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Simple water
treatment Storage
and sedimentation
Storage
is the simplest procedure to improve water quality (certain pathogens do not
survive beyond several days), but it needs large reservoirs. Storage alone is
not always a very effective procedure. During
storage there also occurs purification by sedimentation. Suspended
material settles to the bottom of the reservoir with a proportion of the
pathogens (mainly the largest: helminth eggs and protozoan cysts). Sedimentation
of turbid water is essential before filtration and disinfection. If
natural sedimentation is too slow, flocculation may be necessary
(specialist help is needed for this). In practice, if water in a bottle is
still muddy after an hour, then the natural sedimentation will not be enough. Filtration Passing
water through a permeable bed eliminates a proportion of the pathogens by
retaining them mechanically on the surface or within the filter. Ceramic
candle filters and certain sand filters (known as rapid sand filters) work on
this principle. SLOW
SAND FILTRATION Under certain conditions, in passing water through a bed of sand, particularly effective filtration is achieved by biological purification in addition to the mechanical action of the filter. For this to occur, the filtration must be relatively slow. A deposit is formed at the surface and in the top few centimeters of sand, in which breeds a whole range of bacteria and microscopic plants, forming a skin (called the Schmutzdecke, biological membrane or bacterial film), which works both biologically and mechanically. It acts as a very fine filter which retains or kills most pathogenic organisms: it eliminates eggs, cysts, nearly all pathogenic bacteria and a proportion of viruses. Slow
sand filtration is the only procedure which achieves such an improvement of
water quality in a single operation. A
slow sand filter can operate for weeks or even months without maintenance
(which consists of the removing of a thin layer of sand when the filtration
rate becomes too low). In
practice At
the collective scale, the construction of this type of filter needs the input
of a specialist, and then the maintenance is relatively simple. At
a smaller scale, for example in a feeding center, a small filter may be made
with local materials. Whatever
the size of the filter, the operating principle remains the same. Important The
bacterial layer is fragile and certain precautions should be taken to preserve
it: never let it dry out, and never pass chlorinated water through it. Disinfection
with chlorine Chlorine is a suitable disinfectant for water because it is very powerful without being toxic. It allows the destruction of all viral and bacterial pathogens in water. Chlorine
can also be used to disinfect surfaces in contact with water: the insides of
wells, pumps, pipes, spring boxes, reservoirs, etc., (when putting into
service, after repairs or after accidental pollution). For
this purpose it is used at much higher concentrations than for the
disinfection of drinking water because the pollution may be much greater. In
the situations considered here, gas chlorination is not recommended because it
can be dangerous and is not practical on a small scale. Chlorine-generating
products are preferred: calcium hypochlorite, chloride of lime, sodium
hypochlorite solution. All
these chemicals release chlorine when they are dissolved in water. Chlorine
reacts immediately with all the oxidizable substances which may be present in
the water (organic matter and certain mineral substances as well as pathogens
and other organisms). These
substances consume chlorine. For the chlorination to be effective, sufficient
chlorine must be added to meet this initial chlorine demand. This is confirmed
by checking that an excess of unconsumed chlorine remains in the water
(residual free chlorine). EFFECTIVENESS At
the doses normally used, chlorine destroys all pathogenic viruses and bacteria
in water, but it is ineffective against: -
protozoan
cysts and helminth eggs or larvae, -
pathogens
within suspended particles (as they are thus not in contact with the
chlorine), so it is advisable to filter water prior to chlorination, to remove
eggs, cysts, larvae and suspended particles. PRACTICAL
USE Chlorination is a very
suitable treatment method for emergency situations (e.g. typhoid or cholera
epidemics), because it is very effective, but it is relatively tricky to set
up and needs constant attention. If
water has to be treated, which method should be used? The
choice of treatment method will depend on the appearance of the water, the
supposed degree of pollution, or that measured by bacteriological analysis,
and the technical options. See the following table. In
an emergency, chlorination is often the best technique, but in the long term,
slow sand filtration is generally the most appropriate solution. However, it
is always better to use water which does not need treatment.
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