POWER March 2015 - 23

WATER & WASTEWATER
1. In with the new. Oasys Water's forward osmosis technology is installed to treat flue
gas desulfurization wastewater at the Changxing Power Plant in China. Courtesy: Oasys Water
3. The draw solution thermal recovery
system. Heat is added in the thermal
column to evaporate the draw solution,
leaving behind purified water. Courtesy: Oasys
Water
2. No magic involved. This process diagram shows how a forward osmosis system
produces purified water. Source: Oasys Water
Saline water
Organics,
minerals,
pollutants
Salt
Draw
solutes
Concentrated brine
is not required in the thermal column (Figure
3), less energy is consumed than would
otherwise be necessary. Another advantage
of this arrangement is that no impurities enter
the thermal process, therefore scaling and
foaming are not a problem.
By design, the closed loop system should
not require additional ammonium bicarbonate
to be added. The plant has typical mechanical
components though, such as tanks,
valves, pumps, and piping, so there is always
the potential for leaks or a component failing.
For that reason, Oasys suggests that additional
draw solution be kept on hand.
Benefits of FO
Oasys says its FO system offers some advantages
over other more common water treatment
options. According to Marchewka, in
RO systems used for seawater desalination,
the typical water recovery rate is only about
March 2015 | POWER
Heat
Clean water
50%. In other words, for every two gallons of
seawater taken into a system, one gallon of
purified water is produced and one gallon of
reject water is discharged back to the source.
The FO process can be used to take the reject
from a seawater desalination RO system and
concentrate that to achieve an additional 80%
recovery. Therefore, combining the two systems
can result in an overall recovery of 90%.
RO systems also are limited in the salinity
that they can handle. Once the system reaches
its maximum hydraulic pressure, water can
no longer be pushed through the membrane
to achieve recovery. In contrast, FO technologies
can treat water up to 150,000 ppm of
total dissolved solids-four times the maximum
for conventional RO systems-and concentrate
it to over 280,000 ppm. So not only
can much higher recovery be achieved using
FO-because it is not limited by an osmotic
gradient-but it also operates at a lower preswww.powermag.com
Water
diffusion
Draw
solution
Salt-rejecting
membrane
Recovery
system
sure,
which offers an energy savings.
Thermal systems, such as multiple effect
distillation, multi-stage flash, or mechanical
vapor recompression, offer another option
for desalination of seawater and brine concentrating.
Although thermal systems can be
designed to work well in many situations,
they have limitations of their own.
For one thing, thermal systems are capital
intensive to install. The materials used
have to be capable of handling the corrosive
effects of seawater, so they are frequently
constructed of more expensive alloys. The
energy consumed by a thermal system is also
much higher than in FO systems.
In thermal systems, the feedwater must be
heated to its vaporization temperature, which
requires significant energy. The vapor is then
condensed to produce the distillate. In that
process, impurities in the water can cause
scaling or foaming, resulting in a very maintenance-intensive
operation. As noted previously,
only the draw solution and clean water
enter the thermal recovery column of the FO
system, which eliminates this problem.
Innovative FO Uses
Although FO and RO may sound like rival systems
designed using similar technology-the
membrane portion of an FO system does look
nearly identical to that of an RO system, at
least on the outside-Oasys views its FO system
as more of a complement to RO systems
rather than a replacement for them. It suggests
FO systems are better able to compete directly
with thermal evaporation systems.
" The focus of the company, right now,
is more on industrial high-salinity recov23

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POWER March 2015

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