Choosing the right tanks for a community involves weighing in a number of options.
Gary Brockman, director of
maintenance of Mission Springs Water District in Desert Hot Springs, CA, has
been working with water storage since 1969. When he first started in this
industry, an open dirt reservoir was a common option for community water
storage. You chlorinated the water, and it was good to drink.
“Nowadays, you try to have a
closed system to keep a safe, reliable supply,” says Brockman.
Choosing the right water storage
system is literally a once-in-a-lifetime event. A well-built, well-maintained
water storage tank can last upwards of 100 years. Those responsible for choosing
the tanks for their community have to weigh a number of options before making a
final decision—such as the size, materials, and whether to place the tank
underground or elevated above other buildings.
Water storage tanks are made from
a variety of materials. Concrete and steel are the most popular, particularly
for community reservoirs. Plastic and fiberglass tanks are other options, which
usually fit a smaller or different type of market.
Economics tend to play a key role
in the decisions behind the choice of one material over another. Steel tanks are
less expensive than concrete tanks in the initial cost, but they require more
maintenance than concrete. “It’s a ‘pay-me-now,’ or ‘pay-me-later,’ type of
deal,” says John Thornton, professional engineer at Psomas, a leading consulting
engineering firm. He acknowledges, however, that communities have to install the
water storage system they can afford at the time and budget in for maintenance
in the future.
Because of its lower upfront cost,
steel is a very popular material used for water storage tanks. It is the most
common material used for elevated potable water storage.
“Welded steel tanks have a long
history of service and flexibility of design,” says Kevin Gallagher, vice
president of sales for Caldwell Tanks.
There are a variety of types of
elevated steel tanks, Gallagher explains. The multi-column tank is the most
traditional style of tank and consisted of several columns on an elevated
storage container. The pedisphere style tank is what Gallagher says is commonly
referred to as “the golf ball on the tee.”
“It’s a sleek design and minimizes
the surface area of the water storage area,” he adds.
The fluted tank is a cylindrical
yet conical tank on a corrugated steel column. The composite elevated tank is
similar, but on a reinforced concrete column.
There is also the standpipe tank,
which is a cylindrical shape but stands at ground level.
The purpose of the elevated tanks
is threefold, Gallagher says. It comes down to pressure, capacity, and reserve.
Pressure provides pressure to the system at a constant rate, and the height of
the tank is determined by the site and pressure demands. The capacity of the
tank depends on the amount of water needed at the site.
“Multi-column and pedisphere
tanks, generally speaking, are commonly built up to one billion gallons in
capacity,” explains Gallagher. “Fluted-column and composite elevated tanks are
usually built in capacities larger than 500,000 gallons. So, depending on your
needs of pressure and capacity, plus site selection, the type of tank you’d
choose is narrowed down for you.”
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Photo: Columbian TecTank
Where the tank is located needs to be technically sound, as every place in the country will have its own requirements for a foundation. |
Also taken into account when
selecting the style of elevated tank are soil conditions and environmental
loading. Each style of tank has unique characteristics in terms of distributing
the loads to the foundation system from the capacity of the tank, as well as the
potential for horizontal loading events, such as wind and seismic activity.
Life expectancy and the
flexibility of the steel plate are two of steel’s major selling points. “The
steel plate can be fabricated into almost any shape and size, including
ornamental,” says Gallagher. “A lot of towns have ornamental features specific
to their community, because steel water towers are highly visible water
towers.”
Maintenance of steel tanks is
critical. The American Waterworks Association recommends that steel tanks have a
routine inspection every three to five years.
“You want to have regularly
scheduled maintenance with strategic maintenance,” says Gallagher. “For
instance, the life expectancy of the coating system can be maximized with a
regular inspection and touch ups.”
A full inspection of the water
tank includes a wash out and a clean out. A thorough investigation of the
coating system is then done to make sure it is in good shape. Brockman says that
his department sends divers into the tanks every five years for a thorough
inspection, and, in addition to the coating, paint, and corrosion issues, he
also looks for physical damage to the tank.
“A gunshot can cause a lot of
damage to a tank,” he says, adding that people shooting at steel water tanks is
a relatively common form of damage.
Construction of the steel tanks
takes anywhere from six months to two years, depending on the size and style of
the tank.
There are actually two types of
concrete: reinforced concrete and prestressed. Reinforced contains rebar.
Prestressed concrete is made from a method of compressing the concrete after it
is poured and works best for large loads, like water. With prestressing, you use
less concrete and less steel to contain the same loads. Prestressing also helps
to eliminate cracking.
One of the key benefits of
prestressed concrete storage tanks is their longevity.
“Once you put it in, it’s in, and
it doesn’t require any structural maintenance, says Galit Ryan, vice president
of sales and marketing with DYK Inc.
Another benefit to concrete is
that it requires no coating or special treatment in order to hold water. “What
goes into the design and construction is you build the tank, fill it with water,
and you’re basically done,” says Ryan.
