PHYSICAL - CHEMICAL TREATMENT OF WATER AND WASTEWATER - CHAPTER 17 (END) ppt

Disinfection

Disinfection

is a unit process involving reactions that render pathogenic organisms
harmless. A companion unit process is sterilization.

Sterilization

refers to the killing
of all organisms. Sterilization is not often practiced in the treatment of water and
wastewater. Thus, this chapter will only discuss the unit process of disinfection. This
discussion will include methods of disinfection, factors affecting disinfection, and
the various disinfectants that have been used. Because chlorine is the most widely
used disinfectant, its chemistry will be discussed at length. The design of chlorination
unit operations equipment will also be discussed. The following disinfectants will
also be specifically addressed: ozone and ultraviolet light.

17.1 METHODS OF DISINFECTION AND
DISINFECTANT AGENTS USED

Generally, two methods of disinfection are used: chemical and physical. The chem-
ical methods, of course, use chemical agents, and the physical methods use physical
agents. Historically, the most widely used chemical agent is chlorine. Other chemical
agents that have been used include ozone, ClO

2

, the halogens bromine and iodine and
bromine chloride, the metals copper and silver, KMnO

4

, phenol and phenolic com-
pounds, alcohols, soaps and detergents, quaternary ammonium salts, hydrogen per-
oxide, and various alkalis and acids.
As a strong oxidant, ClO

2

is similar to ozone. (Ozone will be discussed specif-
ically later in this chapter.) It does not form trihalomethanes that are disinfection
by-products and suspected to be carcinogens. Also, ClO

2

is particularly effective in
destroying phenolic compounds that often cause severe taste and odor problems
when reacted with chlorine. Similar to the use of chlorine, it produces measurable
residual disinfectants. ClO

2

is a gas and its contact with light causes it to photoox-
idize, however. Thus, it must be generated on-site. Although its principal application
has been in wastewater disinfection, chlorine dioxide has been used in potable water
treatment for oxidizing manganese and iron and for the removal of taste and odor.
Its probable conversion to chlorate, a substance toxic to humans, makes its use for
potable water treatment questionable.
The physical agents of disinfection that have been used include ultraviolet light

(UV), electron beam, gamma-ray irradiation, sonification, and heat (Bryan, 1990;
Kawakami et al., 1978; Hashimoto et al., 1980). Gamma rays are emitted from
radioisotopes, such as cobalt-60, which, because of their penetrating power, have
been used to disinfect water and wastewater. The electron beam uses an electron
generator. A beam of these electrons is then directed into a flowing water or wastewater
to be disinfected. For the method to be effective, the liquid must flow in thin layers.
17

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740

Several theories have been proposed as to its mechanics of disinfection, including
the production of intermediates and free radicals as the beam hits the water. These
intermediates and free radicals are very reactive and are thought to possess the disin-
fecting power. In sonification, high-frequency ultrasonic sound waves are produced
by a vibrating-disk generator. These waves rattle microorganisms and break them into
small pieces. Ultraviolet light will be addressed specifically later in this chapter.
In general, the effect of disinfectants is thought to occur as a result of damage
to the cell wall, alteration of cell permeability, alteration of the protoplasm, and
inhibition of enzymatic activities. Damage to the cell wall results in cell lysis and
death. Some agents such as phenolic compounds and detergents alter the permeabil-
ity of the cytoplasmic membrane. This causes the membrane to lose selectivity to
substances and allow important nutrients such as phosphorus and nitrogen to escape
the cell. Heat will coagulate protoplasm and acids and alkali will denature it causing
alteration of the structure and producing a lethal effect on the microorganism. Finally,
oxidants, such as chlorine, can cause the rearrangement of the structure of enzymes.
This rearrangement will inhibit enzymatic activities.

17.2 FACTORS AFFECTING DISINFECTION

The effectivity of disinfectants are affected by the following factors: time of contact
between disinfectant and the microorganism and the intensity of the disinfectant,
age of the microorganism, nature of the suspending liquid, and temperature. Each
of these factors are discussed next.

