Page 509 - Water and wastewater engineering

13-4 WATER AND WASTEWATER ENGINEERING

and

.
.
.
[HOCl 3 47 (22 37% ) 77 6%
]

Comments:
1. Note that the concentration of HOCl was not used in the problem solution!
2. For different temperatures, the percentages will be different because K a is a function of
temperature.

Hypochlorite salts dissociate in water to yield hypochlorite ions:

NaOCl Na OCl (13-3)

Ca OCl) 2 Ca 2 2 OCl (13-4)
(

The hypochlorite ions establish equilibrium with hydrogen ions (in accord with Equation 13-2 ).

Thus, the same active chlorine species (HOCl and OCl ) and equilibrium are established in water
regardless of whether elemental chlorine or hypochlorites are used. The significant difference

is in the resultant pH and its influence on the relative amounts of HOCl and OCl existing at
equilibrium. Elemental chlorine tends to decrease pH; each mg/L of chlorine added reduces the
alkalinity by up to 1.4 mg/L as CaCO 3 . Hypochlorites, on the other hand, always contain excess
alkali to enhance their stability and tend to raise the pH somewhat. To optimize disinfecting action,
the design pH is in the a range 6.5 to 7.5.
Free chlorine is relatively stable in pure water. It reacts slowly with naturally occurring organic
matter (NOM) and rapidly with sunlight. The photolytic reaction is with hypochlorite. The reac-
tion products are oxygen, chlorite ion, and chloride ion (Buxton and Subhani, 1971).

Chlorine/Ammonia Reactions. The reactions of chlorine with ammonia are of great signifi-
cance in water chlorination processes. When chlorine is added to water that contains natural or
added ammonia (ammonium ion exists in equilibrium with ammonia and hydrogen ions), the
ammonia reacts with HOCl to form various chloramines. The reactions between chlorine and
ammonia may be represented as follows (AWWA, 2006):

NH 3 HOCl NH Cl H O (13-5)
2
2
Monochloramine

NH Cl HOCl NHCl 2 H O (13-6)
2
2
Dichloramine
NH Cl 2 HOCl NCl 3 H O (13-7)
2
2
Trichloramine