POWER March 2015 - 24

WATER & WASTEWATER
4. A state-of-the-art wastewater
treatment process. The submerged
membrane bioreactor configuration relies on
course bubble aeration to produce mixing and
limit fouling. Courtesy: Ovivo USA
5. Waste not, want not. The Palo Verde Water Reclamation Facility can treat up to 90
million gallons of secondary effluent from the Phoenix metropolitan area and provides all of the
cooling water for the Palo Verde Nuclear Generating Station. Courtesy: Arizona Public Service
ery projects, specifically in zero-liquid
discharge, or near zero-liquid discharge systems, "
said Marchewka.
In addition to the FGD wastewater treatment
system Oasys installed at the Changxing
Power Plant, it has another FO system
already operating in China. That system has
the flexibility to be used for seawater desalination
or for treating cooling tower blowdown,
depending on the plant's needs.
Through a partnership with National
Oilwell Varco (NOV), Oasys' technology
is being deployed in the oil and gas industry
too (see this issue's cover photo).
NOV says the system is suitable for onshore
unconventional shale plays, and it
markets the solution as a means of treating
exploration and production wastewaters. It
touts that these streams can be converted
to freshwater quality, fully treated for reuse
in new drilling and completion fluids
or for surface discharge in remote areas
where disposal options have traditionally
been limited and expensive.
Oasys says it is the first company to deploy
an FO-based brine concentrator. The
company can also imagine using the technology
for things like brackish desalination and
other municipal applications.
One final advantage that really benefits
operators is the FO system's ability to handle
variation. Marchewka noted that the company
has learned from its experience in China that
the water chemistry from the FGD process
is quite variable-seasons, load, and various
other operating parameters all factor in. Although
changes can be problematic for many
systems, because the FO system operates at
lower pressure and pulls the water across the
membrane with the draw solution, it is much
less prone to fouling and scaling, and it can
handle the challenge.
" It actually gives operators a nice benefit
when dealing with fluctuations and
24
changes in water quality and water chemistry, "
says Marchewka.
Utilizing Treated Municipal
Wastewater
Power plants continue to face greater restrictions
in the usage of water from traditional
sources, such as oceans, lakes, rivers, and
wells. In the U.S., regulations like 316(b) are
forcing facilities to consider alternatives to
business as usual. State-of-the-art technology
has made treated municipal wastewater generated
by publicly owned treatment works
(POTW) an attractive source of cooling water
makeup for many power plants.
A study conducted at the University of
Pittsburgh, evaluating more than 400 existing
coal-fired power plants, revealed that 49.4%
of them could have sufficient cooling water
supplied by POTWs within a 10-mile radius
of their plant. If the radius were expanded to
25 miles, the percentage increased to 75.9%.
It also evaluated 110 proposed power plants
and found that 81% of those facilities could
meet their cooling water supply requirements
from POTWs within 10 miles of their proposed
locations. The 25-mile radius satisfied
all but three of the plants.
According to Kaveh Someah, vice president
of global energy for Ovivo USA, the
use of reclaimed water started decades ago
and is gaining in popularity. There are a
number of treatment technologies that must
be considered based on an individual plant's
situation, but one of the more advanced
methods includes the use of a membrane
bioreactor (MBR).
An MBR is a wastewater treatment process
utilizing biological treatment alongside filtration
all in one common tank. MBR systems
are considered the best available technology
for wastewater treatment and reuse applicawww.powermag.com
tions,
because they are reliable, space efficient,
and cost effective. Ovivo-formerly known as
Eimco Water Technologies-worked with a
power plant in Texas to develop a solution that
uses an MBR system to provide makeup water
to the plant's cooling pond.
The Membrane Bioreactor
Treatment Process
At the Texas facility, the screen box design
handles course screening, allowing raw
wastewater to be pumped straight into a finescreening
system to remove particles that
could potentially damage the membranes.
The screened influent enters the equalization
basin, which maintains flow forward up to
the peaking capacity of the membranes.
If sufficient hydraulic pressure is not
available, the plant is designed with an emergency
overflow to a basin located adjacent to
the equalization basin. Once plant flow and
level return to normal, any overflow can be
pumped back to the equalization basin for
feed forward.
From the equalization basin, screened and
equalized wastewater is pumped to the anoxic
basin. The level in the anoxic basin varies,
depending on hydraulic loading conditions.
Control of the MBR plant is based on level in
the anoxic basin.
A programmable logic controller (PLC) receives
a level input and varies the flow rate of
treated water to accommodate influent flow. It
also initiates an intermittent mode to preserve
biology, reduce power consumption during
low plant loading, and protect equipment.
A mixer in the anoxic basin operates continuously
to mix the activated sludge with
incoming wastewater, maintaining a uniform
concentration of mixed liquor suspended solids.
Pumps in the anoxic basin are used for
feeding forward and internal recycling.
POWER | March 2015

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POWER March 2015

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