TECHNICAL PAPER #29
UNDERSTANDING POTABLE
WATER STORAGE
By
Charles M. Ritter
Technical Reviewers
Philip Jones
Irving Starobin
VITA
1600 Wilson Boulevard, Suite 500
Arlington, Virginia 22209 USA
Tel:
703/276-1800 . Fax: 703/243-1865
Internet: pr-info@vita.org
Understanding Potable Water Storage
ISBN: 0-86619-238-7
[C]1985, Volunteers in Technical Assitance
PREFACE
This paper is one of a series published by Volunteers in
Technical
Assistance to provide an introduction to specific
state-of-the-art
technologies of interest to people in developing countries.
The papers are intended to be used as guidelines to help
people choose technologies that are suitable to their
situations.
They are not intended to provide construction or
implementation
details. People are
urged to contact VITA or a similar organization
for further information and technical assistance if they
find that a particular technology seems to meet their needs.
The papers in the series were written, reviewed, and
illustrated
almost entirely by VITA Volunteer technical experts on a
purely
voluntary basis.
Some 500 volunteers were involved in the production
of the first 100 titles issued, contributing approximately
5,000 hours of their time.
VITA staff included Maria Giannuzzi
as editor, Suzanne Brooks handling typesetting and layout,
and
Margaret Crouch as project manager.
The author of this paper, VITA Volunteer Charles M. Ritter,
is a
project engineer with an engineering consulting firm in
Wheat
Ridge, Colorado. Mr.
Ritter specializes in potable water treatment
and distribution and wastewater disposal.
The reviewers are
also VITA Volunteers.
Philip Jones has 15 years experience as a
civil engineer working on water and sanitation
projects. He has
spent seven years working in East Africa and is presently a
consultant based in Washington, D.C., specializing in
environmental
engineering for developing countries.
Irving Starobin is a
chemical engineer, specializing in plastics, who has worked
as a
consultant for UNIDO and has experience in Asia, Europe, and
South America.
VITA is a private, nonprofit organization that supports
people
working on technical problems in developing countries.
VITA offers
information and assistance aimed at helping individuals and
groups to select and implement technologies appropriate to
their
situations. VITA
maintains an international Inquiry Service, a
specialized documentation center, and a computerized roster
of
volunteer technical consultants; manages long-term field
projects;
and publishes a variety of technical manuals and papers.
POTABLE WATER STORAGE
by
VITA Volunteer Charles M. Ritter
I. INTRODUCTION
BASIC THEORY AND APPLICATIONS
Put simply, potable water is drinking water.
Conversely, water
that is not drinkable is termed non-potable.
Water engineers use
the terms in various ways.
The term "potable water supply" can
refer to water in a reservoir or river, for instance, that
may be
too contaminated to drink as is, but which will be treated
to
make it drinkable.
It can also refer to the same water after it
has been treated, or to other water such as from some
bore-holes
and springs, which is naturally pure and does not require
treatment.
Treated or naturally pure water is a scarce and valuable
commodity. Because
of this, it is usually only produced in
amounts necessary to satisfy short-term (i.e., over the next
few
hours or a day) demand, and so particular care is taken to
ensure
that it does not become contaminated.
The term "potable water
storage" refers particularly to storage of this
water. The word
`potable' in this report therefore refers only to water that
is
considered fit to drink.
It may have a wider meaning in other
contexts.
In contrast, the storage of untreated (raw), possibly
contaminated
water is not usually subject to the same standards of
protection, although reasonable precautions should always be
taken to prevent access, for instance, by animals for
drinking or
by humans for washing, to a raw water storage dam that might
hold
several months supply.
The amount of potable water available from a supply source
may
not always be adequate to satisfy demand at a particular
point in
time. Therefore, it
is frequently necessary to hold a sufficient
quantity of water in storage, to be withdrawn during periods
when
consumption exceeds incoming supply.
In addition to supplying water during periods of shortage,
water
storage reservoirs perform other beneficial functions: (1)
maintaining
relatively constant water pressures in the distribution
system; or allowing pumps and treatment processes to run at
constant flow while demand varies; (2) alleviating the need
for
pumps to run continuously; and (3) improving system
reliability.
It should be noted that potable water storage facilities are
not
always needed--or desirable.
