Water storage needs vary greatly, and so do the available systems and tank materials.
Increasingly, onsite water storage
is becoming a more integral part of water conservation. In many locations and
situations, local regulations are requiring commercial facility owners to take
responsibility for treating wastewater onsite and providing their own water
sources for some applications. In both the commercial and residential sector,
many owners are looking for ways to manage the environment and a finite
resource—as well as reap the financial benefits of conserving water.
Several systems and tank materials
are available, and one thing is for sure: No one system or material is ideal for
every water conservation situation. Owners need to consider major factors such
as storage volume and factors affecting the structural integrity of the tank
when choosing a system.
Water Storage of All Shapes and
Sizes
Paul Eldredge, assistant public
works director for the City of Brentwood, CA, reports that three large concrete
storage tanks the city has had constructed in the past few years are key
components in its water conservation plans. Brentwood is located in the East Bay
area of California and has a climate that is more like the state’s Central
Valley than the rest of the Bay Area, he points out. “We have long, hot, dry
summers, and our irrigation use increases in the summer. We’ve got some San
Francisco Bay–Delta winds that can kick up periodically, so the combination of
hot, dry, and windy conditions can dry out the ground quickly.”
Eldredge explains that Brentwood
is separated from the coastal area by a small mountain, “So, we don’t get a lot
of the coastal fog and dampness you’d get if you were closer to the coastal
area,” he says.
In 2004, the city had the first of
two, 4-million-gallon, partially aboveground, concrete tanks constructed.
Eldredge notes that the first tank was to be located in a residential
neighborhood and appearance was important. Given the necessary volume, the city
researched a prestressed concrete tank building system from DYK Incorporated
that is designed to prevent leakage via a method of putting concrete into
compression, a state in which it offers its maximum structural integrity. For
the sake of appearance, the tank was given a façade of architectural stone. A
second concrete tank was to be constructed partially underground, into a
hillside, two years later. Eldredge reports that the tank would serve as a
retaining wall as well as a water storage structure, considerably narrowing the
cost difference between steel and concrete, the tank material ultimately
selected.
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Photo: Containment Solutions
Fiberglass is competitive with concrete in the 2,500- to
10,000-gallon range and is growing its presence in the 10,000-gallon-plus segment. |
The city had its most recently
constructed tank, a 1.5-million-gallon structure at its water treatment
facility, built out of the same prestressed system about two years ago. This
structure has inner and outer tanks, the inner one for final disinfection and
the outer one for storage. All three of the city’s concrete tanks are tied into
its water treatment system.
Eldredge says that he and the city
settled on the prestressed concrete building system after conducting thorough
research and making site visits to look at other DYK tanks in service. He adds
that one factor that made concrete particularly attractive was its performance
in seismically sensitive areas such as the Bay Area. “All of the information we
had received led us to believe that [the tanks] were structurally sound and the
prestressing was a tried and true technology—it wasn’t some new product off the
street,” he says.
The experiences of Michael Stark,
president of Stark Environmental LLC, Columbia, PA, underscore the fact that
some materials are better suited to some water storage situations than other
materials. Stark’s company designs and installs multiple storage systems,
including the Roth MultiTank from Roth Global Plastics Inc. This system is
primarily designed for rainwater collection and storage in residential
applications—which, obviously, have smaller capacity requirements than the City
of Brentwood. Stark notes that polyethylene tanks better suit smaller-capacity
underground rainwater storage applications than some other materials.
Underground rainwater storage
systems, Stark points out, merit the consideration of two factors that
necessitate a tank with high structural integrity and polyethylene provides
sufficient durability in many cases. One factor is the stability of the soil,
the other the height of the water table. A high water table can put the tank in
contact with deleterious groundwater. A low-profile tank not only keeps it above
the water table, but also allows shallower excavations, which are beneficial
from a safety standpoint and can reduce construction costs. Stark, whose company
serves the Mid-Atlantic region, notes that much of the US population lives in
coastal areas where these soil and water table issues are common relative to
water storage system installation.
Concrete and polyethylene are two
of the materials commonly used for water storage tanks. Various materials that
are used have relative benefits and are prevalent in situations that lend
themselves to these benefits.
Different Applications, Different
Materials
Steel is a tried-and-true material
for storage tanks, owing to its competitive cost, Stark points out. “Obviously,
without some coating, you run a lot of potential for rust developing on the
inside of the tank,” he says. “And on the outside, being in contact with the
ground, there could be deterioration if it doesn’t have some kind of coating on
it. No doubt, steel tanks are strong, but they’re fairly heavy” and cannot be
moved into place by a pair of workers, he adds.
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Photo: Tnemec Company Inc.
