WATER QUALITY - Water Quality Engineering in Natural Systems - page 23

from humans or other warm-blooded animals. Desir-
able properties of indicator organisms are as follows:

• The indicator should originate in the digestive tract
of humans and warm-blooded animals.
• The indicator should be present in fecal material in
large numbers.
• The indicator should always be present when the
pathogenic organism of concern is present, and
should be absent in clean uncontaminated water.
• The indicator should respond to natural environ-
mental conditions in a manner similar to the patho-
gens of interest, or survive longer in water than
pathogens.
• The indicator should be easy to isolate, identify, and
enumerate.
• The ratio of indicator to pathogen should be high.
• The indicator and pathogen should come from the
same source.
• The indicator should not be pathogenic.
A number of microorganisms are utilized as indicators,
most notably total coliforms (TCs), fecal coliform s
(FCs), E. coli , fecal streptococci, and enterococci.
Coliforms. The coliform group of organisms is
defined as all the anaerobic and facultatively
anaerobic, Gram-negative, nonspore-forming,
rodshaped bacteria that ferment lactose with gas
formation within 48 hours at 35°C. One of the u.S.
Environmental Protection Agency (uSEPA)-
approved methods for coliform bacteria testing is
the membrane filter (MF) method, which uses a
cellulose membrane material that has pore open-
ings that are approximately 0.47 µ m in diameter.
Water freely passes through the membrane, but
coliform bacteria are retained on the surface.
After the water sample is filtered through the
membrane, it is incubated for 24 hours at 35°C in
a special medium, specific for coliform bacteria.
Coliform bacteria produce colonies with a charac-
teristic metallic green sheen; an example of such
colonies is shown in Figure 2.9. The definition of
coliform bacteria is an operational rather than a
taxonomic definition and encompasses a variety
of organisms, mostly of intestinal origin. The coli-
form group of bacteria meets all criteria for
an ideal indicator. These bacteria are generally
not pathogenic, yet they are usually present
where pathogens are present. Coliform bacteria
are more plentiful than pathogens and can often
stay alive in the water environment for longer
periods of time. Drawbacks to the use of TCs as

Figure 2.8. Algae Anabaena flos-aquae. . Source : Wayne
Carmichael (Wright State university), Mark Schneegurt
(Wichita State university), and Cyanosite (http://www-
cyanosite.bio.purdue.edu).

shown in Figure 2.8. Toxin concentrations during
algal blooms have resulted in illness or death in
mammals, birds, and fish that have ingested a suf-
ficient dose.

Excessive nutrients (nitrogen and phosphorus) in ocean
waters often result in the development of red tides ,
which are algae blooms of such intensity (e.g., 50 × 10 6
cells per liter) that the sea is discolored. The associated
coloration is not always red, but may be white, yellow,
or brown. Many animals, including commercially impor-
tant fish species, are killed or excluded from the area
where red tides occur, either because of clogging of their
gills or other structures, or because of the toxic proper-
ties of the algae. Blooms of Phaeocystis form an unsightly
brown foam that when stranded on the shore can be
mistaken for sewage pollution. Marine algae are a major
component of phytoplankton, which is at the base of the
food chain in the oceans. Along with cyanobacteria,
marine algae are consumed by protozoans and micro-
scopic animals, which are, in turn, eaten by fish.

2.4.2 Indicator Organisms

Testing water samples for a wide variety of pathogens
is usually not practical, and indicator organisms are typi-
cally used to provide a measure of fecal contamination