Page 652 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

19 Disinfection

The term, disinfection, refers to the inactivation of microor- sodium chloride and water as raw material reactants, to form
ganisms. By inference, the inactivation is not 100%, but could Cl 2 , HOCl, ClO 2 , and O 3 as products at the anode of a cell that
be 1-log, 2-log, 3-log, or even 6-logs depending on the initial takes up the electrons produced (MIOX, 2003). The disinfect-
concentration of organisms, detection limit, and other ants that produce a residual include chlorine, chlorine dioxide,
factors. The idea is explained further in Box 19.1 and in the and chloramines. Ozone and UV do not hold residuals.
glossary.
This chapter reviews the fundamentals of disinfection, the
characteristics of each major disinfectant, and the highlights 19.2 HISTORY
of disinfection practice. Each of the disinfectants has its own
The practice of disinfection started about 1900, some 30 years
body of literature and adherents.
after the discovery of microorganisms. This was almost 50
years after Dr. John Snow and coworkers, who in 1854 linked
19.1 FUNDAMENTALS cholera and sewage contamination to the Broad Street Well in
London, establishing, at the same time, a methodology for
Basic notions of microorganisms, diseases, and disinfectants
epidemiology investigations. In 1886, Koch conducted the
are reviewed here. As seen, the idea of disinfection of water
first experiments on disinfection by using pure cultures of
has its roots in the science of microbiology, founded about
bacteria. His work was reviewed by Harriette Chick in her
1880.
1908 paper on the law of disinfection, that is, that
dN=dt ¼ kt, in which k was found to vary with the disinfect-
19.1.1 MICROORGANISMS AND DISEASES ant and with organism, which was the beginning of modern
theory.
Table 19.1 lists three groups of microorganisms, that is,
viruses, bacteria, and protozoa that are causes of waterborne
diseases. Most disease-causing organisms occur in ambient 19.2.1 CHLORINE
waters through fecal contamination, for example, from runoff
Chlorine was discovered in 1774 by a Swedish chemist,
of fecal matter, from land, or from sewage discharges. Cross-
Scheele (Tiernan, 1948, p. 1042). The Greek word for
connections are a possible source of contamination of treated
green, ‘‘chloros’’ was adopted as being descriptive of the
water.
color of the gas. The first continuous application of chlorine
As another variation of microbial contamination, some
bacteria, under conditions of stress, may form ‘‘endospores’’ was in 1908 as sodium hypochlorite, NaOCl at Jersey City,
New Jersey (Doull, 1980, p. 18). Its first use for wastewater
(spores) that may be able to resist heat, chemicals, radiation,
disinfection was in Hamburg in relation to the 1893 cholera
etc., and may survive for years, decades, or even centuries in
epidemic (Isaac, 1996, p. 68). By 1910, of the 619 POTW’s
some cases. The endospores of Bacillus anthracis may remain
(POTW is an acronym from the 1972 Clean Water Act, PL92-
viable for years (Prescott et al., 1993, p. 552). Similarly, some
500, meaning ‘‘publically operated treatment works’’), 22% or
protozoa, for example, aquatic, freeliving, or parasitic forms,
3.6% used chlorine; the percentage grew to 94.9% of 15,000
may ‘‘encyst,’’ which gives protection against environmental
POTW’s by 1979, but declined to 78% in 1990, with 19%
stresses (nutrient deficiency, desiccation, adverse pH, low
using UV (Isaac, 1996, p. 68).
oxygen partial pressure), and provide as a means to transfer
between hosts. Excystation is triggered by a return to favor-
able environmental conditions, for example, ingestion by a 19.2.1.1 Story of Chlorine
host (Prescott et al., 1993, p. 550). As described by Tiernan (1948, p. 1042), the story of chlorine
begins with the Electro Bleaching Gas Co., which was founded
by E.D. Kingsley (who, just prior, in 1906, was superintendent
19.1.2 DISINFECTANTS
of a department store). Kingsley had invested $1000 in a
An array of disinfectant chemicals are available and include chlorine gas–conversion enterprise in Philadelphia; when the
chlorine, ozone, chlorine dioxide, iodine, bromine, and promoter disappeared, he took over the company and went to
UV254 radiation. Others that have been mentioned (Chick, Germany to learn about the method of producing chlorine gas.
1908) include mercuric chloride, silver nitrate, phenol, perox- He returned with a German chemical engineer, imported chlor-
ide, etc. Also included are proprietary disinfectants, for ine-compressing equipment from Germany, and set up the
example, MIOXt, which generates disinfectants onsite using Electro Bleaching Gas Co. next to the Niagara Alkali Co. at

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