222 9 Municipal Purification of Water
that, between 1824 and 1826, chlorine had been intro-
duced into hospitals, especially in obstetrical wards for
the prevention of puerperal fever. A little later, in about
1831, it had been used in water (but not on a continu-
ous scale) during the great cholera epidemic in Europe.
Today, most water disinfection around the world is
done with chlorine (see Table 9. 2 ).
Chlorine has a number of advantages:
(a) It is easily available as a gas (or in compounds as
liquid or powder).
(b) It is reasonably cheap.
(c) It is highly soluble (700 mg/l).
(d) It leaves a residue in solution which, while not
harmful to man, provides protection in the distri-
bution system.
(e) It is highly toxic to microorganisms.
(f) It has several important secondary uses, e.g., oxi-
dation of iron, manganese, and H 2 S, destruction of
some taste- and odor-producing compounds, and
acts as an aid to coagulation.
However, some of its shortcomings must be borne
in mind:
(a) It is a poisonous gas and must be carefully
handled.
(b) It can itself give rise to taste and odor problems,
particularly in the presence of phenol (hence
ammonia was once added to form chloramines,
which have less odor problems).
(c) Suspended materials may shield bacteria from the
action of chlorine.
(d) Chlorine is a powerful oxidizing agent and will
attack a wide range of compounds, including
unsaturated organic compounds as well as reduce
substances, which are found in water, thereby
making it less available to attack microorganisms.
(e) The effectiveness of chlorination is pH-dependent;
chlorination is more effective at pH values of 7.2
and below than above pH 7.6.
There are alternative methods of disinfecting water
and they are compared with chlorine in Table 9.3.9.3.7.1 Reactions of Chlorine with Chemicals
Found in Water
When water is chlorinated, a large number of chemi-
cals may be present in it, especially if the water is
wastewater or effluents from sewage treatments. These
chemicals include nitrogenous compounds (especially
ammonia), carbonaceous compounds, nitrites, iron,
manganese, hydrogen sulfide, and cyanides. Chlorine
combines with many of these compounds in the fol-
lowing manner.
(a) Nitrogen containing compounds
The compounds formed by the reaction of chlo-
rine and a nitrogen-containing compound are
chloramines, which could be either inorganic or
organic.- Ammonia: This is the most important inorganic
compound, which reacts with chlorine; others
are nitrates and nitrites. Chlorine reacts in
dilute aqueous solutions (1–50 ppm.) to form
three choramines:
These chloramines, also known as com-
bined residuals, have disinfectant properties,
but they are far less effective than chlorine,
requiring about ten times the contact time of
chlorine. Furthermore, they confer odor and
taste to water.
The pH of the water determines the relative
amounts of the three kinds of chloramines. At
pH 8.5, monochloramine is the major product.
If it is low, e.g., below pH 4.4, virtually all the
chloramine is in the form of nitrogen trichlo-
ride (NCl 3 ), which imparts a bad taste to drink-
ing water and causes eye irritation in swimming
pools. At the same time, it hardly disinfects.Cl23 2NH NH Cl HCl
Monochloramine+→ +
NH Cl NHC1 HCl 22 2
Dichloramine+→ +
( )
NH 22 Cl NCl 3 HCI
Trichloramine Nitrogen trichloride+→ +
Table 9.2 Usage of chlorine as a water disinfectant in compari-
son with others (From The American Chemistry Council. http://
http://www.americanchemistry.com/s_chlorine/sec_content.asp?CID
=1133&DID=4530&CTYPEID=109. With permission)
(Anonymous 2010 a)
Disinfection
Large systems
(>10,000 persons)Small systems
(<10,000 persons)
Chlorine gas 84% 61%
Sodium hypochlorite 20 34
Calcium hypochlorite <1 5
Chloramines 29 –
Ozone 6 –
UV – –
Chlorine dioxide 8 –