylinder-shaped or dome-shaped digester, a
metallic floating drum with a gas-holder, an inlet pipe, an inlet tank, an outlet pipe, and
an outlet tank with a partition wall. During the discharge of gas, the gas holder is kept
at constant pressure by moving the gas holder up and down with the help of a central
controller pipe. The material of the drum must be protected from corrosion. Gas pro-
duction is increased if the digester drum is painted black or red rather than blue or
white because the temperature of the digester can be increased by solar radiation.
The gas drum should have a sloping roof so that it may be protected from rain.
A drum made of high density polyethylene and glass fiber-reinforced plastic is suc-
cessful, but the construction cost is higher compared to steel. These types of plants
have many advantages such as an integrated arrangement for scum breaking, mainte-
nance of biogas at constant pressure, the volume of gas can be observed by sensing the
drum position of the digester, etc. [65]. The disadvantages are that the drum material is
relatively expensive and maintenance is intensive. The life span of the drum is about
15 years, although in a tropical coastal region it is only around 5 years.