The cost of coatings is a major maintenance consideration for elevated or
partially elevated tanks, as on this steel tank in Rosemont, IL. |
One characteristic of the
polyethylene MultiTanks that appeals to Stark, he says, is the fact that they
incorporate a high percentage of recycled material. Additionally, “Polyethylene,
as plastics go, it’s considered a pretty benign plastic in that it doesn’t
impact the liquids stored in it, and, typically, it also doesn’t bleed things
into the soils around it and doesn’t get attacked by things in the soil.”
Stark’s company also installs
fiberglass tanks. He argues that the advantage of this material over steel is
lighter weight, and, like polyethylene, it does not need an interior lining.
However, fiberglass is more cost-effective in larger sizes, he adds. Stark
points out that precast concrete tanks have a strong presence in larger sizes,
yet some sites are not suitable for truck and crane access, making polyethylene
a better fit. Also, he points out, the price competitiveness of concrete depends
somewhat on the availability of quality aggregates in various regions. A
durability issue in concrete to consider is cracking and repair of alternative
materials such as polyethylene is easier, he adds. Finally, although recycled
concrete is commonly used in both precast and site-cast structures, cement
production is characterized by high-energy use and high emissions, so some might
consider materials like polyethylene to be more sustainable. Concrete becomes
more economically attractive at larger volumes, he concludes.
Peter Young, vice president of
sales and technical services for fiberglass tank manufacturer Containment
Solutions Inc., identifies specific water storage tank capacities that various
materials suit. Polyethylene tanks have a strong presence up to 2,500 gallons;
above this capacity, greater thickness is required. Concrete is particularly
strong in the 2,500- to 10,000-gallon range and traditionally dominated the
market in the 10,000-gallon-plus segment. However, Young says, fiberglass has
been competitive with concrete in the smaller range and is growing its presence
in the larger range, because it becomes more cost-effective with scale. Although
Containment Solutions’ individual tanks have a 50,000-gallon-size limit, he
points out that several can be combined using flange fittings on the bottoms and
manifold pipes that connect the tanks. Steel has traditionally dominated larger
capacities of 250,000 gallons and more. Noting that steel has been a dependable
material for years, he cautions that the interior of the tank should have a
liner to maintain water quality.
Polyethylene was not considered
for the city of Brentwood’s water conservation strategy because of its capacity
needs, says Eldredge, who also acknowledges the cost-effectiveness of steel.
Still, Eldredge says, the initial cost difference between steel and concrete is
narrower than in the past. “The primary disadvantage [of steel] is that we have
to periodically go in there and inspect the coatings or the interior of the tank
and there’s a lot of [operations and maintenance] and capital costs associated
with recoating those tanks and inspecting them and with concrete we do not have
that challenge,” he says, adding that the city also has four large steel storage
tanks.
The choice of concrete came down
to a lower life-cycle cost, Eldredge says. “For the first tank we did, the main
selling point was that we were trying to install this tank; we wanted to have a
[partially buried tank to minimize its visual impact. If there were a
significant cost savings to steel, we would have looked at it more closely.
Concrete is so much more flexible because you can bury it, partially bury it, or
completely bury it and build stuff on top of it. A steel tank is just a tank,
and you try and berm it and plant trees and hope that nobody notices it. I’ve
come to really like the concrete tank system—it provides a lot more options for
engineers and planners. As space becomes more scarce, we’re putting tanks in
areas where people don’t want them, and you can put these into existing
environments with minimal visual impact. When all is said and done, it’s the
visual impact that people are going to care about because people are going to
see the tank every day.”
An advantage of elevated steel
tanks is their use of gravity, which eliminates the need for pumps to move the
water, points out Doug Hansen, director of water tank market sales for Tnemec
Co. Inc., a manufacturer of industrial coatings used on structures such as water
storage tanks. About the only competitor to elevated steel tanks are a composite
structure that have been built since the 1970s, i.e., steel tanks supported by
concrete shafts—designed to eliminate the need for painting the shaft—Hansen
notes. Tom Bloomer, P.E., regional business development manager for DYK, argues
that concrete tanks are much better suited to underground structures. DYK’s
prestressing system makes the concrete in its tanks highly ductile to handle
seismic and static stresses, Bloomer adds. For tanks up to 500,000 gallons,
conventional reinforced concrete typically is used for water storage, but, above
this volume, prestressing is necessary to handle the stresses on the walls,
according to Bloomer.