17.2.1 T

IME



OF

C

ONTACT



AND

I

NTENSITY



OF

D

ISINFECTANT

In the context of how we use the term, intensity refers to the intensive property of the
disinfectant. Intensive properties, in turn, are those properties that are independent of
the total mass or volume of the disinfectant. For example, concentrations are expressed
as mass

per unit volume

; the phrase “per unit volume” makes concentration indepen-
dent of the total volume. Hence, concentration is an intensive property and it expresses
the intensity of the disinfectant. Another intensive property is radiation from an ultra-
violet light. This radiation is measured as power impinging upon a

square unit of area

.
The “per unit area” here is analogous to the “per unit volume.” Thus, radiation is
independent of total area and is, therefore, an intensive property that expresses the
intensity of the radiation, which, in this case, is the intensity of radiation of the
ultraviolet light.
It is a universal fact that the time needed to kill a given percentage of microor-
ganisms decreases as the intensity of the disinfectant increases, and the time needed
to kill the same percentage of microorganisms increases as the intensity of the
disinfectant decreases, therefore, the time to kill and the intensity are in inverse ratio
to each other. Let the time be

t

and the intensity be

I

. Thus, mathematically,
(17.1)t ∞
1
I
m


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Disinfection

741

Note:

I

has been raised to the power

m

, which is a constant. This is to make the
relationship more general.

Letting

k

be the proportionality constant, the equation becomes
(17.2)
In this equation, if

I

m

is multiplied by

t

, and if

I

is expressed as the concentration of
the disinfectant

C

in mg/L, the equation is the famous

Ct

concept with

m

equal to 1
and

t

in minutes.

Ct

values at given temperatures and pH are tabulated in

Ct

tables
used by regulating authorities and by the U.S. Environmental Protection Agency. The
time to kill

t

is synonymous with the time of inactivation of the microorganisms.
The constants may be obtained from experimental data by converting the above
equation first into an equation of a straight line. Taking the logarithms of both sides,
(17.3)
This equation is the equation of the straight line with

y

-intercept ln

k

and slope

m

.
The constants may then be solved using experimental data.
Assume

n

experimental data points, and divide them into two groups. Let there
be

l

data points in the first group; the second group would have

m



−



l

data points.
From analytic geometry,
(17.4)
Substituting Equation (17.4) into Equation (17.2) and solving for

k

produces
(17.5)
Having obtained

m

and

k

, the time

t

can be solved using Equation (17.2) from a
knowledge of the value of

I

. This time is called the

contact time

for disinfection, and
the intensity

I

is called the

lethal dose

. From Equation (17.2) any reasonable amount
of dose is lethal when administered in a sufficient amount of contact time as calculated
from the equation. We call Equation (17.2) the

Universal Law of Disinfection.

Example 17.1

It is desired to design a bromide chloride contact tank to be
used to disinfect a secondary-treated sewage discharge. To determine the contact
time, an experiment was conducted producing the following results:

Contact Time (min/residual fecal coliforms) (No./100 mL)
BrCl Dosage (mg/L) 15 30 60

3.6 10,000 4,000 600
15.0 800 410 200
47.0 450 200 90
t
k

I
m

=
lnt lnk mlnI–=
m
∑
l+1
n
t
i
ln
nl–

∑
1
l
t
i
ln
l
–
∑
l+1
n
I
i
ln
nl–

∑
1
l
I
i
ln
l
–
–=
k
∑
1
l
t
i
ln
l

m
∑
1
l
I
i
ln
l





+exp=

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742

Determine the contact time to be used in design, if it is desired to have a log 2
removal efficiency for fecal coliforms. Calculate the

Ct

value. The original concen-
tration of fecal coliforms is 40,000 per/100 mL.

