POWER March 2015 - 26

WATER & WASTEWATER
6. Managing alternatives. Multimedia filters offer an option for removing suspended
solids, iron, and manganese from incoming water, which can improve RO performance. Courtesy:
U.S. Water
to use the secondary treated water and treat it
further for use for cooling water source and,
with further treatment, for boiler feedwater. "
Membrane Innovations
The RO process is well understood and has
proven to work satisfactorily in many applications.
Even so, membrane manufacturers
continue to improve upon thin-film composite
technology used in their elements.
According to U.S. Water Services Inc. (U.S.
Water), a Minnesota-based integrated water
management solutions provider, a couple of
significant advances have enabled design and
operation improvements in RO systems.
One improvement is in the fouling characteristics
of some membranes. Power plants
are frequently being forced to use poorer
quality water as a source for makeup to circulating
and demineralized water systems. The
latest fouling-resistant membranes have been
designed to meet the more difficult working
conditions while reducing cleaning frequency
and minimizing pretreatment.
Pressure requirements for low-energy eleDiversion
valves on the pump discharge
allow operator-controlled manual wasting of
waste-activated sludge-that is, removing a
portion of it-in order to maintain a proper
mixed liquor suspended solids concentration.
Waste-activated sludge is pumped to a sludge
holding tank that is aerated to prevent septic
conditions. Sludge may be removed via
pump truck, if necessary.
From the anoxic basin, activated sludge is
pumped to the pre-aeration basin. Fine bubble
diffusers evenly disperse air, providing a
residual dissolved oxygen concentration to
prevent premature fouling of the membranes
in the MBR basin. The aerated mixed liquor
gravity feeds into the adjacent MBR basin.
Submerged membranes in the MBR (Figure
4) filter the sludge to produce an extremely
clean effluent referred to as permeate. The
flow rate of permeate is controlled using a
modulating valve to maintain a constant level
in the basin. The membranes foul over time,
so the PLC automatically opens the control
valve to adjust flow until parameters signal
that fouling warrants an in-situ cleaning.
During the cleaning process, the membranes
are relaxed by closing the permeate
control valve and scouring the membranes
with the blower. Excess membrane biofilm
is scoured away to recover flux and improve
performance. A maximum relax time is set to
prevent membrane abrasion.
Permeate from the membranes is pumped
to an in-line chlorine tablet feeder for disinfection
prior to discharge. Disinfected effluent
then flows by gravity to the discharge
26
point. Sludge is processed through a belt
press for dewatering, and dry solids are removed
for disposal. The recovered water is
recycled back into the process for treatment.
The system in Texas is sized to treat
100,000 gallons of wastewater per day, providing
effluent water suitable for makeup to
the plant's cooling pond. Ovivo has many
other systems using various technologies operating
all around the world.
Zero-Liquid Discharge-and Beyond
One of the largest zero-liquid discharge
(ZLD) systems is at the Palo Verde Water
Reclamation Facility in Arizona (Figure 5).
It is a 90 million gallon per day tertiary treatment
plant that reclaims treated secondary effluent
from the cities of Phoenix, Scottsdale,
Tempe, Mesa, Glendale, and Tolleson. According
to Someah, the Palo Verde Nuclear
Generating Station is a ZLD facility and the
only nuclear power station that uses 100%
reclaimed water for its cooling.
Palo Verde's process includes a series of
trickling filters to achieve biological de-nitrification.
Next, first- and second-stage solid
contact clarifiers remove hardness-causing
minerals and calcium from the water. Final
polishing is accomplished in mixed media
gravity filters, after which the softened water
enters the plant's cooling water cycle.
" The technology to treat the water has
come a long way and has advanced drastically
over the last decade, " said Someah.
" Today there are cost-effective technologies
offered by Ovivo that will allow the industry
www.powermag.com
ments have also been improved. Historically,
low-energy elements have had rejection rates
too low to gain much acceptance in the power
industry. The negative impacts of increased
salt ion passage to downstream components,
such as mixed bed demineralizers or electrodeionization
systems, were too great.
However, newer membrane technology is
lowering pressure requirements while keeping
the rejection at, or near, traditional rates
of brackish water membranes. The improvement
allows original equipment manufacturers,
like U.S. Water, to reduce pump and
motor sizes, which saves energy and improves
net plant heat rate.
While membrane improvements are helpful,
the control of microbiological activity is
still extremely important to aide in the longterm
reliability of RO systems. Many facilities
have large water tanks that serve as
process and firewater reserves. Holding times
in these tanks can be very long. As the water
sits relatively stagnant, controlling the microbiological
growth in these tanks needs to be
considered. When they are left unmanaged,
operators often struggle to maintain control
and will be required to clean RO systems
more frequently.
Challenges can also result from active biological
growth on RO membranes or from the
slimy byproduct shed from biofilms upstream
of the RO. U.S. Water strongly recommends
that plants maintain a free halogen level in
the process water tank and upstream multimedia
(Figure 6) or ultrafiltration systems at
all times to help minimize these issues. ■
-Aaron Larson is a POWER associate
editor.
POWER | March 2015

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POWER March 2015

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