Science of Water : Concepts and Applications

134 The Science of Water: Concepts and Applications
DESTRUCTION OF BACTERIA
In water treatment, the destruction of bacteria is usually called disinfection.
√ Important Point: Inhibiting the growth of microorganisms is termed antisepsis, while
destroying them is called disinfection.
Disinfection does not mean that all microbial forms are killed. That would be sterilization.
However, disinfection does reduce the number of disease-causing organisms to an acceptable number.
Growing bacteria are easy to control by disinfection. Some bacteria, however, form spores—
survival structures—which are much more diffi cult to destroy.
WATERBORNE BACTERIA
All surface waters contain bacteria. Waterborne bacteria, as mentioned, are responsible for infectious
epidemic diseases.
Bacterial numbers increase signifi cantly during storm events when streams are high. Heavy
rainstorms increase stream contamination by washing material from the ground surface into the
stream. After the initial washing occurs, few impurities are left to be washed into the stream, which
may then carry relatively “clean” water. A river of good quality shows its highest bacterial numbers
during rainy periods; however, a much-polluted stream may show the highest numbers during low
fl ows because of the constant infl ux of pollutants.
Water operators are primarily concerned with bacterial pathogens responsible for disease. These
pathogens enter potential drinking water supplies through fecal contamination and are ingested by
humans if the water is not properly treated and disinfected.
√ Note: Regulations require that owners of all public water supplies collect water samples and
deliver them to a certifi ed laboratory for bacteriological examination at least monthly. The number
of samples required is usually in accordance with Federal Standards, which generally require
that one sample per month be collected for each 1000 persons served by the waterworks.
PROTOZOA
Protozoans (or “fi rst animals”) are a large group of eukaryotic organisms of more than 50,000 known
species belonging to the kingdom Protista that have adapted a form of cell to serve as the entire body.
In fact, protozoans are one-celled, animal-like organisms with complex cellular structures. In the
microbial world, protozoans are giants, many times larger than bacteria. They range in size from
4 µm to 500 µm. The largest ones can almost be seen by the naked eye. They can exist as solitary
or independent organisms (for example, the stalked ciliates [see Figure 5.5] such as Vorticella
sp.), or they can colonize like the sedentary Carchesium sp. Protozoa get their name because they
employ the same type of feeding strategy as animals; that is, they are heterotrophic, meaning they
obtain cellular energy from organic substances such as proteins. Most are harmless, but some are
parasitic. Some forms have two life stages: active trophozoites (capable of feeding) and dormant
cysts.
The major groups of protozoans are based on their method of locomotion (motility). For
example, the Mastigophora are motile by means of one or more fl agell a (the whip-like projection
that propels the free-swimming organisms—G. lamblia is a fl agellated protozoan); the Ciliophora
by means of shortened modifi ed fl agella called cilia (short hair-like structures that beat rapidly and
propel them through the water); the Sarcodina by means of amoeboid movement (streaming or
gliding action—the shape of amoebae change as they stretch, then contract, from place to place);
and the Sporozoa, which are nonmotile; they are simply swept along, riding the current of the
water.