Failed Water Test
Water used as a source for a small supply of drinking water may be of unknown origin and
come from a catchment prone to consistent or intermittent contamination by faecal material from
domestic and farm animals, wildlife or sanitation systems such as septic tanks. As a consequence,
there is a high probability of pathogenic micro-organisms being present in the source water, and
adequate treatment must be applied before the water is used for domestic purposes. Because no
single treatment can be expected to remove all types of pathogenic agents, a multiple barrier
approach in the form of two or more sequential treatment processes is recommended.
The microbiological quality of drinking water has traditionally been assessed by monitoring for
bacteria called faecal indicator organisms (coliforms, E. coli, and enterococci). The presence of
these organisms is indicative of past faecal contamination and hence the possible presence of
enteric pathogens. Although indicator organisms are generally adequate for monitoring purposes
they cannot completely be relied on to indicate the absence of pathogens. This is especially true
where a pathogen is environmentally more robust, or can survive treatment better than the
indicators. In these circumstances the indicator may be absent even though low numbers of
pathogens still remain.
E. coli as a measure of microbiological safety
Despite the possible shortcomings in the use of faecal indicators as a measure of the
microbiological safety of a water supply, monitoring for coliforms and E. coli is still recommended
and standards of quality are expressed in terms of these organisms. The reason for this is that
monitoring for the pathogens themselves remains rather uncertain because methods of analysis are
relatively insensitive and costly compared to those for faecal indicators. Additionally, the absence
of one pathogen will not guarantee the absence of others. Therefore E. coli and coliforms remain
the best and most sensitive general measure of water quality.
An example of a pathogen with which such a discrepancy can occur is the protozoan parasite
Cryptosporidium. This micro-organism is very much more tolerant of the action of disinfectants
than faecal indicators such as E. coli because it forms resistant spore-like bodies called oocysts.
If disinfection is the sole treatment process used, the numbers of Cryptosporidium oocysts will
remain unchanged whereas the numbers of E. coli may be reduced to undetectable levels. UV Disinfection with a pre-filter will remove oocysts from water.</p)
Treatment: Multi Barrier Approach
UV Disinfection is the treatment method of choice for most owners of a private water supply. This highly effective method of treatment does not alter the taste of the water, nor does it use chemicals
A 5 micron UV pre-filter will remove much of the particulate material present in water, which, if not removed, could otherwise reduce the effectiveness of UV disinfection by creating an oxidant demand and/or shielding microbes from the effects of the disinfectant. This is a good example of why a multiple rather than a single barrier approach is preferred. In addition, the use of multiple barriers will allow some protection to remain even if one process fails.
Acknowledgement: The primary source of information whilst preparing this document is Drinking Water Regulator in Scotland (DWRS).