Page 510 - Water and wastewater engineering

DISINFECTION AND FLUORIDATION 13-5

The distribution of the reaction products is governed by the rates of formation of monochlora-
mine and dichloramine, which are dependent upon pH, temperature, time, and initial Cl 2 :NH 3 ratio.
In general, high Cl 2 :NH 3 ratios, low temperatures, and low pH levels favor dichloramine formation.
Chlorine also reacts with organic nitrogenous materials, such as proteins and amino acids, to
form organic chloramine complexes. Chlorine that exists in water in chemical combination with
ammonia, or organic nitrogen compounds, is defined as combined available chlorine or com-
bined chlorine. The sum of the concentrations of free chlorine and combined chlorine is called
total chlorine.
The oxidizing capacity of free chlorine solutions varies with pH because of variations in

the resultant HOCl:OCl ratios. This is also true for chloramine solutions as a result of varying
NHCl 2 :NH 2 Cl ratios. Monochloramine predominates at high pH levels.

Chlorine Dioxide. Chlorine dioxide is a stable free radical that, at high concentrations, reacts
violently with reducing agents. It is explosive with a lower explosive limit * (LEL) reported vari-
ously between 10 and 39 percent. Thus, virtually all applications require synthesis on-site. Chlorine
dioxide (ClO 2 ) is formed on-site by combining chlorine and sodium chlorite. One of three alter-
native reactions may be employed (MWH, 2005):

2NaClO Cl 2 g() 2ClO g () 2NaCl (13-8)
2 2

2NaClO HOCl 2ClO g () NaCl NaOH (13-9)
2
2

5NaClO 4HCl 4ClO g () 5NaCl 2H O (13-10)
2 2 2
Under alkaline conditions chlorine dioxide forms chlorite (ClO ) and chlorate (ClO ) ions

3
2
(Gordon et al., 1972):

3
2ClO 2OH H 2 O ClO ClO (13-11)
2 2
The typical reaction of chlorine dioxide in water is a one-electron reduction (Haas, 1999):

ClO 2 e ClO 2 (13-12)

Ozone. Ozone is a pungent, unstable gas. It is a form of oxygen in which three atoms of oxygen
are combined to form the molecule O 3 . Because of its instability, it is generated at the point of
use. Ozone may be generated by photochemical, electrolytic, and radiochemical methods, but it
is most commonly generated by a discharge electrode. Either pure oxygen, purchased as liquid
oxygen (LOX), or the oxygen in the air, is dissociated by the impact of electrons from the dis-
charge electrode. The atomic oxygen then combines with atmospheric oxygen to form ozone in
the following reaction:
O O 2
O 3 (13-13)

*Mixtures of gases with air that have concentrations above the LEL can be ignited.