WATER PURIFICATION
The purification of unsafe water requires some trained
supervision if it is to be
done effectively. Such supervision is rarely available in
the villages and the
procedure tends to be neglected sooner or later. Under these
circumstances every
effort must be made to obtain a source that provides
naturally wholesome water
and then to collect that water and protect it against
pollution by the methods
already described. Thus, the necessity for treatment of the
water may be avoided,
and the practical importance of managing this can hardly be
overemphasized.
Water treatment under rural conditions should be restricted
by the responsible
control agency to cases where such treatment is necessary
and where proper plant
operation and maintenance is assured.
If the water needs treatment, this should, if at all
possible, be done for the
whole community and certainly before, or on entry to the
dwelling so that the
water from all the taps in the house is safe. The practice,
common in the
Tropics, of sterilizing (by filtration and boiling) only the
water to be used for
drinking, teeth cleaning, etc., though efficient in itself
(when carefully done) is
frequently nullified by carelessness. Furthermore, children
are likely to use water
from any tap. Contrary to an all too common opinion,
ordinary freezing of water,
though it may retard the multiplication of bacteria, does
not kill them, and ice
from a household refrigerator is no safer than the water
from which it was made.
The principal methods of purifying water on a small scale
are boiling, chemical
disinfection, and filtration. These methods may be used
singly or in combination,
but if more than filtration is needed the boiling or
chemical disinfection should
be done last. Each method is discussed briefly below.
Following this general
introduction are descriptions of a variety of water
purification technologies: boiler
for drinking water, chlorination of polluted water, water
purification plant, and
sand filter.
Boiling is the most satisfactory way of destroying
disease-producing organisms in
water. It is equally effective whether the water is clear or
cloudy, whether it is
relatively pure or heavily contaminated with organic matter.
Boiling destroys all
forms of disease-producing organisms usually encountered in
water, whether they
be bacteria, viruses, spores, cysts, or ova. To be safe the
water must be brought
to a good "rolling" boil (not just simmering) and
kept there for 15-20 minutes.
Boiling drives out the gases dissolved in the water and gives
it a flat taste, but
if the water is left for a few hours in a partly filled
container, even though the
mouth of the container is covered, it will absorb air and
lose its flat, boiled
taste. It is wise to store the water in the vessel in which
it was boiled. Avoid
pouring the water from one receptacle to another with the
object of aerating or
cooling it as that introduces a risk of re-contamination.
Chlorine is a good disinfectant for drinking water as it is
effective against the
bacteria associated with water-borne disease. In its usual
doses, however, it is
ineffective against the cysts of amoebic dysentery, ova of
worms, cercariae which
cause schistosomiasis, and organisms embedded in solid
particles.
Chlorine is easiest to apply in the form of a solution and a
useful solution in one
which contains 1 percent available chlorine, for example,
Milton Antiseptic.
Dakin's solution contains 0.5 percent available chlorine,
and bleaching powder
holds 25 percent to 30 percent available chlorine. About
37cc (2 1/2 tablespoons)
of bleaching powder dissolved in 0.95 liter (1 quart) of
water will give a 1
percent chlorine solution. To chlorinate the water, add 3
drops of 1 percent
solution to each 0.95 liter (1 quart) of water to be treated
(2 tablespoonfuls to 38
gallons), mix thoroughly and allow it to stand for 20
minutes or longer before
using the water.
Chlorine may be obtained in table form as
"Sterotabs" formerly known as
"Halazone"), "Chlor-dechlor" and
"Hydrochlorazone," which are obtainable on the
market. Directions for use are on the packages.
Iodine is also a good disinfecting agent. Two drops of
ordinary tincture of iodine
are sufficient to treat 0.95 liter (1 quart) of water. Water
that is cloudy or
muddy, or water that has a noticeable color even when clear,
is not suitable for
disinfection by iodine. Filtering may render the water fit
for treatment with
iodine. If the water is heavily polluted, the dose should be
doubled. Though the
higher dosage is harmless it will give the water a medicinal
taste. To remove any
medicinal taste add 7 percent solution of sodium
thiosulphate in a quantity equal
to the amount of iodine added.
