2.8.3 Biological Hydrogen Production
MFC systems can be optimized to produce biological hydrogen instead of producing
electrical current. Then, the generated hydrogen could be accumulated for later
applications. Generally, hydrogen production out of protons and electrons produced
by microbial metabolism in MFC is not appropriate in terms of thermodynamics.
But, by using a microbial electrolysis cell (MEC), the thermodynamic barrier would
be solved. MEC system is a reactor for biological hydrogen production by combining MFC and electrolysis, and the anode process of an MEC is the same as that of a
MFC system, but in the cathodic chamber, the electrons combine with the protons to
produce hydrogen (Sun et al. 2008).
Since oxygen as the electron acceptor at the cathode chamber has a higher redox
potential than the microbial anode, electrons flow spontaneously through the external circuit and produce electricity in MFCs, while, in the MEC system, the reduction
reaction of H
+ ions to generate hydrogen at the cathode chamber has a lower redox
potential than the anode and is not possible thermodynamically. Hence, electrons do
not flow spontaneously through the external circuit (Liu et al. 2010).
In addition, in a MEC system as bioelectrochemically assisted microbial reactor,
hydrogen is evolved at the cathode chamber by eliminating oxygen and adding a
small voltage to the circuit. Indeed, in a MFC-MEC coupled system, hydrogen was
produced in an MEC, and the extra power was supplied by an MFC. This microbial
reactors can produce 8 moles hydrogen per 1 mole glucose as substrate (Das 2009).
2.8.4 Wastewater Treatment
One of the most praiseworthy and fruitful applications of MFCs is their functional
use in WWT plants which is much more superior compared with the other introduced processes. Using MFCs in WWT was started by Hoberman and Pommerin in
1991 (Habermann and Pommer 1991). Although various techniques have been
offered for WWT over the last decades, high cost and time-consuming are the
main drawbacks of them which have been introduced (Rahimnejad and Najafpour
2018), and most treatment methods require a high level of operational process. It
should be added here that recovery of energy as electricity in WWT with the help of
MFCs and lower sludge production compared with aerobic process are some of the
most novel advantages of applying MFCs in water treatment (Kim et al. 2007a, b),
since large amount of excess sludge disposal generated in WWT process is needed
and so important. Obviously, all the energy of organic contaminant is utilized by
microorganisms in an aerobic process, while in MFCs huge amount of their energy is
converted to electricity, and just a small amount is dedicated to microbial growth
(Park et al. 2001). Introductory efforts have been shown that MFCs’ yield is 1.5
times more than an aerobic culture (Du et al. 2007). In terms of MFCs’ ingredients,
wastewater is divided into organic and inorganic wastes, and it has been figured out
that by using specific microorganisms in MFCs, they would be able to remove
228
M. Rahimnejad et al.
MFC systems can be optimized to produce biological hydrogen instead of producing
electrical current. Then, the generated hydrogen could be accumulated for later
applications. Generally, hydrogen production out of protons and electrons produced
by microbial metabolism in MFC is not appropriate in terms of thermodynamics.
But, by using a microbial electrolysis cell (MEC), the thermodynamic barrier would
be solved. MEC system is a reactor for biological hydrogen production by combining MFC and electrolysis, and the anode process of an MEC is the same as that of a
MFC system, but in the cathodic chamber, the electrons combine with the protons to
produce hydrogen (Sun et al. 2008).
Since oxygen as the electron acceptor at the cathode chamber has a higher redox
potential than the microbial anode, electrons flow spontaneously through the external circuit and produce electricity in MFCs, while, in the MEC system, the reduction
reaction of H
+ ions to generate hydrogen at the cathode chamber has a lower redox
potential than the anode and is not possible thermodynamically. Hence, electrons do
not flow spontaneously through the external circuit (Liu et al. 2010).
In addition, in a MEC system as bioelectrochemically assisted microbial reactor,
hydrogen is evolved at the cathode chamber by eliminating oxygen and adding a
small voltage to the circuit. Indeed, in a MFC-MEC coupled system, hydrogen was
produced in an MEC, and the extra power was supplied by an MFC. This microbial
reactors can produce 8 moles hydrogen per 1 mole glucose as substrate (Das 2009).
2.8.4 Wastewater Treatment
One of the most praiseworthy and fruitful applications of MFCs is their functional
use in WWT plants which is much more superior compared with the other introduced processes. Using MFCs in WWT was started by Hoberman and Pommerin in
1991 (Habermann and Pommer 1991). Although various techniques have been
offered for WWT over the last decades, high cost and time-consuming are the
main drawbacks of them which have been introduced (Rahimnejad and Najafpour
2018), and most treatment methods require a high level of operational process. It
should be added here that recovery of energy as electricity in WWT with the help of
MFCs and lower sludge production compared with aerobic process are some of the
most novel advantages of applying MFCs in water treatment (Kim et al. 2007a, b),
since large amount of excess sludge disposal generated in WWT process is needed
and so important. Obviously, all the energy of organic contaminant is utilized by
microorganisms in an aerobic process, while in MFCs huge amount of their energy is
converted to electricity, and just a small amount is dedicated to microbial growth
(Park et al. 2001). Introductory efforts have been shown that MFCs’ yield is 1.5
times more than an aerobic culture (Du et al. 2007). In terms of MFCs’ ingredients,
wastewater is divided into organic and inorganic wastes, and it has been figured out
that by using specific microorganisms in MFCs, they would be able to remove
228
M. Rahimnejad et al.