The DYK tanks constructed in
Brentwood have flat concrete membrane floors, cast-in-place walls, and
column-supported two-way flat slab roofs. The tank wall is vertically
post-tensioned with all-thread threadbars and is circumferentially prestressed
using galvanized seven-wire strand and incorporates a flat, two-way slab
concrete roof. The prestressing take the concrete out of tension—a state of
being pulled apart—where it is weakest, while tensioning the steel puts that
material in its strongest state. This construction method, combined with the
tanks’ circular shape, eliminates stress concentrations and uniformly
distributes loads around the tank circumference. In such large structures
relying upon a material that is susceptible to cracking, the steel tensioning is
essential, according to the company. Additionally, Bloomer points out, concrete
tanks are subject to differential temperature and dryness loads because the tank
walls are wet on the inside and dry on the outside, and the temperature varies
between the two sides. To further protect the galvanized steel strands, the
system utilizes the automated application of a one-and-a-half-inch shotcrete
cover coat; the automated application is used to ensure uniform thickness. The
company reports that these tanks have retained their structural integrity amid
major earthquakes such as the 1971 San Fernando Valley, 1989 Loma Prieta, 1994
Northridge, and 2001 Washington State earthquakes.
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Photo: Xerxes Corp.
Flexibility is a major factor for many storage projects. |
 |
Photo: Cal Aerial Imaging
The prestressed concrete tanks—a 2.2 million-gallon potable unit and a 1.8
million-gallon recycled water tank—use compression for structural integrity. |
The roof and floor are separated
from the corewall by neoprene bearing pads, a design intended to provide an
unrestrained connection and reduce bending moments induced by hydrostatic,
thermal, backfill, and seismic forces. This “free-sliding” connection at the
wall base and wall top is designed to enhance the seismic performance of the
tank by allowing the floor, wall, and roof to act independently and a continuous
PVC waterstop between the floor and wall is used to ensure a watertight joint.
Alternatives are available for the roof. A concrete, two-way, flat slab roof has
a slight upward slope to the tank’s center. A grid of equally spaced round
columns supports the flat roof. Free-span concrete or aluminum dome roofs can
also be used and eliminate the need for interior columns, because the dome is
entirely supported by the tank wall.
Bloomer says that the system
offers owners construction flexibility. The tank can adapt aesthetically by
incorporating finishes such as paint, stucco, or exterior insulation and finish
systems (EIFS).
In contrast, the Roth MultiTank
suits the residential-scale rainwater harvesting market sector and allows
builders to earn credit toward the Leadership in Energy and Environmental Design
(LEED) Green Building System. The system collects rainwater from a house’s
gutters and filters out leaves and other contaminants with a two-stage process
before the rainwater enters the underground polyethylene holding tank. The
system suits configurations in which an irrigation system pump is located either
inside the tank or inside the house.
In either pumping configuration,
rainwater enters the storage tank through a “calmed inlet” that prevents the
disturbance and re-suspension of fine sediments that migrate to the bottom of
the tank, and introduces oxygen into the lower layers of the tank so as to
prevent anaerobic conditions from forming. Once a maximum level is reached in
the tank, an overflow siphon skims the water and removes lighter-than-water
particles, maintaining quality water and allowing oxygen diffusion at the water
surface. A pre-charged irrigation pressure tank dispenses the stored rainwater
on a predetermined schedule. According to the company, capturing water on a
1,500 square-foot roof would allow a family to reduce its water bill by 50% and
save about 35,000 gallons of water annually.
Joe Brown, vice president of sales
and marketing at Roth Global Plastics Inc., discusses some unique design
features of the MultiTank. This tank varies in size from 535 to nearly 1,800
gallons and is blow-molded by a computer-controlled, multi-layer machine.
According to Brown, many polyethylene tanks are actually designed for use in
septic systems and, therefore, are full all the time and continually exert
hydraulic pressure on the soil. In contrast, the MultiTank does not rely on
hydraulic pressure inside the tank, but, rather, largely owes its structural
integrity to a thick horizontal rib located between each set of vertical
ribs.
Containment Solutions’ fiberglass
Flowtite Tank, which is often used to store rainwater and graywater treated by
various recycling systems, is another water-storage system that reportedly
allows contractors to earn LEED points. The manufacturer uses steel cylindrical
steel molds called mandrels in fabricating the tank. The rotation of the mandrel
and the application of the materials are controlled by a computer and as the
mandrel rotates, resin, glass, and specially treated silica are precisely
metered onto the mandrel. Using an instrument that magnetically senses the metal
mold surface through the fiberglass-reinforced plastic laminate, the
manufacturer measures the thickness of the laminate at many points throughout
the surface of the tank wall.
Designed to enhance the durability
of some water-storage tanks, Tnemec’s new HydroFlon fluoropolymer polyurethane
coating reportedly has a life expectancy of 15 years or more and is formulated
to retain gloss and color longer than traditional urethane topcoats. The company
says that the coating is available in two-component and single component
formulations and can be brushed, rolled, or sprayed. The product also has 60%
volume solids and high-ultraviolet and saltwater protection, making it
particularly suited to coastal environments.