If the raw water source is itself
drinkable, plentiful, and readily accessible, there is no
advantage to be gained in extracting more than is required,
and
storing it. For
example, an abundant spring to which people come
to collect water, or a well fitted with a hand pump, does
not
require storage. In
fact, the slight-delay of waiting in line
may be highly preferable to risk of contamination that would
accompany the installation of a small well-head storage
tank.
Potable water storage is needed, however, if the rate at
which
water can be extracted from the source varies greatly from
the
rate at which it is consumed.
Pumps, treatment processes, and
the carrying capacity of pipework are most efficient and
simple
to operate when working with a constant flow of water.
Thus, it
is good practice for raw water handling to be based on a
constant
average flow, and for excess potable water to be stored
during
periods of low demand, such as at night, to augment flows
during
periods of high demand, such as in the morning and
evening. In
this way, demand is evened out: storage used for this
purpose is
also referred to as balancing or equalizing storage.
If pumps
and treatment plants can only be operated for part of a day,
say
during daylight, or for one operator shift, then storage is
required to maintain a supply at other times.
Some extra storage
may be provided to maintain a contingency supply in case of
a
breakdown. However,
this should be considered as a short-term
emergency supply only; in dealing with breakdowns, the main
objective should be to repair the system quickly.
One special case, requiring the longer-term storage of
potable
water, is that of rain water, dealt with later.
Typically, potable water is stored after all treatment and
pumping processes have been completed, usually at a point
close
to or within the distribution system, and at an elevation
above
the highest point of discharge.
Thus, the stored water can
continue to flow to consumers by gravity even if there is a
breakdown in the treatment or pumping plant.
The actual location
of the storage will often be obvious, that is, on the
nearest
piece of high ground within or next to the consuming
area. Where
alternatives exist, the location may be governed by the
location
of the pump or intake (so as to avoid a long pumping main),
by
the location of the high use area within the community and
by the
the layout and type of distribution system being built.
Figure 2A
upw2ax4.gif (300x600)
shows the scheme when pumping directly into a storage
tank. Figure 2B
upw2bx4.gif (600x600)
demonstrates when the tank is filled from a more elevated
source, the water then flowing by gravity through a separate
outlet
to the distribution system.
Figure 3 illustrates the situation
upw3x4.gif (486x486)
when water is to be pumped into the distribution system and
allowed to overflow into storage.
In the latter case, some
advantage may be gained by locating storage near the high
usage
area on the opposite side from which the supply enters the
community.
There is no rule as to which system is best; each has
advantages and disadvantages and is more a matter of distribution
design, local engineering practice, and possibly legal
requirements.
Obviously, the second system cannot be used if only
intermittent pumping is done.
A shallow tank with a relatively large areal expanse is
preferred
to a deeper, narrower one.
In piped distribution systems, locating
distribution storage near the high usage area on the
opposite
side from which the supply line enters the community, has
the
advantage of satisfying peak demands through smaller-sized
mains,
and with the lowest pressure loss.
The tank site should be high
enough that maximum head losses can be overcome as water
flows
toward the point of demand, hopefully arriving there with an
adequate positive pressure.
However, a higher tank means a
greater energy requirement for pumping.
In addition, a higher
tank will require higher pressure rated and more expensive
pipe.
It will also result in an increased water loss from leakage.
Therefore, it is important to place the tank at the right
height.
Depending upon the pressure rating of pipes within the
distribution
network, the vertical distance between storage and the low
point in the community should usually not result in a
sustained
pressure greater than 100 pounds per square inch (psi) when
the
system is at rest, i.e., static head and pumping head at
no-flow.
The practice in the United States has been to ensure a
residual pressure of at least 20 psi when the maximum
demand,
i.e., for fighting fires, is applied at critical spots in
the
service area. For
all purposes except firefighting, a small
system such as a school, mission, or hospital complex can
easily
make do with no more than three meters residual head.
If insufficient elevation difference exists in the terrain
surrounding
a community, construction of an elevated tank or standpipe
that is taller than it is wide may be necessary.
Globes,
cylinders, and rectangles are some of the variations of this
type
of storage structure.
The outsides of above-ground tanks and
pipes are subject to weathering.
If suitably situated high ground is not available, the tank
may
be elevated on a tower.
Specialist advice should be sought if
the area is subject to earthquakes or strong winds.
Below-ground
tanks should be constructed above the water table as well as
any
sewage disposal systems in the area.