Iodine compounds for the disinfection of water have been put
into table form, for
example, "Potable Aqua Tablets," "Globaline"
and "Individual Water Purification
Tablets"; full directions for use are given on the
packages. These tablets are
among the most useful disinfection devices developed to date
and they are
effective against amoeba cysts, cercariae, leptospira, and
some of the viruses.
Source:
Small Water Supplies, Bulletin No. 10 London: The Ross
Institute, 1967.
Other Useful References:
Mann, H.T. and Williamson, P. Water Treatment and
Sanitation. London: Intermediate
Technology Publications, 1976.
Iornech Disinfection System, Iornech Ltd., 2063 Lakeshore
Blvd. West Toronto,
Ontario, Canada, (undated).
Manual of Individual Water Supply Systems. Public Health
Service Publication No.
24, Washington, D.C. U.S. Department of Health amd Human
Services, 1962.
Decade Watch newsletter. United Nations Development Program,
Division of
Information.
International Reference Center for Community Water Supply
and Sanitation,
newsletter. P.O. Box 93190, 2509 AD, The Hague, Netherlands.
Boiler for Drinking Water
The boiler described here (Figure 1) will provide safe
preparation and storage of
fig1x140.gif (437x540)
drinking water in areas where pure water is not available
and boiling is practical.
When the unit was used in work camps in Mexico, a 208-liter
(55-gallon) drum
supplied 20 persons with water for a week.
Tools and Materials
208-liter (55-gallon) drum
10mm (3/4") pipe nipple, 5cm (2") long
Bricks for two 30cm (1') layers to support drum
Sand and 1 sack of cement for mortar and base of fireplace
Large funnel and filter medium for filling drum
Metal plate to control draft in front of fireplace
19mm (3/4") valve, preferably all metal, such as a gate
valve, that can withstand
heat.
The fireplace for this unit (see Figure 2) is simple. It
should be oriented so that
fig2x141.gif (600x600)
the prevailing wind or draft goes between the bricks from
the front to the back
of the drum. A chimney can be provided, but it is not
necessary.
When filling the drum, do not fill it completely, but leave
an air space at the top
as shown in Figure 1. Replace the funnel with a filler plug,
but leave the plug
completely loose.
Water must boil at least 15 minutes with steam escaping
around the loose filler
plug. Make sure that the water in the pipe nipple and valve
reach boiling
temperature by letting about 2 liters (2 quarts) of water
out through the valve
while the drum is at full boil.
Source:
Chris Ahrens, VITA Volunteer, Swannanoa, North Carolina
Chlorinating Wells, Springs, and Cisterns
Chlorination, when properly applied, is a simple way to
ensure and protect the
purity of water. Guidelines given here include tables to
give a rough indication of
the amounts of chlorine-bearing chemical needed.
Instructions are also given for
super-chlorination for disinfecting newly built or repaired
wells, spring encasements,
or cisterns. Chlorine-bearing compounds, such as ordinary
laundry bleach
made with chlorine are used because pure chlorine is
difficult and dangerous to
use.
Determining the Proper Amount of Chlorine
The amounts of chlorine suggested here will normally make
water reasonably safe.
A water-treatment system should be checked by an expert. In
fact, the water
should be tested periodically to make sure that it remains
safe. Otherwise, the
system itself could become a source of disease.
Tools and Materials
Container to mix chlorine
Chlorine in some form
Scale to weigh additive
The safest way to treat water for drinking is to boil it
(see "Boiler for Drinking
Water"). However, under controlled conditions,
chlorination is a safe method; it is
often more convenient and practical than boiling. Proper
treatment of water with
chlorine requires some knowledge of the process and its
effects.
When chlorine is added to water, it attacks and combines
with any suspended
organic matter as well as some minerals such as iron. There
is always a certain
amount of dead organic matter in water, as well as live
bacteria, viruses, and
perhaps other types of life. Enough chlorine must be added
to oxidize all of the
organic matter, dead or alive, and to leave some excess
uncombined or "free"
chlorine. This residual free chlorine prevents
recontamination. Too much residual
chlorine, however, is harmful and extremely distasteful.
Some organisms are more resistant to chlorine than others.
