Water Treatment - The Drinking Water - page 240

The unit processes described thus far—screening, coagulation, floccula-
tion, sedimentation, and filtration—together comprise a type of treatment
called clarification . Along with removing turbidity and suspended solids,
clarification also removes many microorganisms from the water; however,
clarification by itself is not sufficient to ensure the complete removal of
pathogenic bacteria and viruses.
Earlier we stated that one of the primary goals of water treatment is to treat
raw water to the point that we can deliver to the consumer a water product
that is perceptually crystal clear. Obviously, the consumer does not want to
drink a glass full of mud, a glass full of slime, a glass full of metal-colored,
foul-smelling water—or even a glass of water that looks like it was dipped
from a creek. Would you? The point is, when the water has been treated to the
point of crystal clarity, the treatment process must be taken a step further—
to the point where the water is completely free of disease-causing microor-
ganisms. To accomplish this, the final treatment process in water treatment
plants occurs— disinfection , which destroys or inactivates pathogens.
Key Disinfection Terms*
Before we move on to a discussion of the major disinfection methods used in
treating water for human consumption, we must first define a few pertinent
terms related to disinfection in general. To begin with, we need to establish
the distinction between primary and secondary disinfection:
• Primary disinfection —The initial killing of Giardia cysts, bacteria, and
viruses
• Secondary disinfection —The maintenance of a disinfectant residual
that prevents regrowth of microorganisms in the water distribution
system between treatment and consumer
Other terms the reader should understand include
• Disinfection —T he inactivation of virtually all recognized pathogenic
microorganisms, but not necessarily all microbial life (which would
be considering pasteurization or sterilization).
• Disinfectant —(1) Any oxidant, including but not limited to, chlorine,
chlorine dioxide, chloramine, and ozone, added to water in any part
of the treatment or distribution process that is intended to kill or
inactivate pathogenic microorganisms. (2) A chemical or physical
process that kills pathogenic organisms in water; chlorine is often
used to disinfect sewage treatment effluent, water supplies, wells,
and swimming pools.
* Adapted from Spellman, F.R., Disinfection Alternatives , Technomic, Lancaster, PA, 1999.