The minimum lateral separation
between water storage reservoirs and sewage disposal
facilities
should be about 30 meters.
To provide good drainage, sur-
VOLUME REQUIREMENTS
One of the primary functions of a water storage structure is
to
provide a reserve capacity that can be utilized to meet
demand
when the normal supply rate is inadequate.
It is, therefore,
necessary to set forth some guidelines for determining how
large
the storage capacity should be.
Per unit water demand is generally used as the basis for
sizing a
tank. For instance,
the average per person consumption multiplied
by total number of people in a community yields one estimate
of
storage volume requirement when halved.
Another recommended method
is to multiply the average daily use by an appropriate
peaking
factor to yield the maximum demand over a one-day period,
and
subtract daily water production (i.e., treatment)
capacity. In
smaller communities, elevated storage should be at least
equal to
one (and preferably two or three) days' requirement during
hot,
dry periods.
A more precise method of calculating volume requirements is
to
construct a mass diagram of water usage on the peak day, and
draw
parallel pump supply lines tangent to the demand curve at
its
most divergent points(*) (see Figure 1).
For this example, the
upw1x3.gif (600x600)
reservoir is assumed to be filling when demand is low, and
emptying
when demand exceeds the production capacity.
It may be necessary
to make some assumptions about the demand curve
configuration.
The practice in the United States is generally to provide a
fire fighting reserve over and above the peak demand
equalization
storage volume.
II. VARIATIONS IN
TECHNOLOGY
CONSTRUCTION MATERIALS
Through the centuries, a variety of construction materials
have
been utilized as the basic element(s) in water storage
structures. Small
reservoirs formed by earthen embankments have
supplied poor quality water to villages in India during the
long
dry season. In
western Sudan, the hollowed-out trunk of the
baobab tree is employed to retain water collected during the
short rainy season.
Brick, masonry, and concrete may be the most
popular materials currently utilized.
These and other modern-day
tank construction materials are discussed below.
---------------------
(*) This method is discussed in greater detail in
Environmental Engineering
and Sanitation by J.A. Salvato, Jr. (see Bibliography).
Foundation pressure under a tank up to about 3 meters deep
is not
very great, and provided that about a foot of topsoil is
removed
(0.3 meters) or maybe .6 meters in soft ground, then no
problems
would be found.
Solid rock is obviously a good foundation.
A
mixture of rock and soil is not good, as the soil will
settle
slightly, but the rock will not, resulting in a cracked
floor and
worse.
Elevated tanks have their weight concentrated over a small
area,
and extra care is then needed to choose a good firm
site. As a
rough guide, if you can park a truck overnight at a site,
and see
only a slight or no dent where the back wheel lay, then the
ground bearing pressure is sufficient.
Typical size ranges suitable for various types of materials
include:
o
ferrocement, less than 1 cubic meter
o
masonry up to about 20 cubic meters
o
reinforced concrete, almost any size, but
hardly worth
the effort
for less than about 5 cubic meters
o
round corrugated galvanized steel, up to 2
cubic meters
o
bolted sectional steel, fiberglass panels, I
cubic
meter up to
almost any size
o
welded steel, 20 cubic meters and up
Brick or Stone Masonry
Hard, dense material such as brick or stone masonry should
be
laid with full cement mortar joints.
Pressed and dried bricks
formed from laterite soils have a soundness comparable to
limesandstone
bricks. A
2-centimeter layer of rich cement mortar
applied to the inside face will render the structure
watertight.
A tank more than about a meter deep may need circumferential
steel reinforcement, which can be laid in the horizontal
mortar
joints, or buttresses spaced around the outside.
The mortar
lining must be carefully cured, like the concrete, by being
kept
damp for several days to a week, otherwise it will
crack. As a
tank fills and empties, and as temperature differences occur
from
night to day, masonry and concrete will expand and contract.
Sliding joints between floor, walls, and roof may be
necessary.
Concrete
Water storage structures made of concrete require internal
steel
reinforcing to provide tensile strength.
The two primary design
methods revolve around (1) working stress, based on the
British
experience, and (2) ultimate strength design (limiting crack
width). (*)
A dense, durable, and impermeable material that will not
erode,
crack, or otherwise permit water leakage that could cause
contamination
of the stored water, or corrosion of the interior
steel is necessary.