Solution:

The percentage corresponding to a log removal can be obtained by
first assuming any original value of the concentration of the microorganisms

x

1

,
computing the next value

x

2

based on the given log removal, and computing the
corresponding percentage. Thus, let

x

1



=

8888888. Then,

Note:

Any number could have been assumed for

x

1

and the answer would still
be 99. Thus, log 2 removal is equal to 99% removal or 99% inactivation.
From the previous table, we produce the following table for the time to effect
a 99% inactivation:

BrCl Dosage
(mg/L) 15 min
%

Inactivation 30 min
%
Inactivation 60 min
%
Inactivation

3.6 10,000 0.75

a

4000 0.90 600 0.985
15.0 800 0.98 410 0.98975 200 0.995
47.0 450 0.98875 200 0.995 90 0.99775

a

0.75

=

(40,000

−

10,000

)/

40,000

BrCl Dosage (mg/L)
Time to 99%
Inactivation (min)

3.6 61.76

a

15.0 31.43

b

47.0 18
8888888 x
2
log–log 2= and x
2
88888.88=
%
8888888 88888.88–
8888888

99==
30 0.90
x 60–
60 30–

0.99 0.985–
0.985 0.90–

=
60 0.985 x
a
61.76=
x 0.99
30 0.98975
x 30–
30 60–

0.99 0.98975–
0.98975 0.995–

=
x 0.99 x
b
31.43=
60 0.995

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Disinfection

743

Use a contact time of 30 minutes and find the corresponding lethal dosage.
let

l

=

1;

n



=

3
Therefore,
Therefore,

17.2.2 A

GE


OF THE MICROORGANISM
The effectiveness of a disinfectant also depends upon the age of the microorganism.
For example, young bacteria can easily be killed, while old bacteria are resistant.
As the bacterium ages, a polysaccharide sheath is developed around the cell wall;
this contributes to the resistance against disinfectants. For example, when using
2.0 mg/L of applied chlorine dosage, for bacterial cultures of about 10 days old, it
takes 30 min of contact time to produce the same reduction as for young cultures of
about one day old dosed with one minute of contact time. In the extreme case are
the bacterial spores; they are, indeed, very resistant and many of the chemical disin-

fectants normally used have little or no effect on them.
BrCl Dosage
(mg/L)
Time to 99%
Inactivation
(min) ln I
i
ln t
i
3.6 61.76 1.28 4.1233
15.0 31.43 2.708 3.4477
47.0 18 3.850 2.8904
t
k
I
m

;= m
∑
l+1
n
t
i
ln
nl–

∑
1
l
t

i
ln
l
–
∑
l+1
n
I
i
ln
nl–

∑
1
l
I
i
ln
l
–

;–= k
∑
1
l
t
1
ln
l

m
∑
1
l
I
i
ln
l




+exp=
m
3.4477 2.8904+()
31–

4.1233()
1

–
2.708 3.850+()
31–

1.28
1

–

–

6.3381
2

4.1233–
6.558
2

1.28–

– 0.477===
k
4.1233()
1

0.477 1.28{}+exp 4.733[]exp 113.60===
t
k
I
m

;30
113.60
I
0.477

;== I 16.3= mg/L Ans
Ct 16.3 30() 489 mg/L min Ans⋅==
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17.2.3 NATURE OF THE SUSPENDING FLUID

In addition to the time of contact and age of the microorganism, the nature of the
suspending fluid also affects the effectiveness of a given disinfectant. For example,
extraneous materials such ferrous, manganous, hydrogen sulfide, and nitrates react
with applied chlorine before the chlorine can do its job of disinfecting. Also, the
turbidities of the water reduces disinfectant effectiveness by shielding the microor-
ganism. Hence, for most effective kills, the fluid should be free of turbidities.
17.2.4 EFFECT OF TEMPERATURE
We have learned from previous chapters that equilibrium and reaction constants are
affected by temperature. The length of time that a disinfection process proceeds is a
function of the constants of the underlying reaction between the microorganism and
the disinfectant; thus, it must also be a function of temperature. The variation of the
contact time to effect a given percentage kill with respect to temperature can therefore
be modeled by means of the Van’t Hoff equation. This equation was derived for
the equilibrium constants in Chapter 11, which is reproduced next:
(17.6)
K
T1
and K
T2
are the equilibrium constants at temperatures T
1
and T
2
, respectively.
is the standard enthalpy change of the reaction and R is the universal gas
constant. If K
T1
is replaced by contact time t
T1
at temperature T1 and K

