Environmental Engineering Reference
In-Depth Information
from heated feedwater in one chamber, transferring the humid air to another chamber
where it comes in contact with a cool surface, thus allowing freshwater collection [63].
Energy consumption in such a system is required by the heat source as well as the pumps
and blowers to move the water and vapor. Recovery rates for these systems range from
5% to 20%, and heat sources such as solar or geothermal supplies are common choices,
especially since the rates of freshwater production are suited for demands of a few cubic
meters per day. Temperature differences between 2°C and 5°C are required, necessitating
heat consumption of approximately 20.9 kJ/m
3
[106].
Researchers have also demonstrated the development of a microluidic synthetic tree
that can drive the motion of water at negative pressures. In addition, this synthetic tree
structure has been shown to draw liquid from subsaturated vapor, not unlike the process
of RO rejecting salt and allowing water to pass through the membrane. Consequently, the
system acts as a tension-based pump that could be developed to extract or purify water
from subsaturated soils [107].
27.3.5 Microbial Desalination Cells
Microbial fuel cells convert biowaste to electricity through microbial activity [108] by
generating electrons available for harvesting in an external load circuit. In an innova-
tive approach, a three-cell system was converted into a water desalination system (Figure
27.15). In a proof-of-concept study, it was shown that the three-cell design with an inte-
grated anion-exchange membrane (AEM) and cation-exchange membrane (CEM) could
generate a small amount of potential (typically less than 1 V) and desalinate water. The
bacteria grow on the anode side and discharge protons into the water; however, the pro-
tons cannot pass through the AEM, so negatively charged ions from the saline water low
through the AEM to balance the positive charges produced. A similar process takes place
on the cathode side, except protons are consumed, requiring positively charged ions from
the saline water to cross the CEM to correct the charge imbalance. Testing of this system
has been conducted with NaCl solutions at concentrations of 5, 20, and 35 g/l, which is
consistent with concentrations seen from brackish water to seawater. The amount of salt
removed from each concentration was at least 88 ± 2% for the 5 g/l case and up to 94 ±
3% for the 20 g/l case [108]. Comparison of the electrons harvested and NaCl removed
(a)
(b)
−Anode
+Cathode
Euent
Euent
e
-
e
-
Na
+
Cl
-
Influent
Influent
Biofilm
AEMCEM
FIGURE 27.15
(See color insert.)
(a) Basic schematic representing a three-chamber cell design for coupling a microbial fuel cell
with a water desalination system. (b) Microbial desalination cell prototype. (From Cao, X., X. Huang, P. Liang,
K. Xiao, Y. Zhou, and B.E. Logan,
Environmental Science Technology
, 43, 7148, 2009.)