Watertightness of the finished structure is
enhanced by a low water-cement ratio (0.45 maximum) within
the
limits of reasonable workability.
A continuous waterstop made of
polyvinyl chloride (PVC) or rubber is cast in the concrete
at all
breaks or joints to prevent the passage of water through
them.
The new concrete should be kept wet and allowed adequate
time for
curing before being placed in service.
Post-tensioned, pre-stressed
concrete is generally not cost-effective unless the tank
is very large.
Building a watertight structure out of concrete is not easy,
and
a mortar-lined masonry tank is usually more successful.
Concrete
work requires a supply of formwork (molds to form the
concrete
shape), which is usually made of wood sheet or planks.
This can
be expensive and requires good carpenters to make waterproof
joints, otherwise the concrete will not be watertight.
Steel
must be accurately fixed inside the formwork.
Altogether, it
requires a more skilled work force than masonry.
A concrete floor slab is relatively easy to build.
The walls,
especially if curved--as they must be for adequate
strength--are
the most difficult part.
A flat slab roof requires support-formwork
that must be strong enough not to move at all during
construction
and subsequent curing, but otherwise is fairly easy to
build. Most
masonry-walled tanks have concrete floors and roofs.
----------------------
(*)Appropriate design procedures are detailed in the
Handbook of
Concrete Engineering and Concrete Sanitary Engineering
Structures
published by the American Concrete Institute and Design of
Liquid-Retaining Concrete Structures by R.D. Anchor; also,
the
April 1981 issue of Concrete International published by the
American
Concrete Institute was devoted entirely to this subject (see
Bibliography).
Ferro-cement
These containers are being built more and more in developing
countries, especially in India.
The technique involves applying a
sand and cement mortar mixture over a framework of steel
rods,
mesh, pipe, chicken wire, etc., to form a lightweight,
watertight
structure. There is
no need for complicated and expensive form-work,
and thin-walled flexible ferro-cement is advantageous in
curved structures such as circular or conical tanks.
Earthen Basins with Impermeable Liners
Plastic film or thin concrete liners can be used to make
earthen
reservoirs watertight.
However, plastic film is very easily torn
or punctured. The
embankments are subject to some natural hazards
such as erosion.
All systems using a flexible membrane should be designed so
as
not to fail structurally if the liner is punctured, and
drains
must be installed if ground water under the liner is a
problem.
For cases where a separate liner is not installed, various
methods of compacting suitable soils or seeding with
bentonite or
chemicals can be employed to improve the soil's water
retaining
characteristics. (*)
Care should be taken to prevent scouring by
water of soil liners around the inlet pipe.
A clay liner can be
protected from drying out with 2.5 meter layer of sand or
gravel.
The disadvantages of an uncovered reservoir described in the
section on `Water Quality Considerations' can be overcome by
spanning the basin with a reinforced concrete slab or
corrugated
metal roof. Other
types of covers or methods of evaporation
control include: (1)
reinforced synthetic rubber supported on
foam floats, (2) polyethylene sheets, and (3) ultra-thin
layers
of long-chain alcohols.
The alcohols are, however, subject to
dispersion by wind and waves.
One variation of the earthen basin is that, instead of being
uncovered, the basin is filled with uniformly sized sand and
acts
as an artificial aquifer (water-bearing formation).
Water still
occupies between 30 and 40 percent of the volume of the
basin,
and purification takes place as the liquid filters through
the
sand. A gravel mulch
layer on top of the sand enhances the
operation of the artificial aquifer by improving percolation
of
rainwater (recharge characteristics) and suppressing
evaporation.
---------------------
(*) See Methods of Creating Low-Cost Waterproof Membranes
for Use in
the Construction of Rainwater Catchment and Storage Systems
by D.
Maddocks.
Smaller artificial aquifers storing less than 25,000 gallons
are
probably easier to design and construct.
To prevent
contamination, such a system must be managed carefully, or
it is
likely to be used for irrigation or stock watering.
Steel Tanks
Several types of steel tanks are available.
For small volumes, 1
cubic meter or so, round corrugated steel or square sheet
steel
(often used as internal roof tanks) tanks, galvanized and
with a
cover may be used.
These are often available from stock at
builders suppliers.
For larger volumes up to several hundred cubic meters, steel
tanks are usually prefabricated in a factory, transported in
sections and erected on site.
