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Rainwater
Harvesting: Comparing Storage Solutions
by Doug Pushard
Storage
tanks, usually the most expensive component of the rainwater harvesting
system, come in a wide variety of sizes and types. When deciding
on the type of tank to use, the main factors to consider include
where you live and your budget. When choosing the size of storage
tank or cistern, consider several variables: rainwater supply
(local precipitation), demand, projected length of dry spells
without rain, catchment surface area, aesthetics, personal preference,
and of course, your budget.
A
myriad of variations on storage tanks and cisterns have been used
over the centuries and in different geographical regions: earthenware
cisterns in India, large pottery containers in Africa, above-ground
vinyl-lined swimming pools in Hawaii, concrete or brick cisterns
in the central United States, and, in Texas and Colorado, galvanized
steel tanks and site-built stone-and-mortar cisterns.
Tanks
can be above or below ground. Factors such as soil, outside temperature
ranges, and cost should be used to determine whether a tank is
placed above or below ground. Some tanks are suited for above-ground
placement (i.e. vinyl-lined swimming pools), where others can
be used both above and below ground (i.e. polyethylene). Some
types of tanks are built to be buried (i.e. polyethylene tanks
designed for burial).
Consequently,
understanding all the information about the options available
is critical to making a good decision about the type of tank to
purchase, since it should prove to be something you live with
for a long time. Below is a general overview of the various tank
types to choose from and some of their characteristics.
 Fiberglass
Fiberglass tanks are light-weight, reasonably priced, and long
lasting. They are built in standard capacities from small 50 gallon
barrels to much larger 15,000 gallon tanks and are available in
both vertical cylinder and low-horizontal cylinder configurations.
Smaller fiberglass tanks (i.e. under 1,000 gallons) are expensive
for their size, so polyethylene might be preferred. Tanks for
potable use need to have a USDA-approved food-grade resin lining
and the tank should be opaque to inhibit algae growth. The durability
of fiberglass tanks has been thoroughly tested and proven to last
for years and can be easily repaired. The fittings on fiberglass
tanks are an integral part of the tank, eliminating one common
potential problem - leaking fittings.
Polyethylene
Polyethylene tanks are probably the most common type of tank being
sold today and are readily available at most farm, ranch supply,
and large landscape retailers. They vary greatly in size, shape,
and color, and can be used above or below ground.
 Most of the tanks stocked by farm and garden supply houses are
usually for above-ground installations. For buried installation,
specially designed and reinforced tanks are necessary to withstand
soil expansion and contraction. Polyethylene tanks are comparatively
inexpensive, lightweight, and long-lasting and are available in
capacities from small 50 gallon barrels to large 10,000 gallon
tanks. They are lighter in weight than other types of tanks, including
fiberglass, and consequently, are cheaper and easier to transport.
Polyethylene
tanks tend not retain paint well, so use pre-painted (i.e. pigmented)
tanks manufactured with opaque plastic. Black and dark colored
tanks will absorb heat and thus, should be shaded or buried. The
fittings of these tanks are aftermarket modifications and are
easy to plumb. However, the fittings are not always tight, and
should be checked for leakage occasionally.
 In-ground
polyethylene
In-ground polyethylene tanks are more costly for two reasons:
the cost of excavation and the cost of a more heavily reinforced
tank.
The
latter is required if the
tank is to be buried more than two feet deep. Burying a tank in
soil with high clay content is not recommended because of the
expansion and contraction cycles of clay.
For
below ground installation, the walls of poly tanks must be manufactured
thicker, and sometimes, an interior bracing structure must be
added.
Swimming
Pools
Above ground swimming pools are commonly used in Hawaii for rainwater
catchment; and although used throughout the islands, they are
not a good rainwater catchment solution. A few of the issues related
to these pools as a storage device are: 1) The pool liner typically
only goes up to the lip of the pool and slips into the pool. 2)
Liners are not food grade quality and may in fact be made to retard
bacterial growth in the pool and can be toxic. 3) Large diameters
of these pools are not easily covered and therefore it is difficult
to keep out debris and animals.
