Environmental Engineering Reference
In-Depth Information
trolyte. Up to 98 % salinity removals from 35 g/L
NaCl were achieved using stacked MDCs consist
of five pairs of cells. These results imply that, for
practical applications, MDCs are more likely to
be used for partial salt removal from seawater.
The requirement of fresh water also depends on
the initial salinity of salt water. MDCs can also be
used for brackish water desalination. Many stud-
ies were performed using acetate as the organic
substrate in anode and phosphate buffer as catho-
lyte. Few other studies also considered real-field
wastewater as anolyte. Microbial oxidation of or-
ganics was the sole mechanism involved in elec-
tric potential in anode, whereas oxygen reduction
reaction (ORR), ferricyanide reduction reaction
and HER were considered for cathodic reduction
mechanism. The maximum CE of MDC mecha-
nism is found to be 80 % (Kim and Logan 2011 )
A system consisting of two membrane-based
bioelectrochemical reactors, an osmotic microbial
fuel cell (OsMFC) containing a forward-osmosis
(FO) membrane and MDC that had ion exchange
membranes was designed to treat wastewater and
to desalinate saline water. Both the reactors were
coupled hydraulically. This design significantly
improved desalination efficiency through both
dilution in the OsMFC and salt removal in the
MDC along with extended organic removal effi-
ciency (Zhang and He 2013 ). Other systems were
also developed with stalk design using more than
one membrane pair between electrodes (Chen
et al. 2011 ) and similar to the stack design used
for electrodialysis (ED) desalinating systems.
The IEM stack consists of alternating AEMs and
CEMs, creating repeating pairs of desalting and
concentrating (concentrate) cells (Chen et al.
2013 ). The MDC stacks should be designed po-
tential energy generated by exoelectrogens with
oxygen reduction and the resistance of individual
cell pairs. Chen et al. ( 2011 ) found that the rate
of desalination with two cell pairs was faster
than that with three cell pairs by increasing the
inter membrane distance compared to electrodi-
alysis systems (0.2-3 mm) (Strathmann 2004 ).
Many MDCs designed were having intermem-
brane distance between 1 and 2.4 cm, resulting
in very high internal resistances (Mehanna et al.
2010a , b ; Chen et al. 2011 ; Luo et al. 2011 , 2012 ;
Qu et al. 2012 ). Performance can be improved by
reducing the internal resistance with minimized
intermembrane distance. The internal resistance
of an MDC also increases with the number of
IEM pairs in the stack. In an ED system, the ap-
plied voltage is controllable depending on the
stack size. In an MDC, however, the voltage used
for desalination is limited to that produced by the
electrode reactions, and therefore the voltage per
cell pair decreases with an increase in the number
of cell pairs (Kim and Logan 2013 ).
Another design, submerged microbial desali-
nation denitrification cell (SMDDC) to in situ
remove nitrate from groundwater and to produce
electric energy along with treatment of wastewa-
ter (Zhang and Angelidaki 2013 ). The SMDDC
can be easily applied to subsurface environ-
ments. When current was produced by bacteria
on the anode, NO 3 and Na + were transferred
into the anode and cathode through anion and
cation exchange membranes, respectively. The
anode effluent was directed to the cathode where
NO 3 was reduced to N 2 through autotrophic de-
nitrification. This design was removed 90.5 % of
nitrate from groundwater in 12 h and generated
3.4 A/m 2 of current density. External nitrifica-
tion was beneficial to the current generation and
nitrate removal rate, but was not affecting total
nitrogen removal (Zhang and Angelidaki 2013 ).
Photosynthetic MDC was designed and operated
using algae as catalyst in cathode (biocathode)
which enhanced the COD removal and utilized
treated wastewater as the growth medium to ob-
tain valuable biomass for high value bioproducts
(Kokabian and Gude 2013 ). The increase in sa-
linity concentrations in anode chamber provide
more favourable conditions for certain types of
microbes than others resulting in enrichment
of selective bacteria with simultaneous elimi-
nation of the bacteria that can withstand saline
conditions (Mehanna et al. 2010 ). Integration of
multiple bioprocess with diverse products can
be beneficial in enhancing the sustainability of
microbial desalination cells. Besides advantages
of MDCs in desalination along with wastewater
treatment at low energy consumption, few limi-
tations were also associated. They can be listed
as salt removal can be very high (> 95 %) but it
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