T2
is replaced
by contact time t
T2
at temperature T2, the resulting equation would show that as the
temperature increases, the contact time to kill the same percentage of microorgan-
isms also increases. Of course, this is not true. Thus, the replacement should be the
other way around. Doing this is the same as interchanging the places in the difference
term between T
1
and T
2
inside the exp function. Thus, doing the interchanging,
(17.7)
Table 17.1 shows the standard enthalpy change as a function of pH for both aqueous
chlorine and chloramines, and Table 17.2 shows the various possible values of the
universal gas constant.
Example 17.2 The contact time for a certain chlorination process at a pH of
7.0 and a temperature of 25°C is 30 min. What would be the contact time to effect
the same percentage kill if the process is conducted at a temperature 18°C?
Solution:
K
T 2
K
T 1
=
∆H
298
o
RT

1
T
2

T
2
T
1
–()exp
∆H
298
o
t
T 2
t
T1
∆H
298
o
RT
1
T
2

T
1
T
2
–()exp=
t

T 2
t
T 1
∆H
298
o
RT
1
T
2

T
1
T
2
–()exp=
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From Table 17.1,
From Table 17.2,
Therefore,
TABLE 17.1
Standard Enthalpy Changes at 25°C
Compound pH ∆ (J/mol)
Aqueous chlorine 7.0 34,332
8.5 26,796
9.8 50,242
10.7 62,802
Chloramines 7.0 50,242
8.5 58,615

9.5 83,736
TABLE 17.2
Values and Units of R
R Value R Units
K Concentration
Units Used ∆H° Units T Units
0.08205 — °K
8.315 °K
1.987 °K
82.05 — °K
H
298
o
L atm
gmmole K °

gmmoles
L

J
gmmole K °

gmmoles
L

J
gmmole

cal
gmmole K °

gmmoles
L

cal
gmmole

atm.cm
3
gmmole K °⋅

gmmoles
L

∆H
298
o
34,332= j/mol 34,332 N m/mol⋅=
R 8.315
J
gmmole K °⋅

8.315
Nm⋅
gmmole K °⋅

==
t
T 2
= 30

34,332
8.315 298()291()

298 291–()exp = 30 0.333[]exp = 41.87 min Ans
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17.3 OTHER DISINFECTION FORMULAS
The literature reveals other disinfection formulas. These include Chick’s law for contact
time, modifications of Chick’s law, and relationship between concentration of disinfec-
tant and concentration of microorganisms reduced in a given percentage kill. Chick’s
law and its modification called the Chick–Watson model, however, are not useful for-
mulas, because they do not incorporate either the concentration of the disinfectant that
is needed to kill the microorganisms or the incorporation of the concentration is incorrect.
The relationship of the concentration of disinfectant and the concentration of the micro-
organisms is also not a useful formula, since it does not incorporate the contact time
required to kill the microorganisms. It must be noted that for a formula to be useful, it
must incorporate both the concentration (intensity) of the disinfectant and the contact
time corresponding to this concentration effecting a given percentage kill. For these
reasons, these other disinfection formulas are not discussed in this book.
The Chick–Watson model needs to be addressed further. Watson explicitly
expressed the constant k in Chick’s law in terms of the concentration of disinfectant
C as
α
C
n
, where
α
is an activation constant and n is another constant termed the
constant of dilution. Chick’s Law, thus, became dN/dt = −
α

