Microbiology of Drinking Water - Public Water Systems - page 88

in size from 0.6 to 1.0 m in width (or diameter) and from 2 to 6 m in length.
Spirilla, or spiral-shaped bacteria, can be found in lengths up to 50 m, whereas
filamentous bacteria can occur in length in excess of 100 m. Table 4-1 summarizes
what is known relative to specific pathogenic bacteria, the disease caused by the or-
ganisms and the host or transmitting agent for the organisms.
Viruses
Viruses are microorganisms composed of a strand of genetic material—deoxy-
ribonucleic acid (DNA) or ribonucleic acid (RNA)—located within a protective protein
coat. Viruses are obligate parasites, unable to carry out any form of metabolism and
are completely dependent upon host cells for replication. Viruses are typically 0.01 to
0.1 m in size and are very species specific with respect to infection, choosing to
attack only one type of host. Although the principal modes of transmission for several
types of viruses, such as hepatitis B virus and poliovirus, are through food, personal
contact, or exchange of body fluids, viruses can also be transmitted through potable
water. Some viruses, such as the retroviruses (including the HIV group), appear to be
too fragile for water transmission to be a significant danger to public health. 1 Viruses
are not classified by species, but rather are grouped by structural characteristic. Table
4-2 lists the viral growth of interest in drinking water, the associated diseases, and
disease pathology.
Protozoa
Protozoa are single-cell eucaryotic microorganisms without cell walls that utilize bac-
teria and other organisms for food. They are typically much larger than bacteria, rang-
ing in size from 2 to 15 m. Most protozoa are free-living in nature and can be
encountered in water. However, several species are parasitic and live on or in host
organisms. Host organisms can vary from primitive organisms, such as algae, to highly
complex organisms, such as human beings. Several species of protozoa known to
utilize human beings as hosts are shown in Table 4-3.
The protozoans of most concern in drinking water are Giardia lamblia and Cryp-
tosporidium parvum . They are typically found in water as resistant spores, cysts, and
oocysts. These forms protect them from environmental stresses and make them far
more resistant to chlorination than are viruses and most bacteria. However, effective
filtration and pretreatment can reduce their density by at least 99 percent. Spores /
cysts / oocysts of these organisms are typically found in surface water or groundwaters
directly influenced by surface waters. The serious consequences of distributing water
contaminated with these organisms is discussed in the next section.
WATERBORNE DISEASE OUTBREAKS
The number of disease outbreaks identified as waterborne in the United States dropped
steadily following the introduction of potable water chlorination. Figure 4-1 shows
the dramatic decline in waterborne disease outbreaks from 1920 through 1960 as the
practice of chlorination became widespread. Of course, improvements in other treat-
ment procedures (flocculation, filtration, etc.) have contributed to the decrease in
waterborne disease. However, disinfection is, and will continue to be, one of the most
important treatment process for the prevention of waterborne diseases.