Two particularly
resistant varieties are amoebic cysts (which cause amoebic
dysentery) and the
cercariae of schistosomes (which cause bilharziasis or
schistosomiasis). These,
among others, require much higher levels of residual free
chlorine and longer
contact periods than usual to be safe. Often special
techniques are used to combat
these and other specific diseases.
It always takes time for chlorine to work. Be sure that
water is thoroughly mixed
with an adequate dose of the dissolved chemical, and that it
stands for at least
30 minutes before consumption.
Polluted water that contains large quantities of organic
matter, or cloudy water,
is not suitable for chlorination. It is best, and safest, to
choose the clearest
water available. A settling tank and simple filtration can
help reduce the amount
of suspended matter, especially particles large enough to
see. Filtration that can
be depended upon to remove all of the amoebic cysts,
schistosomes, and other
parthogens normally requires professionals to set up and
operate.
NEVER depend on home-made filters alone to provide drinking
water. However, a
home-made slow sand filter is an excellent way to prepare
water for chlorination.
Depending on the water to be treated, varying amounts of chlorine
are needed for
adequate protection. The best way to control the process is
to measure the
amount of free chlorine in the water after the 30 minute
holding period. A simple
chemical test, which uses a special organic indicator called
orthotolidine, can be
used. Orthotolidine testing kits available on the market
come with instructions on
their use.
When these kits are not available, the chart in Table 1 can
be used as a rough
tab1x144.gif (600x600)
guide to how strong a chlorine solution is necessary. The
strength of the solution
is measured in parts by weight of active chlorine per
million parts by weight of
water, or "parts per million" (ppm).
The chart in Table 2 gives the amount of chlorine-compound
to add to 1,000 liters
tab2x144.gif (600x600)
or to 1,000 gallons of water to get the solutions
recommended in Table 1.
Usually it is convenient to make up a solution of 500 ppm
strength that can then
be further diluted to give the chlorine concentration
needed. The 500 ppm
solution must be stored in a sealed container in a cool dark
place, and should be
used as quickly as possible since it does lose strength.
Modern chlorination plants
use bottled chlorine gas, but this can only be used with
expensive machinery by
trained experts.
Super-Chlorination
Super-chlorination means applying a dose of chlorine that is
much stronger than
the dosage needed to disinfect water. It is used to
disinfect new or repaired
wells, spring encasements, and cisterns. Table 3 gives
recommended doses.
TABLE 3
RECOMMENDED DOSES FOR SUPER-CHLORINATION*
Application
Recommended Dose
Procedure
New or repaired well
50 ppm
1. Wash casing, pump exterior
and drip pipe
with solution.
2. Add dosage to
water in well.
3. Pump until water
coming from
pump has strong
chlorine
odor for deep
wells, repeat
this a few times
at 1
hour intervals.)
4. Leave solution in well
at least 24
hours.
5. Flush all
chlorine from well.
Spring encasements
50 ppm
Same as above.
Cisterns
100 ppm
1. Flush with water to remove
any sediment.
2. Fill with
dosage.
3. Let stand
for 24 hours.
4. Test for
residual chlorine.
If there is
none, repeat
dosage.
5. Flush system
with treated
water.
* To find the correct amounts of chlorine compound needed
for the required
dosage, multiply
the amounts given under 10ppm in Tables 2 or 3 to get 50ppm
and by 10 to get
100ppm.
Example 1:
A water-holding
tank contains 8,000 U.S. gallons. The water comes from a
rapidly moving
mountain stream and is passed through a sand filter before
storage. How much
bleach should be added to make this water drinkable?
How long should
the water be mixed after adding?
Solution:
In this case 5
ppm are probably sufficient to safeguard the water. To do this
with bleach
requires 13 ounces per 1,000 gallons. Therefore the weight of
bleach to be
added is 13 x 8 or 104 ounces.
Always mix
thoroughly, for at least a half hour. A good rule of thumb is to
mix until you are
certain that the chemical is completely dissolved and
distributed and
then ten minutes longer. In this case, with an 8,000-gallon
tank, try to add
the bleach to several different locations in the tank to
make the mixing
easier. After mixing, test the water by sampling different
locations, if
possible. Check the corners of tank especially.