The segments are welded or bolted
together; this works best if it is done by the supplier as
part
of his duties in case it subsequently leaks.
Welded tanks are
often circular or have more complicated shapes.
They require an
experienced construction crew and skilled welders for a
successful
job. Bolted segment
tanks can be erected by an experienced
crew under the direction of an experienced foreman who can
usually
be provided by the manufacturer.
Although the cost of steel
tanks may appear high, they can often be transported in one
truckload and become competitive when transport costs are
considered.
They come in standard size increments, and can be
arranged to fit almost any requirement.
They are relatively easy
to construct as elevated tanks, either on a steel tower
supplied
as part of the arrangement, or on masonry pillars or walls.
Steel tanks tend to corrode, especially if storing rainwater
or
slightly saline water, or if subject to a salty atmosphere
or
sand-laden winds that wear away paintwork.
A chemist or
competent water engineer can advise on how corrosive your
water
is likely to be.
Simple precautions, such as raising the tank a
few centimeters off damp ground, careful choice of metal
fittings
and careful installation, and painting the inside and
outside can
significantly lengthen tank life. (*)
Silica glass-coated metal panels that are bolted together
circumvent
the periodic maintenance requirements.
These structures are
not suitable for placement below the ground, however.
----------------------
(*)The publication "AWWA Standard D-100-79 for Welded
Steel Water
Storage Tanks," issued by the American Water Works
Association in
1979, sets forth the requirements for welded steel tanks
(see
Steel grain storage bins have been converted to water tanks
using
PVC or other artificial liners.
Wood
A variety of woods, including cypress, fir, pine, and
redwood,
have been used for water storage structures.
One such commercially
available tank is made of staves with tongue-and-groove
joints that are held together by galvanized or
asphalt-protected
steel tension hoops around the circumference.
Like concrete, the
wooden tanks do not require special maintenance, although
their
average life span is shorter.
If wood preservations are used,
they must not contain any toxic chemicals.
Fiberglass and Plastic
Man-made materials such as fiberglass or plastic can also be
used
in the construction of water storage tanks.
However, these tanks
are usually installed only on a very small scale.
Plastic, fiberglass, and various combinations are used to
make
bolted sectional tanks similar to steel tanks.
Damaged sections
can be repaired if suitable resins and fiberglass can be
obtained, or, as with steel segment-bolted tanks, a complete
segment can be replaced.
Small plastic tanks up to about 2 cubic meters made of
polyethylene
or poly vinyl chloride are available.
They are light and
easily handled, but are also easily damaged and difficult to
repair properly.
They may become brittle if exposed to light/sun
and therefore should only be installed indoors.
Miscellaneous
When substandard construction or lack of the proper
materials
results in a tank that is not watertight, liners made from
epoxy,
vinyl, asphalt, or other materials that will resist leakage
can
be applied to the inside.
Care must be taken that any such
materials are safe for drinking water applications.
A reputable
local supplier of construction materials or the ministry
dealing
with water supply or public health should be asked for
guidance.
INDIVIDUAL CISTERNS
Cisterns are used to catch and store rainwater.
Especially in
duction of an individualized water storage technology may be
feasible. Cisterns
should be covered to reduce evaporation and
prevent entry of animals and debris.
And since water quality is
also an important consideration, it may be practical to filter
the water leaving the storage reservoir after a lengthy
detention
period. The
impermeable surface collecting precipitation (often
the roof of a house) must be kept clean, or provision made
to
bypass initial flows around the storage cistern.
Where possible,
water should be extracted from the cistern using a pump or
gravity
pipe, and not by dipping a potentially dirty container into
it.
Rainwater contains appreciable amounts of dissolved oxygen
and
carbon dioxide, which can significantly affect both taste
and
acidity (pH). It is
also comparatively corrosive to iron or
metal.
TANK ACCESSORIES
The addition of a few accessories to the basic storage
structure
will serve to make it more functional and fail-safe.
Piped air
vents are necessary to prevent pressure or vacuum buildup
within
the tank as it is filling or emptying.
These openings should be
covered with a screen material to keep insects, birds, and
other
small animals from entering the reservoir, and should always
point downwards. The
same is true for the outlet ends of drain
or overflow pipes.
These pipes should conduct water far enough
from the tank so that the tank foundation is not adversely
affected.
