Fundamental Principles of Sewage
Chlorination
CHESTER E.  RHINES
Microbiology Section
Basic and Applied Sciences Branch
Division of Water Supply and Pollution Control
U. S. Department of Health, Education, and Welfare
Public Health Service
Robert A. Taft Sanitary Engineering Center
Cincinnati, Ohio
OBJECTIVE
Chlorination has long been considered to have the greatest practical potential
of all disinfection systems for freeing sewage of pathogens. An extremely powerful disinfectant, chlorine operates against all micro-organisms, although there are
marked variations in susceptibility. The great versatility and reliability of chlorine for protection against pathogens have been amply demonstrated in water
supply systems; however, the situation has been very different in sewage chlorination practice. Sewage usually contains many substances that limit the activity
of chlorine, and chlorine treatment has been found to be of little or no benefit in
many plant operations. The Microbiology Section, Basic and Applied Sciences
Branch, Division of Water Supply and Pollution Control, initiated this study to
learn the means of achieving reliable disinfection. The problem has achieved
considerable importance because of extensive use of receiving waters for recreation. This demand for safe recreational waters is constantly increasing, particularly in the large metropolitan areas, where the hazard is greatest.
FACTORS AFFECTING CHLORINE DISINFECTION
Chlorine is an extremely reactive chemical. It irreversibly oxidizes many
common inorganic and organic components of sewage and is itself thereby inactivated. It can combine with many other sewage constituents, forming compounds with markedly reduced disinfection activity. The chlorine that escapes
immediate reduction to chloride in raw domestic sewage forms chloramine compounds. Chlorine is an extremely active disinfectant under acidic conditions, but
much less active under alkaline conditions. Ammonia content and high pH conditions do not destroy chlorine disinfection, but reduce it to an impractically
slow status. Neutralization of alkaline sewage to aid primary effluent chlorination
has not come into use because of excessive cost.
Chlorine must come into contact with a pathogen to destroy it. Domestic
sewage is characterized by the presence of chunks and floes. Organisms within
these chunks and floes are protected. It is thus essential to disperse the particulate
material or remove it prior to chlorination if disinfection is to proceed at a
reasonable rate with a practical chlorine dose. Settling, as utilized to remove
particles from sewage before chlorination in the primary treatment procedure, has
been outstandingly ineffective. Natural stabilizers and detergents interfere with
settling, and gas bubble formation in the unstable raw product causes rising floc-
culant material in a flotation process.
Low  temperatures  are commonly encountered in sewage during the winter
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