Ryan says concrete tanks can
easily last 100 or more years. Her company, for example, has been in business
for over 40 years, and she points out that the concrete tanks constructed then
are holding up well. “Design requirements are more stringent today,” she says.
“Codes have evolved over time. We see how well our tanks from 40 years ago hold
up, so we’re pretty confident that today’s tanks will last.”
The soil underneath the concrete tank needs to be considered from the very beginning, Ryan says, because the
foundation lends itself to the design of the tank. That foundation will dictate the kinds of footing and floor that is needed. The concrete is then poured at
the job site.
The concrete tanks can be designed
and built for almost any size. “We’ve built 100,000-gallon tanks to
40-million-gallon tanks,” says Ryan.
The average, she adds, is usually
in the 2-million-gallon size. Smaller projects take a couple of months to
construct; large tanks can take upwards of 18 months.
Prestressed concrete tanks tend to
be one of the most expensive options, Ryan admits, but only at the time of
purchase. She is quick to point out that concrete usually just costs once.
Coating is vital to steel water
tanks, but it can also be used in concrete tanks.
“Water is corrosive,” says Doug
Hansen of Tnemec, a company that is dedicated to protecting water storage
systems. “An interior tank lining is necessary to protect the steel
surfaces.”
This coating must pass strict
standards from the National Sanitation Foundation (NSF) so the coating doesn’t
affect drinking water.
There is also coating for the
exterior of the tanks, which is meant to protect the outside of the tank, but
can also be dressed up to improve the aesthetics of the tank. “We’ve sold
coating to look like a watermelon or a peach,” says Hansen.
But it also has to be a coating
that stands up to the elements, so it does not have to be painted too
frequently.
A standard interior coating is an
organic zinc-rich primer with an epoxy topcoat. On the exterior, again, an
organic zinc-rich primer is used with anything from polyurethane to polymers.
“The cost and performance of the systems is based on longevity,” adds Hansen.
The higher cost and performance require less maintenance.
He says that in some areas,
particularly in the South, coating is used for concrete tanks. “Concrete is like
steel,” he notes. “You want to protect the concrete for longevity.”
A good coating, with regular
inspection and touch ups, can last up to 25 years.
Fiberglass tanks came about as an
alternative for steel tanks to store petroleum, says David Heiman of Containment
Solutions, but over the past decade, he says the “new frontier for fiberglass”
has been in water and wastewater storage.
“With the growth of population,
there comes a shortage of water, and that increases the need for water storage,”
he adds.
Heiman says fiberglass storage
tanks are most popular in rural areas, although there is a market for fiberglass
in city areas. “These are areas that are away from city municipalities,” he
says. “We put one of our high-volume, high-capacity tanks in the ground and
serve the town’s needs.”
There is a certain engineering
that is required for a fiberglass tank. “It almost looks like a Tylenol pill
with ribs around it, and the ribs are for integrity,” explains Heiman.
“Fiberglass is not as strong as steel, but it is very strong.”
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Photo: Containment Solutions
A well-built, well-maintained water storage tank can last upwards of 100 years. |
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Photo: DYK Incorporated
In certain situations, above ground tanks are a good choice. |
The ribs increase the structural
nature of the tank, keeping it from collapsing. “It would be like putting a huge
piece of plywood on top of two two-by-fours,” he continues. “If you stand in the
middle, it would bow. But if you put another piece in the middle, it becomes
stronger. The ribs are a similar idea; we put ribs every so often in the tank,
and the weight can be distributed evenly throughout the tank.”
Because it is not as tough as
steel or concrete, fiberglass needs careful installation. “Being installed
improperly can lead to some failures,” says Heiman.
Due to the ribs on the tank,
backfill material needs to be used for the installation. “The tank has to sit on
the right materials, and, if it is, it will last practically forever,” he
says.
The important part, continues
Heiman, is making sure to use rocks rather than soil for the backfill. “That’s
more of a compaction issue,” he says. “Dirt won’t properly fill the hole.”
Fiberglass tanks run from 500
gallons to 50,000 gallons, although they could possibly go larger. Heiman says
going smaller than 500 gallons isn’t cost effective for his company, because
that moves into the territory of plastic tanks, and he admits that most buyers
of small tanks would opt for plastic, because it is much cheaper than
fiberglass.
One of the plus sides to
fiberglass is that it is easy to add on to the tank in the future. “Fiberglass
can be grinded down, and we can attach things after the fact,” says Heiman.
However, expense can be a major
sticking point when considering fiberglass. “It’s not sold on being cheap,” he
says. “We’re not the Wal-Mart of the industry. It’s sold on quality and long
term.”
Unlike the other types of tank
materials, plastic water storage tanks are small and used primarily in the
residential applications. These tanks are 2,000 gallons-and-under in size, and
are designed for underground installation.
One of the major pluses to
plastic, says Joseph Brown of Roth Global Plastic, is that it is resistant to
corrosion or chemicals. “What breaks down plastic is sunlight, and since our
tanks are meant to be underground, there is an inherent protection when
installed,” says Brown.
