POWER March 2017 - 40

WATER & WASTEWATER
1. Small but effective. The Hydrotech
Discfilter is a mechanical, self-cleaning filter
that offers a large filter area in a small footprint.
Courtesy: Veolia Water Technologies
problems can increase in closed-cycle cooling
systems when water evaporates and
leaves behind higher concentrations of constituents.
To
control the water quality, power plant
operators have a number of options, including
removal of some of the concentrated,
recirculating water. Flow volumes and
makeup can also be adjusted, and incoming
reclaimed water can be treated prior to being
added to the recirculating system.
What Is the Best Option?
In determining whether use of reclaimed water
makes sense for a power plant, a series
of questions need to be asked about the circumstances.
Some
questions to ask and items to consider
include:
temperature compared to surface waters. Because
secondary effluent is relatively consistent
in quality, the treatment process, and the
design and operation of the water treatment
system, become easier. In addition, the cooling
tower blowdown may be able to be returned
to the municipality, eliminating one of
the waste streams requiring treatment at the
power plant.
Reclamation Requirements
Several sets of regulatory requirements govern
the use of reclaimed water for cooling.
Federally, the Clean Water Act requires a
permit, issued under the National Pollutant
Discharge Elimination System (NPDES)
program, authorizing any discharges of
pollutants to surface waters. The U.S. Environmental
Protection Agency (EPA) implements
the NPDES program and has the
power to authorize states to issue permits
and administer the program.
NPDES permits contain discharge limits
determined by the treatment technology that
the EPA believes is available and affordable,
as well as by the states' water quality standards
and available dilution in the receiving
water bodies. State rules vary according to
factors such as the likely degree of public
exposure to reclaimed water. Where exposure
is high, reclaimed water must be highly
treated. States usually establish limits on
fecal or total coliform bacteria and may require
that wastewater be filtered before it
can be reused as reclaimed water. Turbidity
standards are also frequently established.
On the operational side, unique treatment
issues posed by utilizing reclaimed water
include the need to determine cleanliness
levels that must be achieved to satisfy a
plant's individual operational requirements.
Reclaimed water's chemical elements can
cause problems like mineral scaling, corrosion,
stress cracking, and biofouling. These
40
■ Does the plant need to meet a specific
legal or regulatory performance requirement?
It is imperative that plant management
understands all legal and regulatory
obligations before making modifications.
There is no excuse for breaking the law.
■ What are the costs? The costs of technologies
can vary widely. Investment in disc
filtration, for example, even for a very
large flow may be as low as $500,000 to
$1 million. While ultrafiltration costs may
be three to four times as expensive, other
savings such as from running cleaner water
through the system or lowering chemical
costs may favorably impact life cycle
costs. The costs of freshwater resources
also are rising in some places and beginning
to reflect the true costs of water.
■ What are the benefits? These may include
less immediately tangible, but still important,
benefits to the power company's
image with key stakeholders. For example,
decreasing discharges of secondary
effluent and reducing pressure on water
resources to the benefit of local communities
could be a positive example of
corporate social responsibility for power
companies.
■ Are there publicly owned treatment works
(POTWs) nearby to keep the costs of
transporting wastewater sufficiently low?
A study by the University of Pittsburgh
found that 97% of power plants proposed
in the U.S. could meet their cooling needs
by utilizing secondary treated wastewater
from POTWs located within 25 miles.
■ What is the level of water risk in the local
area? As the effects of climate change
continue to manifest themselves, it is clear
that no place is safe-even in the historically
rain-soaked Northwest, the states of
Washington and Oregon have been faced
with drought emergencies in recent years.
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POWER | March 2017
■ What technical option is the best approach?
Alternatives could include clarifying
systems, disc filters, biological
processes, submerged microfiltration, or
ultrafiltration membranes.
■ Are there special issues that need to be addressed?
For example, some POTWs have
low ammonia levels, while others can be
quite high. Chlorine treatment is one option,
but breaking down the ammonia
requires high levels of chlorine, creating
new risks and adding costs. Biological
systems may negate some of these concerns
although requirements to maintain
a minimum flow circulation even during
planned power outages can be a drawback.
■ Is the power company comfortable operating
the water treatment system, especially
if it's biological? If not, alternative solutions
may be possible, such as having the
POTW host and operate it, or outsourcing
the operation to the system supplier.
Based on the responses to these and other
questions, determining the right combination
of primary and secondary systems to provide
the appropriate level of water purity at a reasonable
cost can be determined with the help
of an expert systems solution provider.
Award-Winning Treatment
A pretreatment strategy was the approach applied
in the growing city of Mankato, Minn.
The city installed a new water reclamation facility
(WRF) to treat effluent from its wastewater
treatment plant (WWTP), which would
supply the cooling tower needs of an electrical
generation plant. In addition to providing
quality reuse water for the energy center, the
WRF needed to meet new state phosphorus
removal regulations.
2. Efficient filter media. Solids catch
on the inside of Hydrotech Discfilter panels.
As solids impede flow, water level increases,
triggering disc rotation and a backwash cycle.
Courtesy: Veolia Water Technologies
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POWER March 2017

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