POWER August 2016 - 37

WATER AWARD
2. The new skid on the block. The membrane brine concentrator system's forward
osmosis skids at the Changxing plant. Courtesy: Oasys Water
stage crystallizer at a target TDS concentration
of 220,000 mg/l or higher. Currently, the
crystallizer processes 2.5 m3
/hour of brine,
operates at pH 8.0-8.5, and concentrates the
brine to saturation. A pusher centrifuge and
hot air dryer system then remove remaining
water to less than 0.5%. Finally, produced
mixed salt crystals of more than 95% (NaCl
+ Na2
SO4
) are bagged and palletized for sale
to chemical manufacturers in the region.
Table 1. MBC design specifications and operational data. Source: Oasys
Water
Parameter
Raw feed flow
Raw feed total dissolved solids (TDS)
MBC brine TDS
MBC product water TDS
Overall water recovery
to Oasys Water, the FO MBC concentrates
wastewater dissolved solids from approximately
60,000 milligrams per liter (mg/l) in
the RO concentrate to 220,000 mg/l or higher.
" The MBC does the brine concentration
work of a thermal or mechanical evaporator
with the simplicity and modularity of a membrane
system, while consuming a fraction of
the energy, " it explained.
How It Works
The FO trains are driven by an osmotic pressure
gradient created across a semi-permeable
membrane to achieve spontaneous and
preferential diffusion of water molecules
from a saline feed into a proprietary draw solution.
As water crosses the FO membrane, it
dilutes the draw solution. At the draw solution
exit of the FO array (Figure 2), it has extracted
the prescribed amount of clean water
from the wastewater stream and a separation
process must be performed to reconcentrate
the draw solution and to separate the clean
water stream for reuse.
To reconcentrate the draw solution, it must
be heated to between 45C and 65C above
37
Design basis
15.0-26.4 m3/h
25,000-40,000 mg/l
>220,000 mg/l
<100 mg/l
79%-88%
Actual conditions
27.0-30.0 m3/h
8,600-9,500 mg/l
>220,000 mg/l
35-50 mg/l
93%-97%
the inlet dilute draw solution temperature to
volatilize and subsequently recondense and
absorb ammonia and carbon dioxide.
For the Changxing system, the energy requirement
is less than 90 kWth/m3
of wastewater
processed, Oasys Water said. The draw
solution recovery was designed to use steam
as the energy source for volatilization and
water cooling for condensation, but the company
noted that mechanical vapor recompression
system designs are also an option.
The freshwater stream exiting the MBC
draw solution recovery loop combines with
the pretreated wastewater stream and passes
through the RO system. It leaves the system
in the second-pass RO permeate stream with
TDS of less than 100 mg/l. It is then stored
in the product water tank and is reused in the
power plant's boiler makeup water system.
The MBC system is designed to recover up
to 23 m3
Lessons Learned
The Changxing Power Station Units 7 and
8 were new builds that came online in late
2014, which meant there was " no actual water
analysis on which to base the design of the
ZLD process train to treat FGD blowdown
wastewater, " noted Oasys Water. " This fact
guided the design team to conservatively design
the pretreatment and the FO brine concentrator
as described, " it said, noting that
since startup, the flow per day to the plant
has been in range of the design basis, but
TDS levels have been lower than expected
(see table). This has resulted in lower flow to
the MBC system and overall higher recovery
prior to the ZLD crystallizer.
Startup and commissioning of the new
plant, and the first-of-its-kind water treatment
technology, was also challenging, as
could be expected. Notably, the Chinese and
American team dealt with language barriers
as it trained operators to run the system.
Startup, commissioning, and training related
to the project that is of much interest
to developers of new power plants and coalto-chemical
projects in China were also complicated
by frequent tours and visitors. But
that interest ultimately paid off for Oasys
and Chinese partner Beijing Woteer, which
have since secured three additional projects
in China. They include another coal power
FGD wastewater project and two projects for
ZLD treatment of streams at coal-to-chemical
process complexes.
Today, the solution continues performing
as expected to transform Changxing's
complex, high-salinity waste stream, which
includes FGD blowdown wastewater and
cooling tower blowdown. Oasys Water's
technology is now transforming 630 m3
per
/hour-or up to 87% recovery-but
it is actually performing at more than 90% in
operation thus far, said Oasys Water.
Notably, the salt byproduct from the process
is also collected and sold. Concentrated
brine exiting the MBC system feeds a twowww.powermag.com
day
of wastewater, reducing both the intake
of local surface water and the outflow of
industrial wastewater. The draw solution recovery,
designed to use steam as the energy
source, also continues to reduce overall energy
requirements and overall costs.
For adopting a novel solution to pressing environmental
concerns, the impressive achievement
at the much-watched Changxing Power
Plant earns POWER's 2016 Water Award. ■
-Sonal Patel is a POWER associate
editor.
POWER | August 2016
http://www.powermag.com
POWER August 2016

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