Page 82 - Water and wastewater engineering

GENERAL WATER SUPPLY DESIGN CONSIDERATIONS 2-25

is a blue skin color, which gives the syndrome its name, cyanosis. This condition is called
cyanosis. Cyanide also causes chronic effects on the thyroid and central nervous system.
The toxic heavy metals include arsenic (As), barium (Ba), cadmium (Cd), chromium (Cr),
lead (Pb), mercury (Hg), selenium (Se), and silver (Ag). The heavy metals have a wide range of
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effects. They may be acute poisons (As and Cr , for example), or they may produce chronic
disease (Pb, Cd, and Hg, for example).

Toxic Organic Substances. There are over 120 toxic organic compounds listed on the U.S.
EPA’s Priority Pollutant list ( Table 2-11 ). These include pesticides, insecticides, and solvents.
Like the inorganic substances, their effects may be acute or chronic.

Microbiological Characteristics
Water for drinking and cooking purposes must be made free from pathogens. These organisms
include viruses, bacteria, protozoa, and helminths (worms).
Some organisms that cause disease in people originate with the fecal discharges of infected
individuals. Others are from the fecal discharge of animals.
Unfortunately, the specific disease-producing organisms present in water are not easily
identified. The techniques for comprehensive bacteriological examination are complex and time-
consuming. It has been necessary to develop tests that indicate the relative degree of contami-
nation in terms of an easily defined quantity. The most widely used test estimates the number

of microorganisms of the coliform group. This grouping includes two genera: Escherichia coli
and Aerobacter aerogenes. The name of the group is derived from the word “colon”. While E.
coli are common inhabitants of the intestinal tract, Aerobacter are common in the soil, on leaves,
and on grain; on occasion they cause urinary tract infections. The test for these microorganisms,
called the Total Coliform Test, was selected for the following reasons:

1. The coliform group of organisms normally inhabits the intestinal tracts of humans and
other mammals. Thus, the presence of coliforms is an indication of fecal contamination
of the water.

2. Even in acutely ill individuals, the number of coliform organisms excreted in the feces
outnumber the disease-producing organisms by several orders of magnitude. The large
numbers of coliforms make them easier to culture than disease-producing organisms.
3. The coliform group of organisms survives in natural waters for relatively long periods of
time but does not reproduce effectively in this environment. Thus, the presence of coli-
forms in water implies fecal contamination rather than growth of the organism because
of favorable environmental conditions. These organisms also survive better in water than
most of the bacterial pathogens. This means that the absence of coliforms is a reasonably
safe indicator that pathogens are not present.
4. The coliform group of organisms is relatively easy to culture. Thus, laboratory techni-
cians can perform the test without expensive equipment.

Current research indicates that testing for Escherichia coli specifically may be warranted.
Some agencies prefer the examination for E. coli as a better indicator of biological contamination
than total coliforms.