Environmental Engineering Reference
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anaerobic biological methods is very slow. More-
over, the cosubstrate addition makes the process
noneconomical. Addition of organic cosubstrate
also leads to the methane formation (van der Zee
and Villaverde
2005
; Mu et al. 2009).
The application of BES for azo dye degrada-
tion in cathode compartment is showing an ad-
vantage of BES processes. It was already known
that in an electrochemical cell, the chromophoric
linkage of azo dyes can be reduced by accepting
the cathodic electrons. The resultant colourless
aromatic amines are more biodegradable (Frijters
et al.
2006
). A similar mechanism prevails in
BES, which acts for the degradation of azo dyes.
(Mu et al. 2009; Ding et al.
2010
). But the azo
dye reduction occurs at high cathodic over po-
tential that imparts system efficiency (Mu et al.
2009). Several dyes such as methyl orange, acid
orange 7, active brilliant red X-3B, amaranth,
congo red, etc. were studied for the degradation
in BES (Table
10.3
). The concentration of dyes
was varied between 10-900 mg/l concentrations
in single and double chamber BES. The reduc-
tion of dyes in a conventional biological reactor
follows different decolorization mechanisms in-
volving enzymes, low molecular weight redox
mediators, chemical reduction by biogenic re-
ductants like sulphide or a combination of these
(Pandey et al.
2007
). The mechanism of dye
degradation in cathode is similar to the anaerobic
anodic degradation, except that there is an addi-
tional mode of electron and proton transfer to the
dye, through the external circuit and the mem-
brane respectively. In BES, the colour removal
was primarily observed due to biodegradation
rather than biosorption by living cells (Sun et al.
2009
). Mu et al. (2009) proposed the decoloriza-
tion mechanism of AO7 (Scheme
10.1
). At the
anode, the substrate is oxidized by bacteria to
produce protons and electrons, which are trans-
ferred to the cathode via proton exchange mem-
branes and external circuit respectively. The azo
bonds of dye are broken at cathode by using pro-
ton and electron generated in anode, resulting in
the formation of toxic intermediates. Ding et al.
(
2010
) reported on methyl orange reduction via
photogenerated electrons in a BES containing an
irradiated rutile-coated cathode.
The performance of a BES for decolorization
depends on the concentration and the type of dye
used. Mu et al. (2009) investigated the effect of
concentration of azo dye acid orange 7 (AO7).
Circuit configuration also showed a consider-
able effect on dye degradation. It was shown that
during closed-circuit operation, decolorization
efficiency decreased from 78 to 35 % with an in-
crease in influent dye concentration from 0.19 to
0.70 mM, while the dye decolorization rate in-
creased from 2.48 to 4.08 mol m
−3
NCC d
−1
with
an increase in the influent dye concentration from
0.19 to 0.70 mM, maintained at constant HRT and
pH. The BES power output increased from 0.31
to 0.60 W/m
3
with increase in AO7 concentration
from 0.19 to 0.70 mM. Sun et al. (
2009
) reported
that the percent decolorization decreased with in-
crease in ABR-X3 (Active Brilliant Red X-3B).
The decolorization rate decreased slightly from
90 to 86 % as ABRX3 concentration increased
from 100 to 900 mg/L within 48 h. It was predict-
ed that decolorization efficiency decreases with
increase in dye concentration. Besides concentra-
tion of azo dye, other factors like operating pH,
structure of dye, HRT, type of wastewater used
in the anode and cathode etc., also influence the
process of dye degradation in BES. These factors
were also found to influence the power genera-
tion capacity of the BES.
10.6
Microbial Desalination
Application of BES in desalination of saline
water and industrial wastewater is found to be a
promising technology that utilizes the microbio-
logical energy from the wastewater treatment to
drive the ions through ion exchange membranes
(IEMs), resulting in desalination (ElMekawy
et al.
2014
). This new method that can reduce or
completely eliminate the electricity requirement
for desalination is called as microbial desalina-
tion cell (MDC). The main feature of the MDC
is that exoelectrogenic microorganisms produce
electrical potential from the degradation of or-
ganic matter, which can then be used to desali-
nate water by driving ion transport through IEMs
(Cao et al.
2009
; Kim and Logan
2013
). When
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