C
n
dt, where N is the
concentration of microorganisms and t is time. Note that C is a function of time.
When this equation was integrated, however, it was assumed constant, thus producing
the famous Chick–Watson model,
where N
o
is the initial concentration of microorganisms. Because the concentration
C was assumed constant with time during integration, this equation is incorrect and,
therefore, not used in this book.
17.4 CHLORINE DISINFECTANTS
The first use of chlorine as a disinfectant in America was in New Jersey in the year
1908 (Leal, 1909). At that time George A. Johnson and John L. Leal chlorinated the
water supply of Jersey City, NJ.
The principal compounds of chlorine that are used in water and wastewater
treatment are the molecular chlorine (Cl
2
), calcium hypochlorite [Ca(OCL)
2
], and
sodium hypochlorite [NaOCl]. Sodium hypochlorite is ordinary bleach. Chlorine is
a pale-green gas, which turns into a yellow-green liquid when pressurized. Both the
aqueous and liquid chlorine react with water to form hydrated chlorine. Below 9.4°C,
liquid chlorine forms the compound Cl
2
· 8H
2
O.
Chlorine gas is supplied from liquid chlorine that is shipped in pressurized steel

cylinders ranging in size from 45 kg and 68 kg to one tonne containers. It is also
shipped in multiunit tank cars that can contain fifteen 1-tonne containers and tank
cars having capacities of 15, 27, and 50 tonnes.
In handling chlorine gas, the following points are important to consider:
• Chlorine gas is very poisonous and corrosive. Therefore, adequate venti-
lation should be provided. In the construction of the ventilation system,
N
N
o

ln
α
C
n
–= t
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the capturing hood vents should be placed at floor level, because the gas
is heavier than air.
• The storage area for chlorine should be walled off from the rest of the plant.
There should be appropriate signs posted in front of the door and back
of the building. Gas masks should be provided at all doors and exits should
be provided with clearly visible signs.
• Chlorine solutions are very corrosive and should therefore be transported
in plastic pipes.
• The use of calcium hypochlorite or sodium hypochlorite as opposed to
chlorine gas should be carefully considered when using chlorination in
plants located near residential areas. Accidental release of the gas could
endanger the community. Normally, small plants that usually lack well-
trained personnel, should not use gaseous chlorine for disinfection.

Calcium hypochlorite is available in powder or granular forms and compressed
tablets or pellets. Depending upon the source of the chemical, a wide variety of
container sizes and shapes are available. Because it can oxidize other materials, calcium
hypochlorite should be stored in a cool dry place and in corrosion-resistant containers.
High-test calcium hypochlorite, HTH, contains about 70% chlorine. (Available chlo-
rine will be defined later.)
Sodium hypochlorite is available in solution form in strengths of 1.5 to 15%
with 3% the usual maximum strength. The solution decomposes readily at high
concentrations. Because it is also affected by heat and light, it must be stored in a
cool dry place and in corrosion-resistant containers. The solution should be trans-
ported in plastic pipes. Sodium hypochlorite can contain 5 to 15% available chlorine.
17.4.1 CHLORINE CHEMISTRY
The chemistry of chlorine discussed in this section includes hydrolysis and optimum
pH range of chlorination, expression of chlorine disinfectant concentration, reaction
mediated by sunlight, reactions with inorganics, reactions with ammonia, reactions
with organic nitrogen, breakpoint reaction, reactions with phenols, formation of
trihalomethanes, acid generation, and available chlorine.
Chlorine has the electronic configuration of [Ne]3s
2
3p
5
and is located in Group
VIIA of the Periodic Table in the third period. [Ne] means that this element has the
electronic configuration of the noble gas neon. The letters p and s refer to the p and
s orbitals; the superscripts indicate the number of electrons that the orbitals contain.
Thus, the p orbital contains 5 electrons and the s orbital contains two electrons, making
a total of seven electrons in its valence shell. This means that the chlorine atom needs
to acquire only one more electron to attain the neon configuration for stability. This
makes chlorine a very good oxidizer. In fact, it is a characteristic of Group VIIA to
attain a charge of −1 when the members of this group oxidizes other substances. The