Example 2:
A new cistern has
been built to hold water between rainstorms. On its initial
filling it is to
be super-chlorinated. How much chlorinated lime should be
added? The
cistern is 2 meters in diameter and 3 meters high.
Solution:
First calculate
the volume of water. For a cylinder, Volume is [D.sup.2 = H
(D is diameter, H
is height and is 3.14.)
--------
4
Here D = 2 meters
H = 3 meters.
V = 3.14 x (2
meters) x (2 meters) x (3 meters)
----
4
V = 9.42 cubic
meters = 9,420 liters (Each cubic meter
contains
1,000 liters.)
From Table 3 we
learn that a cistern should be super-chlorinated with 100
ppm of chlorine.
From Table 2, we learn that it takes 40 grams of chlorinated
lime to bring
1,000 liters of water to 10 ppm C1. To bring it to 100
ppm, then, will
require ten times this amount, or 400 grams.
400 grams
x 9.42 thousand liters = 3,768 grams.
---------------
thousand liters
Source:
Salvato, J.S. Environmental Sanitation. New York: John Wiley
& Sons, Inc., 1958
Field Water Supply, TM 5-700.
Water Purification Plant
The water purification plant described here uses laundry
bleach as a source of
chlorine. Although this manually-operated plant is not as
reliable as a modern
water system, it will provide safe drinking water if it is
operated according to
instructions.
Many factors in this system require operating experience.
When starting to use
the system, it is safest to have the assistance of an
engineer experienced in
water supplies.
Tools and Materials
3 barrels, concrete tanks, or 208 liter (55-gallon) drums
20cm (8") funnel, or sheet metal to make a funnel
2 tanks, about 20 liters (5 gallons) in size
4 shut-off valves
Throttle or needle valve (clamps can be used instead of
valves if hose is used)
Pipe or hose with fittings
Hypochlorite of lime or sodium hypo-chlorite (laundry
bleach)
The water purification plant is made as in Figure 3. The two
tanks at the top of
fig3x148.gif (600x600)
the structure are for diluting the bleach. (The system can
be simplified by
eliminating the concentrate tank; the bleach is then added
directly to the mixing
tank.)
The two smaller tanks on the shelf below are for holding
equal amounts of diluted
bleach solution and water at a constant pressure; this makes
the solution and the
water flow at the same speed into the hoses that lead to the
mixing point. The
mix, which can be seen through the open funnel, is further
controlled by the
valves. If a needle or throttle valve is not available a
throttle action can be
obtained by installing another shut-off valve in series with
Valve #4.
Placing the two barrels at a height of less than 1.8 meters
(6') above the float
valve causes a pressure of less than 0.35kg per square
centimeter (5 pounds per
square inch). Thus, the plumbing does not have to be of high
quality except for
Valve #1 and the float valve of the water hold-up tank, if
the water supply is
under higher pressure.
A trial and error process is necessary to learn how much
concentrate should be
put in the concentrate tank, how much concentrate should
flow into the mixing
tank, and how much solution should be allowed past the
funnel. A suggested
starting mixture is 1/4 liter (1/2 pint) of concentrated
bleach for a mix tank
capacity of 190 liters (50 gallons) to treat 1,900 liters
(500 gallons) of water.
The water in the distribution tank should have a noticeable
chlorine taste. The
amount of bleach solution required depends on how dirty the
water is.
1. Mix concentrated
bleach with water in the concentrate tank with all valves
closed. The
mixing tank should be empty.
2. Fill the pipe
from the mixing tank to the solution tank with water after
having propped
the float valve in a closed position.
3. Let a trial
amount of concentrate flow into the mixing tank by opening
Valve #2.
4. Use a measuring
stick to see how much concentrate was used.
5. Close Valve #2
and open Valve #1 so that untreated water enters the mixing
tank.
6. Close Valve #1
and mix solution in the mixing tank with a stick.
7. Remove the prop
from the float valve of the solution tank so that it will
operate properly.
8. Open wide the
needle valve and Value #4 to clean the system. Let 4 liters (1
gallon) drain
through the system, if the pipe mentioned in the second step is
not permitted to
empty before recharging the mixing tank.)
9. Close down to
needle valve until only a stream of drops enter the funnel.
