WATER QUALITY - Water Quality Engineering in Natural Systems - page 21

hepatitis, gastroenteritis, upper respiratory infec-
tions, and skin rashes. A practical consideration in
the case of viruses is the fact that they are more
resistant to standard water treatment procedures
than are the commonly used indicator organisms.
Therefore, public water supplies and recreational
waters that appear safe based on standard bacte-
rial indicators may in fact contain dangerous con-
centrations of viruses.
Bacteria are single-celled microorganisms that have
a cell wall and a single strand of DNA. Bacteria
are members of the Monera kingdom, reproduce
by binary fission, are not photosynthetic (like
plants), and do not need sunlight to reproduce.
They do not have a well-defined nucleus and do
not contain chlorophyll. The more common shapes
of bacteria are spheres, rod-shaped, spiral and
branching threads, and filamentous. Bacteria sizes
are in the range of 0.1-10 µ m in diameter and
2-4 µ m in length. Most have flagella, a tail-like
structure for movement. Bacteria are responsible
for waterborne diseases, such as cholera and
typhoid. Pathogenic bacteria of particular concern
to humans are Salmonella , Escherichia coli ,
Shigella , and Legionella . E. coli bacteria are
illustrated in Figure 2.5. Opportunistic bacterial
pathogens are not normally a danger to persons in
good health, but they can cause sickness or death
in those who are in a weakened condition. Particu-
larly at risk are newborns, the elderly, and persons
who already have a serious disease. Included
among the opportunistic bacteria are Pseudomo-
nas spp., Aeromonas hydrophila , Edwardsiella
tarda , Flavobacterium , Klebsiella , Enterobacter ,
Serratia , Proteus , Providencia , Citrobacter , and
Acinetobacter .

Protozoans are single-celled microorganisms with a
nucleus but without a cell wall. Protozoans are
members of the Protista kingdom, reproduce by
fission, and feed on bacteria. Protozoans are typi-
cally in the size range of 5 µ m-2 cm, and scientists
have identified approximately 40,000 species of
protozoans. Numerous protozoan species inhabit
the intestinal tracts of warm-blooded animals,
including humans. Giardia lamblia and Cryptospo-
ridium parvum are the protozoans of most concern
in drinking water supply sources. These protozo-
ans, shown in Figure 2.6, are described briefly
below.
Giardia lamblia. Giardia lamblia is a protozoan of
length 9-17 µ m and width 7-12 µ m that causes
the disease giardiasis (also known as “beaver
fever”). Symptoms of giardiasis include skin

Figure 2.5. Escherechia coli ( E. coli ) bacteria. Source : State
of Michigan (2005).

(a) (b)

Figure 2.6. (a) Cryptosporidium parvum ; and (b) Giardia lamblia . Source : State of Michigan (2005).