However, swimming pools are very inexpensive, readily available
and an easy to install solution. If this is the only option available
then take the following precautions: 1) Make sure the liner is
approved by the Food and Drug Administration (i.e. not the standard
liner but one made for storing and holding water). 2) Plan to
have a support system (i.e. a post in the middle of the pool)
to hold up the cover so it does not sag into the water. 3) Make
sure the cover can be firmly attached to the top of the pool to
keep out unwanted guests and debris.
 Wood
For personal aesthetic appeal, a wood tank are sometimes a desirable
choice. Wood tanks, similar to water towers at railroad depots,
were historically made of redwood. However, modern wood tanks
are usually of pine, cedar, or cypress wrapped with steel tension
cables. Wood tanks are lined with plastic to increase longevity.
For potable use, a food-grade liner should be used.
Redwood,
as a tank material, has a great reputation for being durable and
attractive. It contains no resins and has high levels of tannin,
a natural preservative resistant to insects and decay. It is also
a good insulator, keeping the water cooler in the summer and protects
it from freezing the winter.
These
tanks are available in capacities from small 700 gallon tanks
to very large 37,000 gallon tanks, and are usually site-built
by skilled technicians. As with metal tanks, they can be dismantled
and moved to a different location, if required. These tanks are
for above-ground use and not for use in dry, hot climates.
Redwood
is very attractive, but it is expensive and not readily available.
Pine is commonly used and although it does not have some of the
characteristics of redwood, it is readily available and less expensive.
Metal
As with wood, galvanized sheet metal tanks can also be an attractive
option. They are available in sizes that range from small tanks
of 150 gallons to medium-sized 2,500 gallon tanks, and are lightweight
and easy to relocate if required.
 Most
tanks are corrugated galvanized steel dipped in hot zinc to improve
corrosion resistance. These tanks should be lined with a food-grade
liner, usually polyethylene or PVC, or coated on the inside with
epoxy paint. The paint or liner will extend the life of the metal
and, if being used for potable water, must be FDA and NSF approved
for potability.
These
tanks are for above-ground use. Old or recycled tanks may contain
lead and should be avoided. Brass and bronze fittings should not
be connected directly to the tank as they will cause corrosion.
Additionally, care should be taken when cleaning these tanks,
as a film develops naturally on the inside of the tank, which
coats the tank and inhibits corrosion.
 Concrete
The most versatile of tanks, concrete tanks can either be poured
in place or prefabricated. They can be constructed above or below
ground. They can be owner or contractor-built. Poured-in-place
tanks can be very attractive and easily integrated into new construction.
For example, the tank can be placed under a patio, or a basement.
Concrete tanks, once poured, are considered permanent.
One
unique advantage of poured concrete is that the concrete will
over time decrease the corrosiveness of rainwater by leaching
into the water. This advantage of concrete tanks results in a
desirable taste imparted to the water by calcium in the concrete
being dissolved in locations where there is slightly acidic rainwater.
For potable systems, it is essential that the interior of the
tank be plastered with a high-quality material approved for potable
use.
Underground
concrete tanks are prone to cracking and leaking, especially
when in clay soil. Leaks can be easily repaired, although
the tank may need to be drained to make the repair. If building
the tank yourself, it is recommended to involve the expertise
of a structural engineer to determine the size and spacing of
reinforcing steel to match the structural loads of a poured-in-place
concrete. A product that repairs leaks in concrete tanks, Xypex(TM),
is now available and approved for potable use.
Ferrocement
Ferrocement is the term used to describe a steel and mortar composite
material. These tanks can be above or below ground and can be
done by contractors or homeowners. They were developed in third-world
countries to be relatively low-cost and durable. They are listed
separately from concrete, not just because of the materials used
to construct them, but also because they have differing problems
and advantages.
These
tanks are typically built with concrete, but have multiple layers
of wire mesh - typically chicken wire-wrapped around a light framework
of rebar, embedded in the concrete. Walls can be as thin as 1"
and still be strong. Consequently, it can cost less to build than
a concrete-only tank. If buying a ferrocement tank, make sure
it does not contain any toxic compounds in the concrete and that
the wires are not visible on the inside of the tank.
Ferrocement,
like concrete, will need maintenance and repair as cracks appear.
