POWER March 2015 - 22

WATER & WASTEWATER
Water and Wastewater Treatment
Technology Update
Courtesy: U.S. Water
The handling of power plant water and wastewater is becoming increasingly complex.
Fortunately, innovative treatment technologies can help. Recent advances include
forward osmosis, membrane bioreactor wastewater treatment systems, and
reverse osmosis membrane improvements.
Aaron Larson
W
ater is the lifeblood of a thermal
power plant. As such, obtaining
clean and pure makeup water and
dealing with wastewater has been a requirement
since the first steam generating unit
went into operation. As rules and regulations
change, new technology is often necessary to
meet more restrictive guidelines. The desire
for energy savings, more reliable treatment
methods, and solutions to water availability
challenges can also lead to innovations.
Reverse osmosis (RO) is a widely used
technology in the power industry. Developed
in the 1950s, the first commercial RO
plant began operating in 1965. The process
uses a semipermeable membrane to purify
water by applying pressure to overcome osmotic
pressure, forcing water from a region
of high-solute concentration through the
membrane to a region of low-solute concentration.
A newer membrane technology that
22
may not be as familiar to readers is forward
osmosis (FO).
Fast Forward to Forward Osmosis
The first FO water treatment plant was built
in 1998 for use on landfill leachate; today,
research and development continues to refine
the process. While not as common as RO, FO
systems are proving to offer a new solution for
some challenging situations. Boston-based Oasys
Water recently installed a system to treat a
Chinese coal-fired power plant's flue gas desulfurization
(FGD) wastewater (Figure 1).
Lisa Marchewka, vice president of strategy
and marketing for Oasys, explains, " We use
membrane technology, but instead of using hydraulic
pressure to force water through a membrane,
we instead use a high-molarity 'draw'
solution that pulls freshwater across the membrane
rather than pushing it on the surface. "
The key ingredient in the system is the
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draw solution. Oasys uses ammonium bicarbonate,
which is an off-the-shelf product
available in bulk. Although ammonium bicarbonate
is not completely harmless, it is
a relatively safe product that was once used
in homes before modern day baking powder
became available. In fact, Oasys obtains its
product from the well-known baking soda
company Arm & Hammer.
Feedwater enters the FO system at one
end of the membrane module (Figure 2). The
draw solution flows on the opposite side of the
membrane, counter to the direction of feedwater
flow, and pulls water molecules through the
membrane. The draw solution becomes more
and more diluted until it exits the module and
is directed to the thermal process.
In the thermal recovery device, the diluted
draw solution is heated to evaporate only the
draw solutes, leaving behind the clean, purified
water. Because evaporation of the water
POWER | March 2015
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POWER March 2015

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