Emergency Treatment of Drinking Water at Point-of-Use (POU)
UN WHO (World Health Organization) Technical Note for Emergencies No. 5
This note is about simple treatment of drinking water at point-of-use for
people in, or just after an emergency. The options suggested are quick
short-term measures to provide a safe survival level supply of drinking
water from unsafe polluted water sources. The options should be
sustainable until a longer-term safe and cost-effective supply is
available to the population.
 The methods described are suitable for water taken from any source but,
in general, will only remove physical and microbiological pollution.
Pollution by chemicals such as after a spillage of industrial waste will
not normally be removed by these processes and specialist advice should be
taken.
In general terms, treatment of water at household level follows the
processes shown in Figure1. However, depending on the quality of raw
water, some processes may not be necessary.
Figure 1. General steps in the water treatment
processes undertaken at household level
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Straining
Pouring water through a clean piece of cotton cloth will remove a
certain amount of the suspended silt and solids. It is important that the
cloth used is clean, as dirty cloth may introduce additional pollutants.
Specifically made monofilament filter cloths may be used in areas where
guinea-worm disease is prevalent.
Such cloths remove organisms known as copepods, which act as
intermediate hosts for the guinea-worm larvae. The cloth must always be
used with the same surface uppermost. The cloth may be cleaned using soap and clean water.
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Aeration
Aeration is a treatment process in which water is brought into close
contact with air for the primary purpose of increasing the oxygen content
of the water. With increased oxygen content:
- volatile substances such as hydrogen sulphide and methane which
affect taste and odour are removed;
- carbon dioxide content of water is reduced; and
- dissolved minerals such as iron and manganese are oxidized so that
they form precipitates, which can be removed by sedimentation and
filtration.
Figure 2. Aerator trays
The close contact between water and air required for aeration can
be achieved in a number of ways. At a household level, rapidly shake a
container part-full of water, for about five minutes and then stand the
water for a further 30 minutes to allow any suspended particles to settle
to the bottom.
On a larger scale, aeration may be achieved by allowing water to
trickle through one or more well-ventilated, perforated trays containing
small stones, as shown in Figure 2. Again, the water must be collected in
a container and allowed to stand for about 30 minutes to settle suspended
particles.
Figure 3. The three pot treatment system
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Storage and settlement
When water is stored for a day in safe conditions, more than 50% of
most bacteria die. Furthermore, during storage, the suspended solids and
some of the pathogens will settle to the bottom of the container. The
container used for storage and settlement should have a lid to avoid
recontamination, but should have a neck wide enough to facilitate periodic
cleaning. For example a bucket with a lid could be used for this
purpose.
Water should be drawn from the top of the container where it will be
cleanest and contain less pathogens. Storage and settlement for at least
48 hours also eliminates organisms called the cercariae, which act
as intermediate host in the life cycle of bilharziasis
(schistosomoasis), a water-based disease prevalent in some
countries. Longer periods of storage will leader to better water
quality.
A household can maximize the benefit of storage and settlement by using
the three-pot system illustrated in Figure 3.
Filtration
Filtration is the passage of polluted water through a porous medium
(such as sand). The process uses the principle of natural cleansing of the soil.
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Simple up-flow sand filter
Simple household filters may be put together inside clay, metal or
plastic containers. The vessels are filled with layers of sand and gravel
and pipe work arranged to force the water to flow either upwards or
downwards through the filter. Figure 4 shows a modified simple upward
rapid flow filter.
Figure 4. A simple upflow rapid sand filter
A filter such as this could be built from a 200 litres drum. It has
a filter bed made up coarse sand (of about 0.3m depth) of grain size
between 3 and 4mm diameter, and supported by gravel covered by a
perforated metal tray. The effective filtration rate of such a filter
could be as high as 230 litres per hour.
Such filters must be dismantled regularly to clean the sand and gravel
and remove any settled silt. The frequency of cleaning is dependant on the
level of turbidity of the raw water. Furthermore, such filters are not
effective at removing the pathogens. Therefore the water must be
disinfected or stored for 48 hours in order to make it safe.
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Charcoal filters
Charcoal can be quite effective at removing some tastes, odours, and
colour. Ordinary charcoal available locally could be used, but activated
carbon is more effective, though rather expensive. An example of such a
filter is the UNICEF upflow sand filter, illustrated in Figure 5. However,
if the charcoal is not regularly renewed or if the filter is left unused
for some time, there is evidence that it can become the breeding ground
for harmful bacteria.
