|Public Health Technician (MSF, 1994, 192 p.)|
|1. Water in camps of Displaced people|
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.
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 centimetres 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).
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 centre, a small filter may be made with local materials.
Whatever the size of the filter, the operating principle remains the same.
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 (see technical briefs Chlorine-generating products and Chlorination).
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 (see technical brief Chlorine-generating products).
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: see technical brief Monitoring chlorination).
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.
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.