members of this group starting from the strongest oxidizer to the least are fluorine,
chlorine, bromine, iodine, and astatine. This group forms the family of elements called
the halogen family.
All the chlorine disinfectants reduce to the chloride ion (Cl
−
) when they oxidize
other substances, which must, of course, be reducing substances. The chlorine starts
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with an oxidation state of zero and ends up with a −1; it only needs one reduction step.
One the other hand, the hypochlorites start with oxidation states of +1 and end up with
also a −1; thus, they need two reduction steps. Because the chlorine atom only needs
one reduction step, while the hypochlorites need two, the chlorine atom is a stronger
oxidizer than the hypochlorites. As a stronger oxidizer, it is also a stronger disinfectant.
Hydrolysis and optimum pH range of chlorination. As previously mentioned,
chlorine is supplied in the form of liquefied chlorine. The liquid must then be evapo-
rated into a gas. As the gas, Cl
2(g)
, is applied into the water or wastewater, it dissolves
into aqueous chlorine, Cl
2(aq)
, as follows:
(17.8)
Cl
2(aq)
then hydrolyzes, one of the chlorine atoms being oxidized to +1 and the other
reduced to −1. This reaction is called disproportionation. The reaction is as follows:
(17.9)
From Equation (17.9), the hypochlorous acid, HOCl, is formed, which is one of
the chlorine disinfectants. If its formula is analyzed, it will be found that the chlorine

has an oxidation state of +1, as we mentioned before. Note also that hydrochloric acid
is formed. This is a characteristic in the use of the chlorine gas as a disinfectant. The
water becomes acidic. Also, as we have mentioned, the chlorine molecule is a much
stronger oxidizer than the hypochlorite ion and, hence, a stronger disinfectant. From
Equation (17.9), if the water is intentionally made acidic, the reaction will be driven
to the left, producing more of the chlorine molecule. This condition will then produce
more disinfecting power. As will be shown later, however, this condition, where the
chlorine molecule will exist, is at a very low pH hovering around zero. This makes
the chlorine molecule useless as a disinfectant.
HOCl further reacts to produce the following dissociation reaction:
(17.10)
Using Equation (17.9), let us calculate the distribution of Cl
2(aq)
and HOCl.
Expressing in the form of equilibrium equation,
(17.11)
Taking logarithms, rearranging, and simplifying,
(17.12)
pK
H
is the negative logarithm to the base 10 of K
H
.
Table 17.3 shows the ratios of [Cl
2(aq)
]/[HOCl] and [HOCl]/[Cl
2(aq)
] as functions
of pH and the chloride concentrations, using Equation (17.12). The concentration of
1.0 gmmole/L of chloride is 35,500 mg/L. This will never be encountered in the

normal treatment of water and wastewater. Disregarding this entry in the table, the
Cl
2 g()
 Cl
2 aq()
K
Claq
6.2 10
2–
()=
Cl
2 aq()
H
2
O  HOCl H
+
Cl
1–
+++ K
H
4.0 10
4–
()=
HOCl  H
+
OCl
−
+ K
a
10

7.5–
=
K
H
4.0 10
4–
()
HOCl[]H
+
[]Cl
1–
[]
Cl
2 aq()
[]

