D. Assessment of water quality

Is the water potable or not?

The only criteria really of importance to health are the presence or the absence of pathogenic organisms and of toxic concentrations of certain chemicals.

Unfortunately these two criteria cannot be assessed by simple tests.

For example, there is no direct relation between the appearance of a sample of water and its potability. (A cloudy sample may be safe, whereas a clear sample may be both chemically and biologically dangerous.)

In non-industrialised areas the presence of pathogenic organisms is an infinitely more frequent problem than the presence of chemical substances at toxic concentrations.

ln practice, which analysis should be done and when?

TURBIDITY (CLOUDY APPEARANCE)

The presence of suspended solids is always a problem (the water is unpleasant to drink, there is a sediment, etc.), even if it does not necessarily mean that the water is polluted. But it is particularly important to take the turbidity into account if the water needs treatment.

The measurement of turbidity may be done with special equipment (graduated plastic tubes), but generally simply looking through the water in a transparent container gives enough information.

CHEMICAL ANALYSIS

In emergency situations a blind eye is usually turned to the acceptability of the taste and smell of water.

However, a laboratory chemical analysis may be requested:

- if a long term water supply is being planned,
- if there are particular reasons to suspect chemical pollution.

BIOLOGICAL ANALYSIS

In other words, how to tell if water contains pathogens.

- Principle

The pathogenic organisms which may be present in water are too numerous and too various to be identified individually in practice (bacteria, protozoa, helminths, etc.).

As their presence is always linked to fl pollution (except for Guinea worm), it is preferable to look for organisms which are "indicators" of this pollution.

The indicator organisms generally looked for are the fl coliforms, principally Escherichia cold (E. Coli).

These germs are always present in large numbers in the faeces of man and other warm blooded animals.

The presence of fl coliforms an water indicates the possible presence of pathogenic organisms.

The absence of fl coliforms in water indicates the probable absence of pathogens, but is no proof of this (certain pathogens are more resistant than fl coliforms).

The count of fl coliforms (or E. Coli) per 100 ml gives an indication of the degree of fl pollution.

Note:

Other bacteria very similar to fl coliforms live more or less everywhere in the environment. They also belong to the coliform group.

In raw water (untreated), only the presence of fl coliforms is significant for health, because they are the only coliforms which prove fl contamination.

Although in certain cases total coliforms are identified, only fl coliforms are used as indicators in the field.

- Method of bacteriological analysis

The membrane filtration method is generally used. This consists of filtering the water through a membrane which retains bacteria.

The membrane is then incubated at 44ºC for 14 - 18 hours on a specific culture medium.

The count of those colonies which develop with a characteristic appearance gives the number of fl coliforms in the sample of water which was filtered.

Note

When incubating at 37 C all the coliforms develop, so the count is of total coliforms.

There are kits available for field analysis (for example the Del Agua/Oxfam kit or the Milliflex kit from Millipore), composed of portable filtration and incubation systems.

However, these kits are expensive and require good training in their operation and in the reading and interpretation of results.

- How to express the results of analyses?

· Number of coliforms/100 ml if it concerns total coliforms.
· Number of fl coliforms/100 ml or sometimes number of E. Coli/100 ml (the two may be interchanged, as in human faeces E. Coli represents more than 90% of fl coliforms).

When to do a bacteriological analysis?

A bacteriological analysis is rarely necessary. It is generally much more important and useful to do a sanitary inspection (the origin of the water, protection of the water point, its siting in relation to defecation areas, protection of the water during storage and transport, etc.). Such an inspection will often make it possible to see that the water is polluted or has strong chances of being polluted at some time or other.

Analysis only gives an indication of pollution at the precise moment of sampling, and not of the potential risk of pollution.

Moreover, a sanitary inspection may be done by anyone, whereas bacteriological analysis needs equipment and proper training in its use and in the interpretation of results.

A bacteriological analysis should only be considered when:

· Deciding whether or not to treat surface water (see the next section).
· Using properly protected groundwater which, despite everything, is suspected of being the cause of a high incidence of diarrhoeal disease, or of being the origin of an epidemic of diarrhoea, cholera or typhoid.

Caution: in no case is the detection of fl coliforms in water proof that the water is the origin of an epidemic. Detection of heavy pollution indicates that it may be the source of the epidemic, but is no proof of this. However, in this case, action must be taken anyway.

The analysis is done:

· Either by a competent local laboratory within 6 hours of sampling (after this time the samples are no longer representative of the water to be analysed).
· Or in the field by someone trained in the use of field testing kits.

As long as the water is chlorinated and the free residual chlorine level is satisfactory (see brief Chlorination), bacteriological analysis is not necessary nor appropriate.