Installing a valve in the drain line outside the tank
will permit the discharging of the stored contents when
desired.
The drain pipe should never be connected to a sewer line.
A lockable access hatch and ladder permit entry into the
structure.
Like the vent pipe, the hatch should be raised at least one
half meter above the top of a buried tank, and 5 or 6
centimeters
above the top of a surface tank, so contaminated surface
water
flows around or underneath the opening, instead of entering
through it. A
lockable access hatch cover, and fencing around the
tank site will discourage tampering, swimming, or vandalism.
----------------------
(*)A summary of the different types of cisterns that have
been used
over the years is contained in "Cistern Based Water
Supply in
Rural Areas in Low Developed Countries" by G. Schulze
(see Bibliography).
Overflow pipes should be one size larger than the inlet, and
never fitted with a valve.
Outlet pipes raised several
centimeters off the tank floor allow the accumulation of
silt
which can be flushed out during periodic maintenance
cleanings.
WATER QUALITY CONSIDERATIONS
Water quality may be either beneficially or adversely
affected by
detention in a storage reservoir.
Turbidity is often reduced as
water passes through a basin.
This process, known as sedimentation,
could be responsible for removing significant numbers of
bacteria and other particulates.
Transmission of some parasites,
which must contact the host organism within 24 to 48 hours
to
remain viable, is effectively prevented during storage and
detention.
On the other hand, large uncovered reservoirs are
susceptible to
contamination because algae build up in the surface
layer. If the
incoming water contains a proper supply of nutrients, algae
production will be enhanced by sunlight, and solids will
accumulate
at a faster rate than sedimentation can remove them.
The
bacteriological quality is then affected because algae and
other
solids protect various pathogens from the disinfecting
chemical.
Excessive algae growth can be controlled, to some extent,
through
regular applications of copper sulfate.
However, this chemical is
not always available, and building a roof over the tank is
preferable
to avoid the problem completely.
Other potential sources of pollution that pose a greater
threat
if the reservoir is uncovered are birds, animals, insects,
humans, and windblown and atmospheric contaminants.
Moreover,
chlorine tends to dissipate faster in an uncovered
reservoir,
making maintenance of a sufficient residual impossible.
Proper construction of accessories and even the tank itself
will
reduce the potential for the introduction of contaminants
into
drinking water. For
instance, vent pipes must extend above the
flow level of any surface drainage, because it may be
contaminated
and drain lines should not be directly connected to
sewers. The completed
structure should be as watertight as
possible, and situated above any underground seepage.
Interior
liners must be non-toxic and impart no taste to the water;
this
includes all interior paints, resins, compounds used for
filling
cracks, formwork releasing agents, and any additives mixed
with
the concrete.
Tanks should be drained as often as necessary (at least once
per
year) for maintenance.
The operations technician should inspect
the interior of the tank, repair any leaks, and remove any
silt
or plant life that has collected there.
Two different procedures for disinfecting a storage tank
before
placing it in service are described in the American Water
Works
Association's Standard D-105-80 (see Bibliography).
One method
involves filling the tank with a concentrated chlorine
solution
(10 milligrams per liter) and letting it stand full for 24
hours,
after which time the disinfection water is drained as waste.
The second method is useful where water is scarce, and using
rather than discarding the chlorine solution is
desired. The
steps in this procedure are as follows:
1. Thoroughly coat
(with sprayer) interior of surfaces with a
strong solution
containing 200 milligrams per liter of
chlorine.
2. Fill drain piping
with 10 milligrams per liter chlorine solution.
3. Allow 30 minutes
of contact between all surfaces and the
chlorine
solution.
4. Permit fresh
water to enter the tank, and purge drain piping
of the
disinfection water.
5. Close drain valve
and fill tank to maximum level.
With either method, the tank's inside surfaces should be
thoroughly
cleaned and swabbed before disinfecting.
After disinfection,
the water should be tested for proper bacteriological and
aesthetic qualities to assess its suitability for public
consumption.
Because of the hazards involved in spraying the strong
chlorine solution, the workmen must be adequately protected
with
the proper clothing and breathing apparatus.
One person should
remain outside, connected by a rope to a co-worker inside
the
tank. All workers
should be free of intestional diseases.
They
should wash their boots--or feet--before entering the tank
(and
not (wash them in the tank water through the access hatch).
Different chlorine compounds and the amounts needed for preparing
a 50 milligram per liter solution are given in Table 1.
