As fresh water sources
diminish in some areas of the U.S. and around the world, water conservation is
becoming increasingly important. Wastewater reclaim is a vital tool in these
conservation efforts, where at least two main themes have emerged: effluent use
from publicly owned treatment works [(POTW), i.e., municipal wastewater] as industrial
plant makeup, and the recovery/reuse of wastewater generated within the
industrial plant. This article examines potential difficulties with these
processes and considers methods for avoiding pitfalls.
Growing
reclaim water use. It is well-known that drought conditions in the
southwestern U.S. continue to increase in severity over the long term.
Accordingly, conserving water by using alternatives to fresh water has become vital.
California has mandated that industrial plants take POTW effluent for makeup. To
various degrees, this philosophy is being adopted at other locations within the
country. However, the variability and complexity of POTW effluent chemistry and
quality makes reuse a challenging task. Many wastewater plants employ primary
and secondary treatment, which remove suspended solids and pathogens before
discharge; however, tertiary treatment is often missing, allowing elevated
concentrations of many dissolved solids to remain. TABLE 1 offers a comparison between four
fresh water supplies and four reclaim waters (listed by another common name,
“grey water”).
Compare the
surface water supplies, highlighted in blue, with the grey water supplies, highlighted
in orange. The grey water exhibits much higher concentrations of the major
cations (calcium, magnesium, potassium, sodium), anions (chloride, nitrate,
phosphate, sulfate) and, of course, conductivity. Ammonia in surface waters is usually
low but often appears in the double digits as mg/l for reclaim water, as in the
Sonora and Pittsburg samples in TABLE 1, unless the POTW employs a nitrification step to
convert the ammonia to nitrate. Not shown is total organic carbon (TOC), which
is also typically much higher in reclaim water than fresh water. Municipal
wastewater effluent may have significant concentrations of suspended solids. The
nitrogen species, phosphate and organics serve as nutrients and food for
microorganisms.
Dealing
with the “nasties” in reclaim water. Anyone who has dealt with cooling water
treatment knows that microbiological fouling control can be a severe challenge.
Consider systems with cooling towers. Hordes of microbes enter from both the
air flow and makeup water to the tower. The tower and associated cooling system
provide warm, wet conditions for the development and growth of microbiological
colonies. Once colonies become established, some organisms secrete a protective
polysaccharide (slime) layer that protects the microbes (FIG. 1). The slime,
plus any silt and other debris that it captures, can quickly and severely foul
heat exchangers throughout the system (FIG. 2).
A
well-designed, maintained and operated chemical feed program is necessary for
fouling control. An oxidizing compound (chlorine, chlorine-activated bromine,
chlorine dioxide, etc.) serves as the primary biocide, with perhaps periodic
feed of a non-oxidizing biocide as a supplement.2 However,
throughout modern industrial history, severe fouling has repeatedly occurred from
upsets in treatment system design or operation. Now, imagine what could happen
with a makeup stream that has the chemistry of the grey water samples shown in TABLE 1. The elevated
levels of nitrogen and phosphorus, combined with plenty of organic food, can
lead to explosive microbiological growth. Furthermore, fouling is not limited
to cooling networks and can affect other systems, including pretreatment for
high-purity water production, service water lines, etc. For example, large
organic molecules can severely foul RO membranes, apart from microbiological issues.
This is a
primary challenge for POTW effluent reuse as industrial plant makeup. In cases
where the stream contains a constant elevated suspended solids concentration,
clarification might be in order. Clarifier design has significantly evolved from
the large, circular types that were ubiquitous during the last century. A
common rise rate (effluent flow divided by the surface area at the top of the
clarifier) of 1 gpm/ft2 or slightly less is a common standard for
these older designs, as the gentle flow allows particles to settle. However, in
systems such as the ballast type (ballast material is typically micro-sand or
magnetite), after coagulant injection, the influent encounters the
ballasted recirculating flow in the flocculation zone (FIG. 3).
The coagulated/flocculated
solids attach to the ballast material. The microsand or magnetite is much
heavier than the sludge, therefore the settling efficiency increases
dramatically. Rise rates of 25 gpm/ft2 or higher have been reported
for some of these technologies. The ballast material is recovered in
hydrocyclones (via the underflow stream) for reuse, while the sludge exits with
the hydrocyclone overflow for disposal.
If the makeup
water is high in hardness, lime softening clarification may be a consideration,
although this process generates large quantities of sludge that must be
disposed. Lime softening, and the resultant elevated pH, will precipitate
phosphate and hardness that could otherwise cause downstream difficulties. However,
the added complexity of lime softening and sludge disposal diminishes its
viability for many applications.