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Photo: Xerxes Corp.
In the future, several forces will drive change in water storage, including
materials and maintenance. |
Any discussion of available tank
materials must include steel. One company, the Tank Connection Affiliate Group,
says that it provides all four types of steel tank construction: bolted rolled,
tapered panel (RTP), field-weld, shop-weld, and hybrid tank (combining bolted
flat-panel steel, field-weld, and concrete). Focusing on bolted RTP, the
manufacturer reports that standard liquid tank capacities range from 25,000 to 8
million gallons. The company claims that the system’s structural integrity comes
from its design, proprietary coating systems, and field installation procedures
that utilize a synchronized, hydraulic screw jack process, allowing field crews
to install tanks at grade level.
Another steel tank manufacturer,
Columbian TecTank, designs and fabricates bolted and factory-welded tanks. Its
environmentally controlled process uses continuous tunnel design, beginning with
surface preparation followed by the coating application, and final thermal
curing at baking temperatures up to 400°F. The company also uses a proprietary
epoxy powder coating for interior surfaces that provides corrosion resistance
and immersion performance, as well as flow, impact, and abrasion-resistant
properties. The company points out that the system conforms to NSF Standard 61
for potable water. Columbian also has an exterior coating system that includes a
special primer for corrosion resistance with a urethane topcoat for color
retention.
The exteriors of the company’s
factory-welded tanks are coated with a special primer and a urethane or a zinc
base/epoxy intermediate/performance urethane finish topcoat for high-corrosion
resistance and color retention. The elimination of field welding reduces
erection costs, according to the company. Standard tank capacities range from
4,000 to 3 million gallons, and material options include factory-coated carbon
steel, galvanized steel, stainless steel, and aluminum. Roof options include
1:12 sloped steel, aluminum geodesic dome, and open top with wind girder.
According to Tom Tietjen of Xerxes
Corp., Fiberglass (FRP) underground tanks are rapidly gaining favor among water
collection/conservation system designers and owners for the variety of
advantages they offer over the traditional products that have been used. While
competitively priced or less expensive to install, Tietjen points out that FRP
tanks provide unmatched corrosion resistant properties not found with materials
in alternative (concrete or steel) tanks. Additionally, they are a lightweight,
factory-produced product that can be shipped one piece in very large capacities
(12-foot diameter and up to 65,000 gallons) directly to a jobsite and installed
easily and quickly.
FRP tanks provide an excellent,
long-term, 30-plus-year track record of watertight performance, having become
the standard for the retail petroleum industry where storage of hazardous
materials underground is taken very seriously. Tietjen notes that today FRP
tanks are routinely being specified for a wide variety of water and wastewater
applications, some associated with Green Building designs such as rainwater
collection or plumbing graywater systems. Projects for water with potable
applications can be specified with an NSF listing, explains Tietjen, assuring
the specifier of compliance with the strict NSF standard. FRP tanks are
available from manufacturing facilities located throughout North America.
Forces Driving Change
Experts identify several forces
that will drive change in water storage in coming years, from applications to
materials to maintenance.
Young reports that Containment
Solutions has identified three major growth markets for its system: fire
suppression, decentralized wastewater treatment, and rainwater harvesting. He
says that the company supplies many tanks for dedicated sprinkler use in a
building, and that some municipal building codes now require a dedicated supply
and not the municipal supply in some situations. Similarly, some new commercial
buildings are not allowed to utilize a municipal wastewater treatment system and
must treat the water onsite instead. This market sector also encompasses the car
wash industry, which uses large volumes of water. Rainwater harvesting seems to
be growing by the week, Young argues. For now, the focus is on reusing captured
rainwater for irrigation, but he also can see rainwater being used for toilets
in the future. He adds that water storage is a growing component of companies’
business; 10 years ago, it accounted for only about 1–2% of business, and now it
makes up about 25%.
Brown also sees rainwater
harvesting as a source of growth for Roth, albeit gradual. He adds that, in
North America, water is still plentiful and inexpensive enough that economic
drivers of growth do not yet exist as in other parts of the world.
Bloomer of DYK notes that storage
capacities have consistently increased in recent years, adding that his company
was recently involved in construction of a 40-million-gallon tank.
In
terms of system materials and design, Hansen says he thinks that steel tanks
will still be prevalent, but owners will increasingly seek ways to prolong their
service lives, and that a major steel tank rehabilitation market does and will
exist. Owners will put a lot of emphasis on durability-enhancing coatings in
this market, because their cost accounts for 15–25% of a tank’s maintenance
costs. In terms of dollars, he says, coating a 500,000-gallon tank might cost
$140,000, so owners favorably view any potential extension in recoating. Also,
according to Hansen, taking a tank out of service might take five to 15
weeks.