III. CHOOSING THE
TECHNOLOGY RIGHT FOR YOU
A number of factors should be considered in selecting the
most
appropriate storage structure for a particular
location. Cost is
Table 1.
Quantity of Disinfectant Required to Give
a Dose of 50 mg/1 Chlorine
Ounces of
Disinfectant/
Diameter U.S.
Gallons 10-Foot Depth
of Water
of Well, of
Water 70 Percent
25 Percent
5-1/4 Percent
Spring, per
foot Calcium
Calcium
Sodium
or Pipe of
Water Hypo-
Hypo-
Hypo-
(inches)
Depth chlorite[a]
chlorite[b]
chlorite[c]
2
0.163
0.02
0.04 0.20
4
0.65
0.06
0.17 0.80
6
1.47
0.14
0.39 1.87
8
2.61
0.25
0.70 3.33
10
4.08
0.39
1.09 5.20
12
5.88
0.56
1.57 7.46
24
23.50
2.24
6.27 30.00
36
52.88
5.02
14.10 66.80
48
94.00
9.00
25.20 120.00
60
149.00
14.00
39.20 187.00
72
211.00
20.20
56.50 269.00
96
376.00
35.70 100.00
476.00
[a] Ca(OC1), also
known as high-test calcium hypochlorite.
A
heaping
teaspoonful of calcium hypochlorite holds approximately
1/2 oz.
[b] CaC1(OC1).
[c] NA(OC1), also
known as bleach, (brand names include Chlorox,
Dazzle, etc.),
can be purchased at most supermarkets, drug,
and grocery
stores.
Source: J.A.
Salvato, Jr., Environmental Engineering and Sanitation
(New
York: Wiley-Interscience, 1972).
probably the most important consideration, because
sufficient
funds, either from a local source or foreign development
aid, are
necessary before anything of a permanent nature can be
built.
Since the unemployment rate in most developing countries is
high,
labor-intensive technologies offer certain advantages over
more
costly mechanization-based schemes.
In addition, materials used in construction should be
available
locally, whether imported from outside the country or
produced
indigenously. The
purchase of locally-derived materials may boost
a region's economy, and ensure that proper means for repair
or
replacement are available.
If foreign goods are utilized, they
should be simple, rugged, and reliable so that they will not
require much maintenance attention or repair work.
Because of the
need to inspect and paint them regularly, metal tanks are
probably
not the best solution.
Local customs and cultural effects are other important
factors to
consider. If water
has traditionally been collected by the women
at a local gathering spot, it is probably advantageous to
integrate
them into the planning, and perhaps build a large communal
system rather than individual storage cisterns.
Conversely, if
different segments of the community will not associate or work
with one another, building a large public water storage
facility
may be difficult, not to mention pointless.
This is unfortunate
in light of its advantages--the inherent economies of scale,
and
the fact that it is easier to monitor and maintain water quality
in a reservoir serving the whole community.
The choice of storage systems depends on community resources
and
needs. A well-built
concrete or masonry tank should last for at
least 20 years. A
well-maintained steel tank may last for 10
years. Some low-cost
simple but dependable technologies include:
1. Earthen basins
with impermeable liners and whatever covers
can be fashioned
over the tops of them;
2. Ferro-cement
containers constructed with a variety of possible
materials
available that will lend tensile strength to
the cement;
3. Artificial
aquifers may be the least resource-intensive,
utilizing instead
large amounts of cheap labor.
To choose the technology right for you, consider the
following
questions.
1. How much storage
do you need?
2. Where do you need
it?
3. What types of
tank would satisfy (1) and (2)?
4. Which of the
options from (3) do you have the resources to
build and
maintain?
5. From what is
left, choose the cheapest.
Having made your choice, try to find someone else who has
already
tried it, and see what advice they have to offer.
Their advice
will probably be among the best you can obtain, but if they
have
any unsolved problems, VITA may be able to offer a solution.
Attention to the points raised in this report, together with
a
more detailed investigation of your chosen technology will
help
ensure a long lasting and reliable storage system.
The lack of good, dependable, environmentally protected
stores of
water is a serious problem in many underdeveloped regions of
the
world. Improving
this situation will require a substantial infusion
of effort and money.