It is important to ensure that the ferrocement mix does not contain
any toxic components. Some sources recommend painting above-ground
tanks white to reflect the sun's rays, reduce evaporation, and
keep the water cool.
 Stone
and Mason
Hand made stone or mason tanks are not as common as they once
were. Increasing labor costs, decreasing costs and increasing
availability of other types of tanks has limited their use to
areas where labor is very, very cheap (i.e. third-world countries)
or where budget is not an issue.
The
mass of the stone gives these tanks two distinct advantages: it
keeps the water cool in hot climates and they can be very attractive.
As with ferrocement tanks, care should be taken to make sure the
mix does not contain toxic materials.
These
tanks are custom-built, so they can be as large as designed. Most
tanks are designed to be circular, since this shape is more structurally
sound. These tanks, if properly constructed and maintained, will
last for decades.
Plastered
Tire Cistern
Another
type of hand-made tanks are plastered tire cisterns. They were
pioneered with the earthships in the Taos, New Mexico area and
some have been in use for decades. They are simply a circle of
buried tires with a wire mesh inside covered with plaster. Just
like stone or cement cisterns, they will need periodic maintenance
to repair cracks on the inside.
 These
tanks are meant to be buried and can be very economical for large
tank sizes (i.e., 10,000+ gallons/37,900 liters), especially if
owner-built. In earthships, tanks are typically built as an integral
part of the home and can provide cooling in hot climates. In cooler
climates, the tops and sides should be insulated to prevent cooling.
Care should be taken in building a plastered tie cistern to ensure
the wire mesh and tires are thoroughly covered with plaster.
As
with stone and mason cisterns, these tanks are custom-built, so
they can be as large as designed. They are typically designed
to be circular, but since they are buried, they can be almost
any shape. These tanks, if properly constructed and maintained,
will last for decades.
Summary
A
summary of cistern materials is below. Keep in mind that the tank
is one of the most important components of the system, typically
being the most expensive and the most permanent. Prices on each
type of tank can vary widely depending on your locale, local labor
costs, and the price of raw materials. Check with distributors
and other rainwater harvesters in your area prior to making your
final choice.
|
|
Expected
Life
|
Availability
|
Transportability
|
Expected
Maint.
|
Build
Your Own
|
|
Fiberglass
|
++
|
++
|
++
|
--
|
---
|
|
polyethylene
|
++
|
+++
|
+++
|
--
|
---
|
|
Below
Ground polyethylene
|
+++
|
++
|
+++
|
--
|
---
|
|
Cement
|
++
|
+
|
---
|
++
|
+
|
|
Ferrocement
|
++
|
+
|
---
|
++
|
++
|
| Plastered
Tires |
+++ |
-
|
---
|
++
|
++
|
|
Stone
|
+++
|
-
|
---
|
+
|
+
|
|
Wood
|
+
|
-
|
++
|
+
|
-
|
--
Low/No
+ High/Yes
General
Guidelines
General
guidelines for all tanks include:
- Before
you start, conserve, conserve, conserve. Cutting your water
usage will reduce the size of tank you need and save you money.
- Remember
- water is very heavy. (i.e., 500 gallons weighs over 2 tons!)
- Make
sure the tank is easy to access and maintain.
- Tank
should be opaque or darker, either upon purchase or painted
later, to inhibit algae growth.
- For
potable systems, storage tanks must never have been used to
store toxic materials.
- Tanks
must be covered and vents screened to discourage mosquito breeding.
- Tanks
used for potable systems must be accessible for cleaning.
- Install
first-flush and screening devices prior to water reaching the
tanks to keep it as fresh and clean as possible.
- Keep
tops of tanks free of debris to make it harder for animals to
reach the top of the tank.
- Buried
tanks should be located in well-drained soil and location.
- Water
weighs about 8 pounds per gallon so plan your pad, if any, before
installing your tank.
- Plan
where storage tank overflow should be piped or directed to.
Keep it away from underneath your holding tank to prevent pad
erosion and to keep animals away.
Related Resources:
List of tank
providers by state - HarvestH2o
NSF
on Rainwater Collection
Information
on Plastered Tire Tanks
Water Storage Tanks, Cisterns, Aquifers, and Ponds
Pictures of Building a Ferrocement Tank
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