Chemical Engineering August 2014 - 60

Environmental Manager
At a combined-cycle power plant
in the Northwestern U.S., another
interesting initial difficulty occurred
in application of this process.
Normal MF or UF operation is
a cyclic process with permeate production
during a user-established
period (20 minutes is common), followed
by a backwash period of one
minute or so, followed again by permeate
production and so on. Typically,
a small port,ion of the permeate
is collected in a separate tank at
the beginning of the process for use
during the backwash step.
However, most modern MF and
UF units are now equipped with automatic,
chemical-enhanced backwash
(CEB) systems. After a certain
number of regular backwash cycles,
a CEB backwash is automatically
initiated. First the membranes are
cleaned with a dilute caustic or
bleach solution to remove organics
and microbiological organisms.
This is followed by a rinse step,
and then a dilute citric-acid wash
to remove iron particulate matter.
When the UF in this particular application
was first commissioned,
the membranes quickly developed a
layer of calcium silicate scale during
the CEB caustic stage. The higher
pH had greatly reduced the silicate
solubility, producing scale that was
very hard to remove. In this application,
the magnesium concentration
of the blowdown was relatively
low - otherwise magnesium
silicate deposition would also have
occurred (this was observed at another
facility using this setup along
with a CEB procedure). To address
this, the operators made a switch
to softened water as the backwash
supply for the CEB procedure.
This case and others clearly emphasize
that pilot testing should be
strongly considered before installing
such systems. Some water-treatment
equipment vendors have suggested
the use of upstream multi-media
filters to help condition the blowdown
stream, but direct observation
has shown that these filters may be
completely ineffective in preventing
chemical fouling of membranes, for
the reasons offered here.
Another important factor to consider
with these treatment systems
regards redundancy, either via additional
storage capacity or standby
equipment. With a properly operating
system, the final waste stream
is obviously very much reduced. But
if the system goes offline for any
reason, the entire blowdown volume
plus additional plant wastewater
streams can be too much for the
final treatment process, particularly
if it involves thermal evaporation.
Summary
Once-through cooling is no longer
an option for many power plants
and chemical process industries
(CPI) plants. Most often, cooling
towers are the preferred choice, although
air-cooled condensers are
appearing more frequently. For
new and existing plants with cooling
towers, increasingly stringent
effluent guidelines are requiring
plant owners, operators, and
technical personnel to evaluate
water and wastewater treatment
modifications and additions. Many
factors will influence the final
technology selection, including
the following:
* States may impose guidelines beyond
those set by EPA
* Regulations are becoming more
stringent for additional wastewater
constituents, such as TDS,
sulfate, phosphate, ammonia and
some heavy metals
* Restrictions on discharge of specific
impurities, such as phosphate,
can greatly influence the choice of
cooling-tower treatment program.
For many operators, all-polymer
treatment programs are emerging
as a preferred alternative
* Discharge chemistry control may,
in part at least, have to be addressed
by modifications to the
water-treatment steps taken for
the makeup water
* Increasingly, plants in some areas
of the U.S., either from a mandate
or by necessity, are selecting reclaim
(gray) water for makeup.
These supplies often have variable
water quality, which greatly influences
cooling tower operation and
the ultimate chemical makeup of
the blowdown
60 ChemiCal engineering www.Che.Com august 2014
* Installation of wastewater-treatment
systems may be required to
comply with new guidelines
* It is quite possible that other
wastewater constituents may
be regulated in the future, such
as additional salts, heavy metals,
chloride and bromide, which
are under scrutiny. Selection of
ZLD as a proactive measure can
prepare a plant for such future
eventualities
* Installation and operation of a
wastewater-treatment system to
achieve ZLD is not a simple process.
Many factors can influence
system performance, including
the following:
The variable quality of the plant
makeup water, especially if unreated
or reclaim sources are
the supply
The potential for fouling of the
wastewater-treatment system
that can arise from standard
chemicals that are widely utilized
for water treatment
Change in the final waste
stream that comes from the
primary treatment process. If
such techniques as evaporation
ponds or deep-well injection are
not possible, thermal evaporation
may be the only choice. ■
Edited by Suzanne Shelley
References
1. Post, R. and Buecker, B., Power Plant Cooling
Water Fundamentals, Pre-conference
seminar for the 33rd Annual Electric Utility
Chemistry Workshop, June 11, 2013, Champaign,
Ill.
2. Personal conversation with Dan Janikowski,
Plymouth Tube Co.
Author
Brad Buecker is a process
specialist in the Process Engineering
and Permitting group
of Kiewit Power Engineers
(9401 Renner Blvd, Lenexa,
KS, 66219; Phone: 913-9287311;
Email: brad.buecker@
kiewit.com). Kiewit provides
consulting and engineering
services for industrial water
and wastewater projects. He
has over 33 years of experience
in the power industry, much of it with City
Water, Light & Power (Springfield, Ill.) and at
Kansas City Power & Light Company's La Cygne,
Kan., generating station. Buecker has written
many articles and three books on steam generation
topics, and he is a member of the American
Chemical Society, AIChE, the American Soc. of
Mechanical Engineers, the Cooling Technology
Inst., and the National Assn. of Corrosion Engineers.
He holds a B.S. in chemistry from Iowa
State University, with additional course work in
fluid mechanics, heat and material balances, and
advanced inorganic chemistry.
http://www.Che.Com
Chemical Engineering August 2014

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