Water treatment is any process that improves the quality of water to make it appropriate for a specific end-use. The end use may be drinking, industrial water supply, irrigation, river flow maintenance, water recreation or many other uses, including being safely returned to the environment. Water treatment removes contaminants and undesirable components, or reduces their concentration so that the water becomes fit for its desired end-use. This treatment is crucial to human health and allows humans to benefit from both drinking and irrigation use.

1. WATER SUPPLY ENGINEERING
(ENVIRONMENT ENGINEERING I)
CHLORINATION OF WATER
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ASST. PROF. PRACHI DESSAI

2. Chlorination
Addition of chlorine disinfectant to drinking-water to kill or inactivate microorganisms
Chlorine is a powerful disinfectant that kills most harmful microorganisms associated with waterborne disease
Chlorine does not kill all harmful microorganisms
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MOST EFFECTIVE EFFECTIVENESS OF CHLORINE
LEAST EFFECTIVE
BACTERIA VIRUSES PROTOZOA
ASST. PROF. PRACHI DESSAI

3. Properties of chlorine
Chlorine may be found in three forms
Powder Liquid Gas
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ASST. PROF. PRACHI DESSAI

4. Powder
Properties of chlorine: Powder
I. Appearance: White powder, granules or tablet
II. Strength: Requires mixing with water to make a
liquid solution before use (see ‘Chlorine liquid’
below)
III. Stability: May lose strength over time; more stable
than chlorine liquid
IV. Application: Typically used for small-sized water
treatment plants (< 5 000 cubic meters per day)
V. Examples: Bleaching powder (approx. 35 % active
chlorine)
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ASST. PROF. PRACHI DESSAI

5. Properties of chlorine: Liquid
I. Appearance: Pale yellow to clear liquid
II. Stability: May lose strength over time; less stable than
chlorine powder
III. Application: Typically used for small- to medium-sized
water treatment plants (i.e., <10 000 cubic meters per
day)
IV. Examples:
Sodium hypochlorite (10 to 15 % active chlorine)
Domestic bleach (5 to 10 % active chlorine)
Chlorine liquid solution prepared from chlorine powder
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Liquid
ASST. PROF. PRACHI DESSAI

6. Properties of chlorine: Gas
I. Appearance: Green-yellow gas
II. Stability: Most stable
III. Application: Typically used for medium- to large-
sized water treatment plants (i.e., >10, 000 cubic
meters per day)
IV. Example: Chlorine gas (liquefied)
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Gas
ASST. PROF. PRACHI DESSAI

7. Types of chlorination
The different types of chlorination adopted in water treatment are as follows:
Plain chlorination
Pre-chlorination,
Post-chlorination,
Break point chlorination,
Super chlorination,
De-chlorination.
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ASST. PROF. PRACHI DESSAI

8. TYPES OF CHLORINATION
Plain chlorination:
i. When the raw water contains turbidity less than 10 NTU, obtained from unpolluted lakes or
reservoirs, the water could be supplied to the public without any treatment except chlorination.
ii. Such chlorination is called plain chlorination. The dosage of chlorine for plain chlorination is
about 0.5 mg/L.
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ASST. PROF. PRACHI DESSAI

9. TYPES OF CHLORINATION
Pre-chlorination
i. Addition of chlorine to the raw water prior to treatment to produce residual chlorine after
meeting chlorine demand.
ii. Aids in coagulation, controlling algae problems in sedimentation basins, reducing odor
problems, and controlling mud-ball formation in filters.
iii. Increases safety in disinfecting heavily contaminated water.
iv. Pre-chlorination is generally applied to the water before coagulation.
v. It improves the coagulation and reduces load on filters. It also reduces taste, colour, odour, algae
and other organisms.
vi. The chlorine dose for pre-chlorination should be 0.1 to 0.5 mg/L.
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ASST. PROF. PRACHI DESSAI

10. TYPES OF CHLORINATION
Post chlorination
i. Applying chlorine in the end, when all other treatments are completed but before the water reaches the
distribution system.
ii. At this stage, chlorination is meant to kill pathogens and to provide a chlorine residual in the
distribution system.
iii. Part of the treatment process, either used in combination with prechlorination or used as the sole
disinfection process.
iv. Leave a residual-chlorine of about 0.1 to 0.2 mg/L after a contact period of 20 to 30 minutes.
v. This residual chlorine will ensure the disinfection of water if at all any recontamination occurs in the
transmission and distribution system.
vi. Chlorine dose should not be generally greater than 2.0 mg/L as the excess residual concentration of
chlorine may damage the pipelining and pump impellers.
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ASST. PROF. PRACHI DESSAI

11. TYPES OF CHLORINATION
Super chlorination
i. Indicates the addition of excessive amount of chlorine (i.e., 5 to 15 mg/L) to the water.
ii. Special cases when the water is highly polluted, or during epidemics of water borne diseases.
iii. The huge quantity of chlorine, which is added in super chlorination, is such as to give about 1 to 2
mg/L of residual beyond the break point in the treated water.
iv. Sometimes even higher doses may be used and the resultant -water is rechlorinated after the end of the
desired contact period, by using dechlorinating agents.
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ASST. PROF. PRACHI DESSAI

12. TYPES OF CHLORINATION
De-chlorination
i. De-chlorination means removing the chlorine from the water.
ii. This is generally required when super-chlorination has been practiced.
iii. The de-chlorination process may either be carried out to such an extent that sufficient residual
chlorine of 0.1 to 0.2 mg/L only remains in water after de-chlorination.
iv. The common de-chlorinating agents are sulphur dioxide gas, acivated carbon, sodium
thiosulphiate, sodium metasulphate, and ammonia.
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ASST. PROF. PRACHI DESSAI

13. Breakpoint Chlorination
The entire chlorine added will be utilized in
reacting with organic substances (including
bacteria). Hence there will not be any residual
chlorine (line AB) as initial chlorine demand.
When chlorine is further added to water, it
reacts with the ammonia present in the water, so
as to produce combined chlorine residual
(chloramines).
The combined chlorine residual, increases with
addition of dosage (curve BC) until a maximum
combined residual is reached (point C).
ASST. PROF. PRACHI DESSAI 13

14. Breakpoint Chlorination
If the addition of the chlorine is continued
beyond the point C, the chlorine reacts with
organics and ammonia naturally, found in the
water and therefore, the residual chlorine content
suddenly falls down, as shown in the curve CD.
The point D at which the total chlorine demand
is satisfied, as any chlorine added to water
beyond this point, breaks through the water and
appears as residual chlorine. This point D is
called break point.
ASST. PROF. PRACHI DESSAI 14

15. Breakpoint Chlorination
The addition of chlorine beyond break point is
called break point chlorination. The residual of
free chlorine, appearing after break point, is not
usually removed except by sun light and
therefore, it takes care of the future
recontamination of water.
The breakpoint chlorination is the most
common form of chlorination, in which enough
chlorine is added to the water to bring it past the
breakpoint and to create some free chlorine
residual.
ASST. PROF. PRACHI DESSAI 15

16. Recommended Readings
i. Mark J. Hammer; Water and Waste Water Technology; Prentice Hall of India.
ii. S. K. Garg; Water Supply Engineering; Khanna Publ.
iii. B. C. Punmia, A. K. Jain; Water Supply Engineering; Laxmi Publication.
iv. G. S. Birdie; Water Supply Engineering and Sanitary Engineering; Dhanpat Rai.
v. R. C. Rangwala; Water Supply Engineering, Charotar Publ. House.
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