WATER QUALITY - Water Quality Engineering in Natural Systems - page 24

streptococcal species from humans and eliminates
some predominant streptococcus species in live-
stock and wildlife feces. unfortunately, most live-
stock and wildlife feces are still sources of the
enterococcus group species, so enterococcus den-
sities do not exclusively indicate human fecal con-
tamination. Enterococci are usually found at lower
densities than E. coli in human feces and sewage
effluents, and this fact may compromise their use-
fulness as indicator organisms. On the other hand,
enterococci are particularly resistant to saline
environments that make them more desirable
indicators in marine waters.

The reliability of TC and FC bacteria as indicators of
the presence of pathogens in water depends on the per-
sistence of pathogenic microorganisms relative to coli-
form bacteria. In surface waters, pathogenic bacteria
tend to die off faster than coliforms, whereas viruses and
protozoans tend to be more persistent. In recreational
lakes and streams, FC levels of less than 200 CFu/100 ml
are usually considered acceptable. If no coliforms are
detected, the water is presumed to be uncontaminated
by sewage.
E. coli and enterococci are both suited for predicting
the presence of pathogens in freshwater environments,
and enterococci is more suited for predicting the pres-
ence of pathogens in marine environments (uSEPA,
2002). These preferences are widely reflected in regula-
tory water-quality standards, where typical geometric-
mean standards are 126 MPN/100 ml for E. coli and
33 CFu/100 ml for enterococci in freshwater and
35 CFu/100 ml for enterococci in marine waters.
In groundwaters, FC concentration is not a reliable
indicator of contamination by pathogenic microorgan-
isms, since microorganisms larger than viruses seldom
travel appreciable distances, and there is no relation
between the presence of FCs and pathogenic microor-
ganisms. The presence of viruses is of particular concern
in groundwaters, where the viruses originate from septic
tank effluent and can move through porous media
because of their small size (typically, 0.01-0.03 µ m).
Since viruses are difficult to detect in groundwater and
FCs are not a reliable indicator, protection of drinking
water intakes (wells) from viral contamination is usually
achieved by requiring minimum setback distances for
viral sources, such as septic tanks and minimum disin-
fection requirements.

Figure 2.9. Membrane filter test for coliforms. Source : State
of Oregon (2005).

an indicator include their regrowth in water, thus
becoming part of the natural aquatic flora. As a
rule, water with any detectable coliforms is unsafe
to drink.
Fecal coliforms. Fecal coliforms provide stronger
evidence of the possible presence of fecal patho-
gens than do TCs. FCs are a subgroup of TCs,
distinguished in the laboratory through elevated
temperature tests (43-44.5°C, depending on the
test). Although the test determines coliforms of
fecal origin, it does not distinguish between those
of human and animal origin. The MF method can
also be used to detect the presence of FC bacteria.
After the water sample is filtered through the
filter, it is incubated in a special medium at a tem-
perature of 44.5°C, and FCs appear as blue colo-
nies. The concentration of FCs is typically much
lower than that of TCs. As frames of reference,
human feces contain on the order of 10 7 FCs* per
gram (Droste, 1997), and rivers polluted by com-
bined sewer outfalls may have FCs in excess of
10 6 CFu/100 ml.
E. coli and Enterococci. E. coli and enterococci are
commonly used as indicators of recreational
water quality instead of FCs because of the better
correlation with gastroenteritis in swimmers. E.
coli is the species of the FC group most commonly
associated with wastes from warm-blooded
animals, and the enterococcus group is a subgroup
of fecal streptococcus that includes predominant

2.4.3 Biological Integrity

Biological integrity is defined as the ability of a water
body to support and maintain a balanced, integrated,

* Commonly, stated as colony-forming units (CFu).