10. Open valve #3.
The flow into the funnel and the taste of the water in the
distribution tank
should be checked regularly to ensure proper treatment.
Source:
Chris Ahrens, VITA Volunteer, Swannanoa, North Carolina
Sand Filter
Surface water from streams, ponds, or open wells is very
likely to be contaminated
with leaves and other organic matter. A gravity sand filter
can remove
most of this suspended organic material, but it will always
let rivus and some
bacteria pass through. For this reason, it is necessary to
boil or chlorinate water
after it has been filtered.
By removing most of the organic matter, the filter:
o Removes large
worm eggs, cysts, and cercariae, which are difficult to kill
with chlorine.
o Allows the use
of smaller and fixed doses of chlorine for disinfection, which
results in
drinkable water with less taste of chlorine.
o Makes the water
look cleaner.
o Reduces the
amount of organic matter, including living organisms and their
food, and the
possibility of recontamination of the water.
Although sand filtration does not make polluted water safe
for drinking, a
properly built and maintained filter will make chlorination
more effective. Sand
filters must be cleaned periodically.
The household sand filter described here should deliver 1
liter (1 quart) per
minute of clear water, ready for boiling or chlorinating.
Tools and Materials
Steel drum: at least 60cm wide by 75cm (2'x 29 1/2")
Sheet metal, for cover: 75cm (29 1/2") square
Wood: 5cm x 10cm (2" x 4"), 3 meters (9.8') long
Sand: 0.2 cubic meter (7 cubic feet)
Gravel
Blocks and nails
Pipe, to attach to water supply
Optional: valve and asphalt roofing compound to treat drum
The gravity sand filter is the easiest type of sand filter
to understand and set up.
It uses sand to strain suspended matter from the water,
although this does not
always stop small particles or bacteria.
Over a period of time, a biological growth forms in the top
7.5cm (3") of sand.
This film increases the filtering action. It slows the flow
of water through the
sand, but it traps more particles and up to 95 percent of
the bacteria. The water
level must always be kept above the sand to protect this
film.
Sand filters can get partially clogged with organic matter;
under some conditions
this can cause bacterial growth in the filter. If the sand
filter is not operated
and maintained correctly, it can actually add bacteria to
the water.
The drum for the sand filter shown in Figure 4 should be of
heavy steel. It can
fig4x151.gif (600x600)
be coated with asphalt material to make it last longer.
The 2mm (3/32") hole at the bottom regulates the flow:
it must not be made
larger.
The sand used should be fine enough to pass through a window
screen. It should
also be clean; it is best to wash it.
The following points are very important in making sure that
a sand filter operates
properly.
o Keep a
continuous flow of water passing through the filter. Do not let the
sand dry out,
because this will destroy the film of microorganisms that forms
on the surface
layer of sand. The best way to ensure a continuing flow is to
set the intake
so that there is always a small overflow.
o Screen the
intake and provide a settling basin to remove as many particles
as possible
before the water goes into the filter. This will keep the pipes
from becoming
plugged and stopping the flow of water. It will also help the
filter to
operate for longer periods between cleanings.
o Never let the
filter run faster than 3.6 liters per square meter per minute (4
gallons per
square foot per hour) because a faster flow will make the filter
less efficient
by keeping the biological film from building up at the top of
the sand.
o Keep the filter
covered so that it is perfectly dark to prevent the growth of
green algae on
the surface of the sand. But let air circulate above the sand
to help the growth
of the biological film.
o When the flow
becomes too slow to fill daily needs, clean the filter: Scrape
off and discard
the top 1/2cm (1/4") of sand and rake or scratch the surface
lightly.
After several cleanings, the sand layer should be returned
to its original thickness
by adding clean sand. Before doing this, scrape the sand in
the filter down to a
clean level. The filter should not be cleaned more often
than once every several
weeks or even months, because the biological growth at the
top of the sand
makes the filter more efficient.
Source:
Hubbs, S.A. Understanding Water Supply and Treatment for
Individual and Small
Community Systems. Arlington, Virginia: VITA Publications,
1985.
Wagner, E.G. and Lanoix, J.N. Water Supply for Rural Areas
and Small Communities.
World Health Organization, 1959.
