TM 5-814-3/AFM 88-11, Volume III
CHAPTER 17
DISINFECTION
17-1. General considerations.
Disinfection is a process in which pathogenic organisms are destroyed or inactivated. This process may be
accomplished by physiochemical treatment or addition of chemical reagents. Improved coliform and virus
removal can be obtained by utilizing flash mixing and acid feed for pH reduction. Chlorine, as liquid chlorine
or in the form of chlorine compounds, is the most common chemical used to disinfect wastewater treatment
plant effluents. Calcium hypochlorite or sodium hypochlorite will only be used as chlorinating agents for very
small installations (less than 0.02 million gallons per day). Ozone has been an effective disinfectant when used
in the water treatment field and its use as a disinfectant for wastewater is being seriously considered. This
interest has developed mainly because zonated effluents have normally shown no toxic effects on the
receiving water biota as have residual chlorine compounds; however, for certain industrial wastes, epoxides
have been found. The major disadvantage of ozonation is the high capital and operational cost associated with
its generation.
17-2. Chlorination.
a. Purposes of chlorination. It is recommended that unless dechlorination is required, chlorine or chlorine
derivatives be used for disinfection of wastewater. The principal purposes of wastewater chlorination include:
(1) Disinfection of primary, secondary, and advanced or tertiary plant effluent.
(2) Oxidation of ammonia and organic matter contribution to biochemical oxygen demand.
(3) Destruction and control of iron-fixing bacteria and slime-forming bacteria.
(4) Destruction and control of filter flies and slime growth on trickling filters.
(5) Control of septic conditions and resulting odors.
(6) Control of algae and related organisms.
It should be noted that chapter 14 of this manual provides a detailed discussion on chlorination of waste pond
effluent since pond effluent has unique chlorine demand requirements. The discussion in this chapter should
complement that in chapter 14.
b. Limitations of chlorine. Although chlorine is an effective disinfectant when in actual contact, the
chlorine may not always come in contact with the microorganisms that are inside the organic matter. There
is a danger of a false security, which prevails among the general public (and, to some extent, among engineers
and plant operators), in the impression that chlorine will remove all health hazards from wastewater. Chlorine
disinfection involves a very complex series of events and is influenced by the kind and extent of reactions with
chlorine-reactive materials, temperature, pH, suspended solids concentrations, and the viability of test
organisms. It is essential that the design of the various elements of the treatment plant be such that effective
treatment will reduce the need for disinfection to a minimum. Both coliform and virus removal can be
improved over conventional practices by acid feeding to reduce the pH to between 5.3 and 5.8 and by
providing flash mixing of the chlorine and wastewater.
c. Design parameters. EPA guidelines for chlorination of wastewater treatment plant effluent require
a detention period of 30 minutes in the contact chamber to provide maximum disinfection. Table 17-1 should
be used to estimate chlorine dosage requirements.
17-1
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