8.6 Reactor Design
Bouallagui et al. (2005) have explained different types of bioreactors which are
commonly used in the industry: “batch, continuous one-stage system, or continuous
two-stage/multi-stage systems.” Some additional modifications are made to the
existing models to design “anaerobic sequencing batch reactor (ASBR), upflow
anaerobic sludge blanket (UASB) reactor, tubular reactor, plug-flow systems, and
anaerobic filters.” Khalid et al. (2011) have opined that among all the reactors. Batch
reactors are quick, economical, and simple to operate. The digester tanks used are
made of steel and concrete. Among the different types of construction materials
being used, concrete constructions are more advantageous compared to others.
Generally, the digester tanks are constructed with a lifetime of about 15–20 years.
Hydrogen sulfide formation may lead to corrosion of the tank. The mixing system is
important as it maintains a homogenous digestate during the process of anaerobic
digestion. Longer stirring times are required during the initial phases of operation
compared to the later phases of the operation. Ward et al. (2008) have suggested that
the design of digester has to address three major issues for competent and economical formation of biogas. Firstly, it should have the capability to handle a high
organic loading rate. Secondly, it needs to have a short hydraulic retention time,
and lastly it should be able to produce higher volumes of good quality biogas. In this
process, the highest methane production is seen in the beginning. Figure 8.3 shows
the construction of a thermophilic anaerobic digester used for methane production.
In the process of continuous digestion, these are fed continuously; after digestion,
the digestate is discharged leading to a steady state for constant gas production rate.
These types of systems are dependent on substrates which can be pumped into the
system without any mechanical hindrance. If it is not possible, a semi-continuous
process is experimented where the feedstock is fed at several times. “Continuous
stirred-tank reactors (CSTR) (Fig. 8.4) and plug-flow reactors (PFR)” are the two
most commonly used reactors, while others are less used. The plug flow reactors are
generally used for dry digestion with the feedstock which contains a lot of solid mass
(Patinvoh et al. 2016a). On the other hand, CSTRs are used only in systems where
there is continuous supply of feedstock to the reactor such as in wet digestion
systems. The decision to use either of the other mentioned systems depends upon
the solid contents which are present in the feedstock. Mostly, CSTR design is used in
single-stage systems favoring acidogens and methanogens. These are economical
and easy to operate (Vandevivere et al. 2003). In the case of two-stage reactors, the
process of acetogenesis is separate from that of acidification and takes places in two
stages. The first phase favors growth of acidogens, and the pH is generally kept low
and acidic. In the case of second phase, the pH is increased favoring the growth of
methanogens (Ince 1998). Chaudhary (2008) has observed that the rate-limiting
issue in the second stage is the growth rate of microorganisms. Hence, biomass
retention times are longer in this phase (Verma 2002). Chaudhary (2008) have
noticed that these kinds of systems are more stable compared to single-stage
systems. Griffin (2012) has opined that better process control and optimization can
8 Bioprocess Parameters for Thermophilic and Mesophilic Biogas Production: Recent. . . 241
Bouallagui et al. (2005) have explained different types of bioreactors which are
commonly used in the industry: “batch, continuous one-stage system, or continuous
two-stage/multi-stage systems.” Some additional modifications are made to the
existing models to design “anaerobic sequencing batch reactor (ASBR), upflow
anaerobic sludge blanket (UASB) reactor, tubular reactor, plug-flow systems, and
anaerobic filters.” Khalid et al. (2011) have opined that among all the reactors. Batch
reactors are quick, economical, and simple to operate. The digester tanks used are
made of steel and concrete. Among the different types of construction materials
being used, concrete constructions are more advantageous compared to others.
Generally, the digester tanks are constructed with a lifetime of about 15–20 years.
Hydrogen sulfide formation may lead to corrosion of the tank. The mixing system is
important as it maintains a homogenous digestate during the process of anaerobic
digestion. Longer stirring times are required during the initial phases of operation
compared to the later phases of the operation. Ward et al. (2008) have suggested that
the design of digester has to address three major issues for competent and economical formation of biogas. Firstly, it should have the capability to handle a high
organic loading rate. Secondly, it needs to have a short hydraulic retention time,
and lastly it should be able to produce higher volumes of good quality biogas. In this
process, the highest methane production is seen in the beginning. Figure 8.3 shows
the construction of a thermophilic anaerobic digester used for methane production.
In the process of continuous digestion, these are fed continuously; after digestion,
the digestate is discharged leading to a steady state for constant gas production rate.
These types of systems are dependent on substrates which can be pumped into the
system without any mechanical hindrance. If it is not possible, a semi-continuous
process is experimented where the feedstock is fed at several times. “Continuous
stirred-tank reactors (CSTR) (Fig. 8.4) and plug-flow reactors (PFR)” are the two
most commonly used reactors, while others are less used. The plug flow reactors are
generally used for dry digestion with the feedstock which contains a lot of solid mass
(Patinvoh et al. 2016a). On the other hand, CSTRs are used only in systems where
there is continuous supply of feedstock to the reactor such as in wet digestion
systems. The decision to use either of the other mentioned systems depends upon
the solid contents which are present in the feedstock. Mostly, CSTR design is used in
single-stage systems favoring acidogens and methanogens. These are economical
and easy to operate (Vandevivere et al. 2003). In the case of two-stage reactors, the
process of acetogenesis is separate from that of acidification and takes places in two
stages. The first phase favors growth of acidogens, and the pH is generally kept low
and acidic. In the case of second phase, the pH is increased favoring the growth of
methanogens (Ince 1998). Chaudhary (2008) has observed that the rate-limiting
issue in the second stage is the growth rate of microorganisms. Hence, biomass
retention times are longer in this phase (Verma 2002). Chaudhary (2008) have
noticed that these kinds of systems are more stable compared to single-stage
systems. Griffin (2012) has opined that better process control and optimization can
8 Bioprocess Parameters for Thermophilic and Mesophilic Biogas Production: Recent. . . 241