It is hoped that the suggestions made
herein will be valuable in stimulating new ideas, selecting
the
most suitable technology from among the various alternatives
available, and applying the correct criteria to locate and
size
storage facilities.
BIBLIOGRAPHY/Suggested Reading List
American Concrete Institute.
Concrete International. Vol. 3, No.
4.
Detroit, Michigan:
American Concrete Institute, April
1981.
American Concrete Institute.
"Concrete Sanitary Engineering
Structures." Report No. ACI 350R-83. Detroit, Michigan:
American
Concrete Institute, 1983.
American Concrete Institute.
Handbook of Concrete Engineering.
ACI-82 Manual of
Practice. Detroit, Michigan:
American Concrete
Institute, 1982.
American Water Works
Association. "AWWA Standard
D-100-79 for
Welded Steel
Water Storage Tanks." Denver, Colorado:
American
Water Works
Association, 1979.
American Water Works
Association. "AWWA Standard
D-105-80 for
Disinfection of
Water Storage Facilities." Denver, Colorado:
American Water
Works Association, 1980.
American Water Works
Association. Water Distribution
Operator
Training
Handbook. Denver, Colorado: American
Water Works
Association,
1976.
American Water Works Association.
Water Quality and Treatment.
Third
Edition. New York, New York:
McGraw-Hill, 1971.
Anchor, R.D. Design
of Liguid-Retaining Concrete Structures.
New
York, New
York: Wiley and Sons, 1982.
Brown, J.H.
"Flexible Membrane: An
Economical Reservoir Liner and
Cover." Journal of the
American Water Works Association.
Vol. 71, No. 6,
June 1979.
Feachem, R.G.; McGarry, M.G., and Mara, D.D. Water, Wastes
and
Health in Hot
Climates. New York, New York:
Wiley and Sons,
1977.
Great Lakes-Upper Mississippi River Board of State Sanitary
Engineers.
"Recommended Standards for Water Works." Albany, New
York:
Great Lakes-Upper Mississippi River Board of
State
Sanitary
Engineers, 1976.
Hartog, J.P.
"Ferro-Cement Construction." Unpublished paper prepared
for VITA,
1984. Arlington, Virginia:
VITA, 1984.
Helweg, O.J. and Smith, G.
"Appropriate Technology for Artificial
Aquifers."
Ground Water. Vol. 18, No. 3, May-June 1978.
Huisman, L.
"Low Cost Technology for Public Water Supplies in
Developing
Countries." Opening Remarks. Low
Cost Technology--Specialized
Conference of
International Water Supply
Associations,
Berlin, West Germany, March 31-April 1, 1981.
Ryden, D.E.
"Evaluating the Safety and Seismic Stability of
Embankment
Reservoirs." Journal of the
American Water Works
Association.
Vol. 76, No. 1. Denver,
Colorado: American
Water Works
Association, January 1984.
Maddocks, D. Methods
of Creating Low Cost Waterproof Membranes
for Use in the
Construction of Rainwater Catchment and Storage
Systems.
London, England:
Intermediate Technology Publications,
Ltd., February
1975.
Salvato, J.A., Jr. Environmental Engineering and
Sanitation. New
York, New
York: Wiley-Interscience, 1972.
Schulze, G.
"Cistern Based Water Supply in Rural
Areas in Low
Developed
Countries." Low Cost Technology--Specialized Conference
International
Water Supply Association. Berlin,
West Germany,
March 31-April 1, 1981.
Sharma, P.N. and Helweg, O.J. "Optimum Design of Small
Reservoir
Systems."
Journal of irrigation and Drainage
Division--American
Society of Civil
Engineers. Vol. 108, IR4, December
1982.
Sherer, K.
"Technical Training of Peace Corps Volunteers in Rural
Water Supply
Systems in Morocco." Water and Sanitation for
Health Project
(WASH) Field Report No. 43. Washington, D.C.:
U.S. Agency for
International Development, May 1982.
Silverman, G.S.; Nagy, L.A.; and Olson, B.H.
"Variations in
Particulate
Matter, Algae, and Bacteria in an Uncovered,
Finished Drinking-Water Reservoir."
Journal of the American
Water Works
Association. Vol. 75, No. 4. Denver,
Colorado:
American Water
Works Association, April 1983.
Upmeyer, D.W.
"Estimating Water Storage Requirements." Public
Works. Vol. 109,
No. 7, July 1978.
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