YSM Issue 86.1

ENVIRONMENTAL ENGINEERING
A schematic of PRO. When the dilute and concentrated solutions mix, they generate
osmotic pressure that spins a turbine to produce electricity. Courtesy of Menachem
Elimelech.
material must allow water to diffuse freely
across it but simultaneously block the passage
of salts and other dissolved substances. These
ideal conditions are difficult to attain: small
amounts of salt from the concentrated chamber
are able to pass through the membrane
into the dilute chamber, and water from the
dilute stream may flow into the concentrated
stream as well. The net effect of these factors
is to increase the salt concentration of the
dilute stream and reduce the overall driving
force for osmosis, a phenomenon known as
internal concentration polarization.
An additional challenge with membranebased
electricity production is membrane
fouling: natural water sources contain organic
material, bacteria, and other contaminants
that can become trapped in the pores of
the membrane and lower its efficacy over
time. Since water treatment is an energyconsuming
process, the Elimelech group is
working to find fouling-resistant materials.
“Current membranes that produce a very
high water flux have some inherent surface
roughness…and organic matter likes to stick
to it,” says Elimelech. “The key is to make
more smooth membranes that organic matter
will not attach to.”
Reverse Electrodialysis (RED)
Unlike PRO, which relies on water transport,
reverse electrodialysis captures energy
from the movement of ions. Ions, charged
particles formed when a salt dissolves in
water, are abundant in seawater. When seawater
mixes with freshwater, ions naturally
diffuse into the less concentrated freshwater
to create energy. Just as the name implies, this
is the opposite of electrodialysis, which uses
energy to force ions against their concentration
gradient.
RED uses two types of semi-permeable
membranes: anion-exchange membranes,
which only allow the passage of negativelycharged
ions, and cation-exchange membranes,
which only allow the passage of
positively-charged ions. The RED system is
set up with alternating salt water and fresh
water channels separated by membranes.
Each salt water channel lies between two
fresh water channels, bounded by an anionexchange
membrane on one side and a cationexchange
membrane on the other. A typical
RED apparatus consists
of many stacks
of these alternating
membrane pairs. As
salt water and fresh
water mix, anions
and cations diffuse
in opposite directions
toward two
electrodes on either
end of RED apparatus.
Electrodes
receive the ions and
convert this energy
into an electrical
current carried by a
connecting wire.
Research has
revealed that the
maximum amount
of energy that RED
can theoretically
produce depends on
the salinity, or salt
concentration, of the water source. Whereas
typical seawater can produce just under 1
kilowatt-hours of energy, highly concentrated
salt water sources like the Dead Sea can generate
over 14 times that amount. To optimize
power density, researchers are also working
to redesign spacers, structures that provide
mechanical support between membranes.
Conventional spacers interfere with ion transport,
but newly-developed conductive spacers
are permeable to ion flow.
A major challenge to implementing widespread
RED systems is the cost of the ionexchange
membranes, but researchers hope
that prices will go down as global demand
increases. Better production technologies and
more efficient membranes will also contribute
to a lower cost.
Microbial Fuel-Cells (MFC)
Thanks to the advent of new technologies,
modern methods of acquiring energy have
become remarkably diverse. In addition to
the water from oceans and rivers, scientists
have found that wastewater can be a valuable
resource. In particular, this energy can be
converted into electricity-using bacteria. Since
current wastewater treatment plants already
use bacteria to remove organic material from
the water, microbial-fuel cell (MFC) technology
can transform these treatment plants into
the power plants of the future.
Elimelech suggests that the energy created by PRO and RED
could be reused to desalinate water, creating a closed-loop system.
Courtesy of Menachem Elimelech.
www.yalescientific.org
January 2013 | Yale Scientific Magazine 15