Clarification
does nothing for nitrogen-based impurities, particularly ammonia and
nitrite/nitrate. Also, many organics may be carried through a clarifier. Therefore,
even with clarification for suspended solids and possibly phosphate removal, many
nutrients and food can still enter plant water systems. Accordingly, for POTW
effluent from a facility with only secondary treatment, the industrial plant
may need to strongly consider independent tertiary treatment. Proven methods
have been in place for decades. The activated sludge process—in which, following
large solids and grit removal, the waste stream flows into a large basin or
basins filled with beneficial microorganisms that consume the organics and
nitrogen/phosphorus nutrients—is
common. The term activated comes from the fact that air is injected into the
basin, often at numerous locations, to provide an aerobic environment for the
microbes. A common variation on this technology is the trickling filter, in
which beneficial organisms attach to fixed media that wastewater flows over. This
arrangement gives the microbes a stable base to carry out waste removal.
A difficulty
with both technologies is that the treatment processes are slow and require
large pond volumes or media surface area. Among several advanced systems, two
have emerged as leading candidates: membrane bioreactors (MBR) and moving-bed
bioreactors (MBBR). A basic MBR schematic is shown in FIG. 4.
The fundamental
process is similar to conventional activated sludge, wherein the beneficial
organisms consume food and nutrients that enter the main vessel from the mixing
zone. A recycle stream helps to bring active, well-established microbes to the
inlet of the mixing zone. However, modern units have a much smaller footprint
than older, traditional activated sludge systems. Another major difference of
MBR from conventional activated sludge is the use of microfilter membranes
rather than a clarifier to separate solids from the effluent. The
microfiltration process produces a very clear stream, essentially free of
suspended solids. Conscientious care of the filters is necessary to maintain reliability.
This includes periodic membrane cleaning.
One deficiency
of this most basic MBR process is that while ammonia in the stream is converted
to nitrite/nitrate, the nitrogen remains. These species can still serve as a
nutrient in industrial plant water systems. The problem can be solved, if
necessary, by expanding the MBR unit to include anoxic or anaerobic reaction
chambers, where other beneficial microorganisms convert nitrite/nitrate to
elemental nitrogen. However, this step adds complexity to the process.
The prime
feature of MBBR is a mechanically stirred main reaction vessel that contains
mobile plastic media. The media (shown in FIG. 5) serves as sites for the beneficial
microorganisms to attach and then consume the nutrients and food from the
influent.
In some
respects, MBBR can be thought of as an advanced version of the trickling bed
wastewater treatment process, but with mobile rather than fixed reaction sites.
Due to the presence of circulating solid media in the reaction vessel, microfiltration
membranes cannot be placed in this compartment. Rather, the microfilters must
be external to the reactor.
A common concern
when evaluating system installation is staffing requirements to operate the
unit. Two potential solutions stand out: if the plant is located close to the
POTW, it may be possible to place the MBR or MBBR at the POTW and have that
staff operate it; or many reputable manufacturers offer
“build-own-operate-maintain (BOOM)” programs and will, for an annual fee, manage
all equipment and operating details.
Consider
another issue with the reclaim water chemistry outlined in TABLE 1: the rather high chloride
concentrations. Common materials for combined-cycle and co-generation steam
surface condenser tubing are either 304 or 316 stainless steel. Chloride can
induce serious pitting in these metals, which may severely shorten tube life. The
recommended chloride limits for clean tubes have been gradually reduced as more
data has become available about corrosion potential. One top industry expert
now suggests chloride limits of 200 ppm for 304 SS and 400 ppm for 316 SS.3
Standard pretreatment processes do not remove chloride, and it is evident from TABLE 1 that if any
of those reclaim waters were selected as makeup, the concentration increase induced
by cooling tower cycling would quite likely cause a chloride exceedance. Note
the emphasis above on clean tubes. Deposits would decidedly increase the
corrosion potential.
Internal
water conservation. Multiple waste streams may be generated at large industrial
facilities. Cooling tower blowdown is one example that has received increased attention
for water conservation. FIG.
6 outlines a treatment process at a combined-cycle power plant in a
semi-arid region of the U.S.
The system
recovers 90% of the blowdown as high-purity reverse osmosis (RO) permeate, with
the remainder discharged to an evaporation pond. The configuration allows the
plant to operate as a zero-liquid-discharge (ZLD) facility.
Keys to the
process are:
Lessons learned
and modifications made during the commissioning phase of this system include:
As many readers
may have observed, this particular ZLD application is somewhat simplified because
the final concentrated wastewater stream can be disposed of by natural
evaporation. That option is not viable at many locations, and more intensive
methods such as evaporation/crystallization may be needed to reach ZLD.4
Per the example above, pre-concentration reduces the wastewater volume ahead of
final treatment. Of the remaining waste stream, perhaps 90% can be recovered in
a brine concentrator and recycled to the plant. The crystallizer converts the
remaining liquid stream to a solid product. These final steps can be energy
intensive and often require precise chemical feed to reduce the scaling
potential of the concentrated waste stream. Equipment costs may be high for exotic
materials to resist corrosion. Regular equipment cleaning is usually necessary
to remove scale and restore proper heat transfer.
