Environmental Engineering Reference
In-Depth Information
10.2
Basic Mechanisms of BES
on the energy-efficient or energy-gaining pro-
cesses for the waste remediation. Bioprocess en-
gineering certainly comes under environmentally
benign treatment strategies. Bioelectrochemical
systems (BES) are multidimensional systems that
can accomplish significant change in wastewa-
ter treatment by considering them as renewable
energy based repository units (ElMekawy et al.
2014 ). Microbial fuel cells (MFCs) and microbial
electrolysis cells (MECs) are two examples of
BES, which are rapidly developing towards envi-
ronmental sustainability. Conventional treatment
processes cannot handle some of the wastewa-
ter components, especially coloured compounds
(dyes), complex organic and inorganic chemicals,
toxic substances, etc. due to the metabolic limi-
tations of the microbes. Similarly, the existing
electrochemical process also has some limitations
in treating this type of waste in terms of energy
input and additional waste generation. At this
point, BES combines both biological and electro-
chemical processes for waste remediation along
with the energy generation in terms of electricity,
hydrogen or other useful chemicals. This multi-
faceted application of BES has been attracting
several researchers across the globe. Figure 10.1
shows the increasing interest in this field of re-
search in terms of publications from the past de-
cade. Combination of multiple disciplines, viz.
environmental science, biotechnology, microbi-
ology, electrochemistry, etc., involved in the de-
velopment of this particular area. Microbial elec-
troremediation is aimed at the use of biological
energy generated during BES operation for the
extended treatment of wastewaters and specific
pollutants present in wastewater. Various types of
bioreactors have been designed and operated in
the literature for the targeted processes. Several
microbial species are reported in such biopro-
cesses for their specific function towards waste/
pollutant treatment. In this chapter, basic princi-
ples of microbial electroremediation processes at
both the electrodes (anode and cathode) of BES
are discussed in detail. Further to this, different
types of wastewater used in BES are discussed
followed by a comprehensive discussion on the
specific pollutant, viz. nutrients, metals, dye, re-
moval and microbial desalination processes.
Energy generation in microbial metabolism, in-
cluding both anabolism and catabolism, is com-
bination of fermentation (substrate oxidation) and
respiration (reduction) processes. This process
requires an electron source (substrate) which falls
in the metabolic flux of the microbe (can be uti-
lized by the microbe) and a strong/weak electron
sink (acceptor) to complete the electron transport
chain. Separating these two processes (fermenta-
tion and respiration) by an ion permeable mem-
brane (optional) in a system equipped with elec-
trodes (artificial electron acceptors) creates an
environment to harness the energy generated by
the microbe in the form of current density, against
the potential difference generated between these
two processes (Venkata Mohan et al. 2014a ). The
microbes utilize the available substrate (fermen-
tation) generating the reducing equivalents [pro-
tons (H + ) and electrons (e )] at anode. Protons are
transported to cathode through the solution elec-
trode interface across ion selective membrane,
generating a potential difference between anode
and cathode against which the electrons will flow
through the circuit (current) across the external
load (Pant et al. 2012 ). The reducing equivalents
generated during BES operation have multiple
applications in the energy generation as well as
waste remediation areas. Broadly, BES applica-
tion can be classified as a power generator, waste-
water treatment unit and system for the recovery of
value-added products. Reducing equivalents gen-
erated from substrate metabolism gets oxidized in
presence of an electron acceptor at a physically
distinct component of BES (cathode) and results
in power generation. Alternatively, when the
waste/wastewater functions as an electron donor
or acceptor, its remediation gets manifested either
through anodic oxidation or cathodic reduction
under defined conditions (Pant et al. 2010 ). Very
recently, reduction of some substrates or carbon
dioxide (CO 2 ) as electron acceptors during BES
operation is also being reported, increasing its
commercial viability (Srikanth et al. 2014 ). The
current chapter fully focuses on the remediation
aspects of BES with respect to different wastewa-
ter, specific pollutants and desalination.
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