==
Cl
2 aq()
[]
HOCl[]
1 0
{ pK
H
−pH+log Cl
1–
[]}
10
{3.40−pH+log Cl
1–

[]}
==
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concentration of Cl
2(aq)
is already practically nonexistent at around pH 4.0 and above.
In fact, it is even practically nonexistent at pH’s less than 4, except when the pH is
close to zero and chloride concentration of 0.1 gmmol/L; but, 0.1 gmmol/L is equal
to 3,500 mg/L, which is already a very high chloride concentration and will not be
encountered in the treatment of water and wastewater. Practically, then, for conditions
encountered in practice, at pH’s greater than 4.0, [HOCl] predominates over Cl
2(aq)
.
Now, using Equation (17.10), let us calculate the distribution of HOCl and OCl
−
.
Note that from the previous result, HOCl predominates over Cl
2(aq)
above pH 4.0,
Cl
2(aq)
being practically zero. Thus, above this pH, the distribution of the chlorine
disinfectant species will simply be for HOCl and OCl
−
. Expressing Equation (17.10)
in the form of the equilibrium equation,
(17.13)
Taking logarithms, rearranging, and simplifying,
(17.14)

pK
a
is the negative logarithm to the base 10 of K
a
.
Table 17.4 shows the ratios of [OCl
−
]/[HOCl] and [HOCl]/[OCl
−
] as functions
of pH using Equation (17.14). This table shows that HOCl predominates over OCl
−
at pH’s less than 7.5. Also considering Table 17.3, we make the conclusion that
for all practical purposes, HOCl predominates over all chlorine disinfectant species
TABLE 17.3
Ratios of and as Functions pH and Chloride Concentration
[Cl
−
]
(gmmoles/L) pH pH
1.0 0 2.51(10
3
) 3.98 (10
−4
) 1 251 3.98(10
−3
)
2 25.12 3.98(10
−2
) 3 2.51 0.398

4 0.251 3.98 5 0.0251 39.81
6 2.51(10
−3
) 398 7 2.51(10
−4
) 3.98(10
3
)
10
−1
0 251 3.98(10
−3
) 1 25.12 0.0398
2 2.51 0.398 3 0.251 3.98
4 0.025 40 5 2.51(10
−3
) 398
6 2.51(10
−4
) 3.98(10
3
) 7 2.51(10
−5
) 3.98(10
4
)
10
−3
0 2.51 0.398 1 0.251 3.98
2 0.025 40 3 2.51(10

−3
) 398.11
4 2.51(10
−4
) 3.98(10
3
) 5 2.51(10
−5
) 3.98(10
4
)
6 2.51(10
−6
) 3.98(10
5
) 7 2.51(10
−7
) 3.98(10
6
)
10
−5
0 0.0251 39.81 1 2.51(10
−3
) 398
2 2.51(10
−4
) 3.98(10
3
) 3 2.51(10

−5
) 3.98(10
4
)
[Cl
2(aq)
]
HOCl

[HOCl]
[Cl
2(aq)
]

[Cl
2(aq)
]
HOCl

[HOCl]
[Cl
2(aq)
]

[Cl
2(aq)
]
HOCl

[HOCl]

[Cl
2(aq)
]

K
a
10
7.5–
H
+
[]OCl
−
[]
HOCl[]

==
OCl
−
[]
HOCl[]

10
pH−pK
a
10
pH−7.5
==
TX249_frame_C17.fm Page 749 Friday, June 14, 2002 4:49 PM
© 2003 by A. P. Sincero and G. A. Sincero
in all pH ranges up to less than 7.5. At exactly pH 7.5, the concentrations of HOCl

and OCl
−
are equal and above this pH, OCl
−
predominates over all chlorine disinfectant
species. This reality is more than just a theoretical interest, because HOCl is 80 to
100% more effective than OCl
−
as a disinfectant (Snoeyink and Jenkins, 1980). We
now conclude that the optimum pH range for chlorination is up to 7.0. Beyond this
range, OCl
−
predominates and the disinfection becomes less effective.
The three species Cl
2(aq)
, HOCl, and OCl
−
are called free chlorine. Although
Cl
2(aq)
is a stronger oxidizer than the other two, it is not really of much use, unless
the chlorination is done under a very acidic condition.
Example 17.3 At pH 6.0, calculate the mole fraction of HOCl.
Solution: At pH 6.0 the mole fraction of Cl
2(aq)
is practically zero. From
Table 17.4, the mole ratio of HOCl to OCl
−
at this pH is 31.62.
Therefore,