Condensate
return. At times, condensate return chemistry affects water
conservation, but also often has a strong influence on equipment performance
and reliability. Consider the basic co-generation schematic in FIG. 7.
The diagram includes
automatic condensate return dump valves that are activated by conductivity
excursions—a
feature common for many systems. The size and frequency of upsets may vary
greatly from plant to plant, or even between the condensate streams within a
plant. Dumping can result in considerable water loss. Furthermore, this water
would already have gone through makeup treatment and energy input from the
boiler(s), making dumping costly from that standpoint. If condensate polishing
is considered, the types of impurities in the condensate will dictate polisher
design. Conventional ion exchange resins are effective for mineral salt and
hardness removal, subject to resin temperature limitations.
Granular
activated carbon (GAC) is a possibility for some organic compounds,
particularly for larger organic molecules. Ion exchange resins, without the
active sites, may also be effective for organic removal, as the millions of
beads with their many minute passageways offer an enormous surface area for
organic adsorption. Direct media filtration may work well on particulates such
as iron oxide corrosion products; however, a better approach is addressing the
root cause(s) of corrosion with proper condensate chemical treatment, whenever
possible.
As a final thought,
consider a reverse scenario to the discussion above. Years ago, author Buecker and a colleague investigated a
fouling issue at an organic chemicals plant that produced a primary product
plus four closely-related derivatives. Steam generation came from several 550-psig
drum-type boilers in which solids deposition in the superheaters and the associated
tube overheating required tube bundle replacement every 1.5 yr–2 yr. An extracted
superheater bundle had internal deposits up to approximately ¼ in. deep. It was
also discovered that foam was being issued from every boiler-saturated steam
sample line. The plant had minimal steam generation analytical chemistry
equipment—with
tests performed by operators, not chemists—so internal data was sparse. However, condensate
return chemistry data provided by the plant’s water treatment vendor showed condensate
total organic carbon (TOC) concentrations as high as 200 ppm. Compare this to
the recommended upper limit of 0.5 ppm in the ASME industrial steam generator
guidelines.5 The excessive organic contamination in the boilers
generated the foam that carried over to the superheaters. Condensate dumping
would have produced a large and potentially hazardous waste stream and required
a substantial upgrade to the makeup treatment unit to provide the lost volume. However,
no condensate polishers were in place to remove the organic compounds. So,
without some modifications, the superheater fouling was destined to continue.
Takeaways.
Water conservation is
becoming increasingly important, as many parts of the world face drying
conditions. Various methods are available to recover and recycle both external
and internal wastewater streams for industrial plant use. However, pretreatment
to varying degrees is necessary to minimize the fouling, scaling and corrosion
potential of makeup supplies and internally recycled streams. Costs of unit
shutdowns can be much greater than the expenditures for design, installation
and operation of reliable reclaim water treatment systems.
Please remember
that each system is different and has unique treatment needs. Due diligence is
necessary to determine the feasibility for utilizing the methods discussed in
this article. Always consult your equipment manuals and guides and contact
a water treatment professional before making changes to your systems and
treatment processes. HP
REFERENCES
BRAD BUECKER is President of Buecker &
Associates LLC, specializing in consulting and technical writing/marketing. Most
recently, he served as Senior Technical Publicist with ChemTreat Inc. Buecker
has more than four decades of experience in or supporting the power and water
treatment industries, much of it in steam generation chemistry, cooling water
treatment, air quality control, and results engineering positions with City
Water, Light & Power (Springfield, Illinois) and Kansas City Power &
Light Company’s (now Evergy) La Cygne, Kansas station. His work has also
included 11 yr with two engineering firms, Burns & McDonnell and Kiewit,
and 2 yr as Acting Water/Wastewater Supervisor at a chemical plant. Buecker earned
a BS degree in chemistry from Iowa State University with additional course work
in fluid mechanics, energy and materials balances, and advanced inorganic
chemistry. He has authored or co-authored more than 250 articles for various
technical trade magazines, and has written three books on power plant chemistry
and air pollution control. He is a member of the ACS, AIChE, AIST, ASME, AWT, NACE
(now AMPP), the Electric Utility Chemistry Workshop planning committee, the
Power-Gen International planning committee, and he is active with the
International Water Conference. He may be reached at beakertoo@aol.com.
RAJENDRA P. KALAKODIMI is the Director of Cooling Water for ChemTreat Inc. in Ashland, Virginia. He received an MS degree in physical chemistry at Andhra University and a PhD in electrochemistry at the Indian Institute of Science in Bangalore in 2003. While in India, Dr. Kalakodimi served as the Engineering Technical Leader at the GE India Technology Centre in Bangalore and as Product Manager for Chemicals and Monitoring Solutions for GE Water. He has more than 20 patent filings, 20 international publications and various conference presentations.