Figure 5. The Unicef upflow charcoal filter
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Ceramic filters
Water may be purified by allowing it to pass through a ceramic filter
element. These are sometimes called candles. In this process, suspended
particles are mechanically filtered from the water. The filtered water
must be boiled or otherwise disinfected. Some filters are impregnated with
silver which acts as a disinfectant and kills bacteria, removing the need
for boiling the water after filtration. Ceramic filters can be
manufactured locally, but are also mass-produced. They can be costly but
have a long storage life and so can be purchased and stored in preparation
for future emergencies. The impurities held back by the candle surface
need to be brushed off under running water, at regular intervals. In order
to reduce frequent clogging, the inlet water should have a low turbidity.
Figure 6 shows a variety of ceramic candles.
Candle filters
Figure 6. Ceramic filters
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Disinfection
It is essential that drinking water be free of harmful organisms.
Storage, sedimentation and filtration of water reduce the contents of
harmful bacteria but none of them can guarantee the complete removal of
germs. Disinfection is a treatment process that ensures drinking water is
free from harmful organisms or pathogens. It is recommended that this be
the final treatment stage, as many of the disinfection processes will be
hampered by suspended solids and organic matter in the water. There are
various methods of achieving disinfection at household level:
Disinfection by boiling - Boiling is a very effective though
energy consuming method to destroy various pathogens such as viruses,
spores, cysts and worm eggs. The water should be brought to a rolling boil
for at least five minutes and preferably up to a period of twenty minutes.
Apart from the high energy costs involved in boiling, the other
disadvantage is the change in taste of water due to the release of air
from the water. The taste can be improved by vigorously stirring the
water, or shaking the water in a sealed container after it has cooled. A
better water quality can be obtained by storing the boiled water, as
described earlier.
Disinfection using chlorine - Chlorine is a chemical most widely
used for the disinfection of drinking water because of its ease of use,
ability to measure its effectiveness, availability and relatively lower
cost. When used correctly, chlorine will kill all viruses and bacteria,
but some species of protozoa and helmithes are resistant. There are
several different sources of chlorine for home use; in liquid, powder and
tablet form. Chlorine is commonly available to households as liquid bleach
(sodium hypo chlorite), usually with a chlorine concentration of 1%.
Liquid bleach is sold in bottles or sachets, available on a commercial
basis.
Chlorine must be added in sufficient quantities to destroy all the
germs but not so much as to affect the taste adversely. The chemicals
should also have sufficient contact time with the pathogens (at least 30
minutes for chlorine). Deciding on the right quantity can be difficult, as
substances in the water will react with the disinfectant at different
rates. Furthermore, the strength of the disinfectant may decline with time
depending on how it is stored. It is therefore recommended that in
emergency situations, chlorine solutions be centrally dispensed to the
users by qualified personnel. Displaced people should receive standard
containers for collecting/storing water, as well as simple dropper tubes
or syringes. Technical staff should provide the instructions for mixing
the chlorine solution, at the point of dispensing. See Note 1 Cleaning
and disinfecting wells for details of preparing chlorine
solutions.
Solar disinfection - Ultra-violent rays from the sun are used to
inactivate and destroy pathogens present in water. Fill transparent
plastic containers with water and expose them to full sunlight for about
five hours (or two consecutive days under 100% cloudy sky). Disinfection
occurs by a combination of radiation and thermal treatment. If a water
temperature of least 50oC is achieved, an exposure period of
one hour is sufficient. Solar disinfection requires clear water to be
effective.
An enhanced example is the SODIS system, whereby half-blackened bottles
are used to increase the heat gain, with the clear side of the bottle
facing the sun, as shown above.
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Other water treatment chemicals
A number of commercially produced chemicals have been developed to
holistically treat water at household level in emergency situations.
Studies have shown that some of these powders significantly remove
pathogenic bacteria, viruses and parasites from water. They also enable
the particles to flocculate together, so they then to sink to the bottom
of the container. Commercially available sachets typically treat 10 litres
of water. The water should be allowed to stand for at least 5 minutes
before it is strained. It should be allowed to stand for a further 30
minutes before it is used for human consumption.
Further information
Shaw, Rod (ed.) (1999) Running Water: More technical briefs on
health, water and sanitation, ITDG, UK.
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This information has been prepared by WEDC
Author: Sam Kayaga
Series Editor: Bob Reed
Editorial contributions and illustrations: Rod Shaw
Graphics: Ken Chatterton
Design and print layout: Glenda McMahon, Sue Plummer
and Rod Shaw
Adapted for the web by: Kay Davey and Andy Larkum
Water, Engineering and Development Centre,
Loughborough University,
Leicestershire, UK.
Phone: +44 1509 222885
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