Environmental Microbiology of Aquatic and Waste Systems

224 9 Municipal Purification of Water

(d) Cyanide
The destruction of cyanide by chlorine is most
effective at high pH values (8.5–9.5), which can be
created when necessary by the addition of NaOH.
Sodium cyanate is produced, which decomposes,
releasing nitrogen.

2

2

2Cl 4NaOH 2NaCN 2NaCNO 4N

(sodiu

aCl

2H O m cyanate)

++→ +

+

The complete reaction is thus:

(e) Manganese
Chlorine reacts with manganese at pH 7–10, thus

(f) Methane
Methane is produced by anaerobic bacteria.
Besides its unpleasant odor, methane could be
explosive. It is best removed by aeration, but chlo-
rine also reacts with it.

9.3.7.2 The Present Practice of Water

Chlorination

The current practice of chlorination is to add sufficient

chlorine to oxidize all organic matter, iron, manganese,

and other reducing substances in water as well as oxi-

dize free ammonia in raw waters, and leave unused

chlorine as free residual chlorine rather than the less

active combined residual chlorine or chloramines. The

process is known as “super-chlorination,” “break-point

chlorination,” or “free residual chlorination” (see

Fig. 9. 2 ).

The reactions of chlorine with progressively large

doses of chlorine are shown in Fig. 8.2. Free residual

chlorine is not molecular chlorine reacting as a dis-

solved gas except at pH values below 5. When chlorine

is dissolved in water, it dissociates to form two germi-

cidal compounds, hypochlorous acid (HOCI) and the

chlorite ion (OCI). The undissociated molecule of

hypochlorite ion is 100 times more germicidal than the

chlorite ion. At pH 5.6, the chlorine forms hypochlo-

rous and hypochloric acid, thus:

Free residual chlorine

In earlier chlorination practice, only 0.1–0.2 ppm.

of chlorine was added. The result was that though there

33 ( ) ( )

22

2FeCl 3Ca HCO 2 2Fe OH 3

6CO 3CaCl

+→

++

23

2

5Cl 10NaOH NaCN 2NaHCO

10NaCI N 4H O

+ +→

+ ++

42 2

24 2

MnSO Cl 4NaOH MnO 2NaCI

Na SO 2H O

++ → +

++

2CH4 22++ → +Cl H O 4HCl H O 2

Cl 22 +→+H O HCl HClO

HClO→+H ClO

I = Destruction of chlorine by reducing compounds; no residual chlorine and no

disinfection

II = Formation of chloro-organic compound and chloramines, used in earlier

chlorination practice

III = Destruction of chloramines and chloro-organic compounds; breakpoint zone;

chloramines oxidized.

IV = Formation of free chlorine and presence of presence of undestroyed chloro-

organic compounds

Fig. 9.2 Modern (breakpoint) chlorination (Modified from Tebbutt 1992 ; http://water.me.vccs.edu/concepts/chlorchemistry.html))
(Anonymous 2010 b)