The tanks are also lightweight and
easy to transport, and they provide a clean environment for potable water. “They
are also relatively easy to install, because you don’t need the heavy equipment
like you would for concrete,” says Brown.
Installation involves excavation
to create a vault for the tank and a bedding material like sand or fine gravel.
Backfilling needs to be done in layers, 8 inches at a time, to create an
envelope around the tank.
Plastic water storage tanks are
often used to harvest rainwater at private homes for irrigation purposes. “The
South and Mid-Atlantic regions have been under droughts, so we’ve seen an
increase of homeowners who want to catch their rainwater to save for a later
date,” says Brown. This is done through a filtration system through the home’s
gutters.
Thornton says when determining the
size of the tank, the most important decision is making sure it is large enough
for fire flow conditions.
“That’s going to provide the
largest quantity of sizing the tank,” he says. The fire flow, he adds, is
mandatory. The National Fire Code, perhaps amended with local code, will dictate
the amount of water needed.
Operational flow conditions, which
is meant to meet the peak water use times in any particular community, is
another consideration. That’s an additional amount of water over and beyond the
water stored for fire flow. And usually there is a third category, which is
emergency storage in case the regular supply is cut off.
“Those are the issues that define
the size of your tank,” says Thornton. “Once you do that, then you need to
determine the location for it.”
The type of location Brockman
looks for has elevation to it, and that has to do with the type of water system
he uses to move the water. There are two kinds of systems, Brockman says: a
pressure system that works like a motor home—you turn the tap on, and a pump
comes on and keeps the pressure up—or a gravity system where you take the water
and boost it to a higher elevation, and the differential in the elevation
creates the pressure in the pipe.
“In California, the criteria we
use is to make sure the source of supply is 100 feet above the first point of
service,” says Brockman. “The reason for that is that it creates about 45 pounds
of pressure. And where you want that to shut off is at about 80 to 90 pounds at
the bottom end.”
Thornton agrees with Brockman that
elevating the tank is the best option, but adds, “If you are out in the plains
of Nebraska, for example, that may be difficult. So you have to go through an
exercise to determine the optimum site that fits into the hydraulics of your
water system.”
Where the tank is located also
needs to be technically sound, and every place in the country will have its own
requirements for a foundation. “In other words, you aren’t going to put a tank
on an earthquake fault or an historical landslide,” says Thornton.
There are also environmental
issues to consider. For example, there may be animal or plant habitats that need
to be addressed. “That’s one of the first steps to take care of when picking a
site,” he adds.
The site then needs to be
prepared. Thornton says the site should be place in the cut of a hill, as
opposed to put on fill.
“The site needs to be big enough
for construction,” he says. “There is a lot of large equipment involved. If it
is a steel tank, you need room to store the steel plates and a place for the
crane to come in.”
Another issue to consider is
whether to put the tank above ground (at grade), or below ground (buried). There
is also a distinction between at grade and elevated tanks. “Reservoirs and
standpipes are above ground, but are called ground storage tanks, and the column
tanks are elevated,” explains Gallagher.
Steel tanks are rarely put below
ground, because of the risk of corrosion; concrete tanks, on the other hand, are
commonly put below ground.
“Whether it is above ground or
below ground often has to do with aesthetics,” says Thornton.
Some communities think of elevated
storage tanks as a NIMBY (“not-in-my-backyard”) issue; other communities think
of how to best utilize the land itself. It is not unusual for an urban area to
put the water storage system under ground, and then use the land above for
playgrounds or tennis courts.
Brockman adds that placing a tank
under ground makes it less susceptible to earthquakes. In desert areas, the
extreme heat has to be taken into consideration when planning for a new tank, as
well as the hot/cold temperatures in the Northeast and Midwest.
Building for earthquakes is a
topic that was repeated frequently by the tank manufacturers. After all, seismic
activity doesn’t happen only in California, but is a consideration in the
Pacific Northwest, East Coast, and Midwest, as well. Ryan says planning for
earthquakes is something that is part of the plan and design of concrete tanks.
“It’s similar to a base isolation system,” she says. “The concept is to allow
the wall and roof to move relative to the foundation.”
Storage tanks that are to be used
for wastewater have a few considerations that are different for potable water.
While concrete tanks are extremely durable for potable water, for example,
sulfuric acid can eat away at the material, so a coating may be required.
“There is sometimes a mixing
system to process the wastewater,” says Ryan. “The design will be for a floor
that slopes so the low point will be in the middle.
Hansen says the NSF has stricter
requirements for the coating used for drinking water. “Even though the systems
might overlap, the cost for NSF approval and performance for potable water
rating will be the difference,” he says.
In fiberglass tanks, there is no
difference in the materials between one used for wastewater versus one used for
potable water. However, they do use different options and accessories on the
tank. For plastic tanks, the same basic materials are used for both potable
water and wastewater, but, for the potable tanks, Brown says his company uses an
NSF-61 food-grade resin approved by the Food and Drug Administration.
“We’re all in this together,” says
Heiman. “We’re all looking for ways to maximize environmental safety and green
building.”