5.
6 DISINFECTION
Objective
- to destroy pathogenic microorganisms
- to produce safe drinking water
2
Disinfectant Properties
destroy bacteria/pathogens within a practicable
period of time
no negative effect
not change water properties
have residual in a sufficient concentration to provide
protection against recontamination
easily determined in the treated water
reasonable cost
safe and easy to store, transport, handle and apply
3
Disinfections Method
Heat
very effective
suitable for small scale
Ozone
more effective than chlorine
no residual remain it is necessary to use a
small amount of chlorine after ozonation to
provide continued protection against
regrowth in the distribution system
high cost
4
Disinfections Method
Ultraviolet Radiation
effective
leaves no residual in water
Chlorination
the most common application
available in granular, powdered, liquid or
gases forms
have residual in water
production of Trihalomethanes (THMs
carcinogens)
5
Chlorine Building
6
Chlorina4on
Filtration water before
treated with chlorine
Water after treated with
chlorine
Chlorina4on
Pipe with water
added with chlorine
Contains pipe with
water from
filtration tanks
Chlorina4on
Cylinders of chlorine gas
- Each cylinder 930kg, can
be used for 11-12 days, with
4kg of usage each hour.
- Contact time 20 minutes
- Using 0.6 ppm of chlorine
- Treat 36600m3 water per day
Chlorina4on Equipment
Chlorinator
Chlorine Cylinders
10
Chlorine combines with water
H 2O + Cl2 HCl + HOCl
HOCl H+ + -OCl (pH > 8)
H+ + -OCl HOCl (pH < 7)
HOCl hypochlorous acid
-OCl hypochlorite ion
HOCl is more effective than -OCl
11
The reaction of chlorine with ammonia
HOCl + NH3 H2O + NH2Cl
(monochloramine)
HOCl + NH2Cl H2O + NHCl2
(dichloramine)
HOCl + NHCl2 H2O + NCl3
(trichloramine)
12
The proportion of chloramines depends on:
- pH 4.5 8.5 - monochloramine and
dichloramine
> 8.5 - monochloramine
< 4.4 - trichloramine
- temperature
- time
13
- initial chlorine to ammonia ratio Cl2 : NH3
(molar)
ratio Cl2 : NH3 = 1:1 chloramine
ratio Cl2 : NH3 = 3:2 N2 gas
3Cl2 + 2NH3 N2 + 6HCl
14
Combination of chlorine with ammonia
nitrogen or organic nitrogen compounds is
defined as combined chlorine.
Combined chlorine less reactive
15
Factors affecting the process are:
types of chlorine used
pH
contact time (20 min.)
presence of other reducing agents
such as suspended solids, iron, and
manganese.
16
Disinfection practice is based on
establishing a given amount of
chlorine during treatment and then,
maintaining an adequate residual.
- chlorine effectively kill bacteria
- protozoans and viruses are more
resistant to chlorine
17
Breakpoint Chlorination Curve
18
Breakpoint Chlorination Curve
As chlorine is added, readily oxidizable substances, such as Fe+2, Mn
+2, H2S, and organic matter, react with the chlorine and reduce most
of it to the chloride ion (Point A). After meeting this immediate
demand, the chlorine continues to react with the ammonia to form
chloramines between points A and B. Between point B and the
breakpoint, some chloramines will be converted to nitrogen
trichloride (NCl3), the remaining chloramines will be oxidizes to nitrous
oxide (N2O) and nitrogen (N2), and the chlorine will be reduced to the
chloride ion. With continued addition of chlorine, most of the
chloramines will be oxidized at the breakpoint. The main reason for
adding enough chlorine to obtain free chlorine residual is that usually
disinfection can then be ensured.
19
Example 1
For disinfecting water supply, it is required to
treat 500,000 litres of daily supply with 0.5 ppm
of chlorine. If the disinfectant is available in
the form of bleaching powder containing
30% of available chlorine, calculate the
amount of bleaching powder (sodium
hypochlorite) required to treat the daily flow
of water.
20
Solution
0.5 ppm = 0.5 mg/L
Quantity of chlorine
= 500,000 L x 0.5 mg/L x 1 kg/106 mg
= 0.25 kg
For the bleaching powder with
available chlorine of 30%, amount
of bleaching powder required
= 0.25 kg x 100/30 = 0.833 kg
21
Example 2 :
An experimental test on chlorine
intake yields the following results
(table right):
Determine graphically,
the chlorine dosage at
breakpoint chlorina4on.
Calculate the chlorine
demand when the
chlorine dosage is 1.2
mg/L.
22
0.9
0.8
0.7
0.6
Cholirne Residual (mg/L)
0.5
0.4
0.3
0.2
0.1
0
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8
Chlorine Dosage (mg/l)
