eliminate the pathogenic bacteria in water. A
potable water supply requires zero or very
low bacterial concentration to avoid disease
transmission. The total number of coliforms,
instead of the presence of specific pathogens,
is often used as an indicator for sanitaryqual-
ity and the efficiency of disinfection. There
are many chemical disinfectants and physical
methods that can be incorporated for disin-
fection.
For public health reasons, treated waste-
waters should be disinfected before final dis-
charge. Addition of a chemical disinfectant
to water provides a maximal time of contact
between the chemical and organisms, assur-
ing efficient bactericidal action (Wang et al.,
2003). Less disinfection is required as a result
of the removal of microbes by primary and
secondary wastewater treatment and by
death of pathogenic microorganisms from
extended exposure to natural environments.
A variety of chemical disinfectants are avail-
able for use in water treatment. Examples are
chlorine, iodine, bromine, quaternary ammo-
nium, and ozone. Chlorine, as gaseous chlo-
rine or solid components such as calcium or
sodium hypochlorite is a most common
chemical used for disinfection due to low-
cost, high efficiency, and ease of application.
Pre-chlorination, or source water chlorina-
tion, is designed to minimize operational
problems associated with biological slime
formation on filters, pipes, and tanks and to
lessen potential taste and odor problems.
Post-chlorination, or terminal disinfection is
a primary exercise for microbial reduction in
product water. According to Haas (1990),
the addition of chlorine either immediately
before the clear well or immediately before
the sand filter is most common. Because of
the potential reaction of disinfectants with
organic matter, it is more practical to disin-
fect at the end of wastewater treatment.
Table 12–2, which relates the typical micro-
bial population and load in domestic waste-
water, illustrates the amount of contamina-
tion that can occur from wastewater of food-
processing operations.
Chemical oxidants; ultraviolet, gamma,
and microwave irradiation; and physical
methods, such as ultrasonic disruption and
thermal application, are used as disinfectants.
Chlorination has received less emphasis in
recent years because of potentially carcino-
genic organohalides in chlorinated waters.
In addition, over-chlorination of wastewater
effluents can be toxic to fish. Chlorination
and other chemical treatments do not kill all
microorganisms. Certain algae, spore formers,
and viruses (including pathogenic viruses)
survive chlorination treatment.
Antimicrobial agents, such as sanitizers,
incorporated in a food plant’s sanitation pro-
gram can present a challenge since they may
destroy microorganisms involved in waste-
water treatment. Sanitizers, pH, flow, BOD
loading, solids, temperature, and other toxic
materials, under certain conditions, can
adversely affect the operation of a waste treat-
ment plant. However, when sanitizers are
properly used according to label directions,
they do not normally interfere with the delicate
microbial process in most treatment plants.
Yet, accidental or mass discharge due to a spill
of any sanitizer or chemical may complicate
the treatment process. The sanitizer used by232 PRINCIPLES OFFOODSANITATION
Table 12–2Microbial Characteristics of Domes-
tic WastewaterQuantity per 100 mL
Microorganism Wastewater
Total bacteria 109 -10^10
Coliforms 106 -10^9
Fecal streptococci 105 -10^6
Salmonella typhosa 101 -10^4
Viruses (plaque- 102 -10^4
forming units)
Source:Arceivala, 1981.