Expression of chlorine disinfectant concentration. Now that we have detailed
the various reactions of the chlorine disinfectants, it is time to unify the concentra-
tions of the chlorine species. By convention, the concentrations of the three species
are expressed in terms of the molecular chlorine, Cl
2
. The pertinent reactions are
written as follows:
(17.15)
(17.16)
(17.17)
TABLE 17.4
Ratios of and as Functions of pH
pH pH
0 3.16(10
−8
) 3.16(10
7
) 1 3.16(10
−6
) 3.16(10
6
)
2 3.16(10
−6
) 3.16(10
5
) 3 3.16(10
−5
) 3.16(10
4

)
4 3.16(10
−4
) 3.16(10
3
) 5 3.16(10
−3
) 316.2
6 3.16(10
−2
) 31.62 7 0.316 3.16
7.5 1.0 1.0 8 3.16 0.316
9 31.62 3.16(10
−2
) 10 316.2 3.16(10
−3
)
11 3.16(10
3
) 3.16(10
−4
) 12 3.16(10
4
) 3.16(10
−5
)
13 3.15(10
5
) 3.16(10
−6

) 14 3.16(10
6
) 3.16(10
−7
)
[OCl
−−
−−
]
[HOCl]

[HOCl]
[OCl
q
]

[OCl
−−
−−
]
[HOCl]

[HOCl]
[OCl
−−
−−
]

[OCl
−−

−−
]
[HOCl]

[HOCl]
[OCl
−−
−−
]

mole fraction of HOCl
HOCl[]
HOCl[]OCl
−
[]Cl
2 aq()
[]++

=
31.62
31.62 1 0++

0.97 Ans==
Cl
2 aq()
H
2
O +  HOCl H
+
Cl

−1
++
NaOCl  Na
+
OCl
−
+
Ca OCl()
2
 Ca
2+
2OCl
−
+
TX249_frame_C17.fm Page 750 Friday, June 14, 2002 4:49 PM
© 2003 by A. P. Sincero and G. A. Sincero
From these reactions,
(17.18)
(17.19)
(17.20)
Whenever a concentration of a chlorine disinfectant is mentioned, the above equa-
tions are implicitly referred to, and this concentration is the equivalent Cl
2
concen-
tration. Of course, the equivalent Cl
2
concentration of the chlorine gas disinfectant
is Cl
2
.

Reaction mediated by sunlight. Aqueous chlorine is not stable in the presence
of sunlight. Sunlight contains ultraviolet light. This radiation provides the energy
that drives the chemical reaction for breaking up the molecule of hypochlorous acid.
The water molecule breaks up, first releasing electrons that are then needed to reduce
the chlorine atom in HOCl to chloride. The overall reaction is as follows:
(17.21)
The O
2
comes from the break up of the water molecule, oxidizing its oxygen atom
to the molecular oxygen.
The previous reaction in the presence of sunlight is very significant. If the disin-
fectant is to be stored in plastic containers, then this container must be made opaque;
otherwise, the chlorine gas will be converted to hydrochloric acid, and the hypochlo-
rites will be converted into the corresponding salts of calcium and sodium.
Example 17.4 A solution of sodium hypochlorite containing 50 kg of NaOCl
is stored in a transparent plastic container. It had been stored outdoors for quite some
time and then used to disinfect a swimming pool. How effective is the disinfection?
What material is used instead for the disinfection? And how many kilograms is it?
Solution: Because the solution is stored outdoors and in a transparent con-
tainer, the following reaction occurs:
From this reaction, no disinfectant exists in the container and the disinfection is not
effective. Ans
The material used instead for disinfection is the salt NaCl. Ans
The amount of sodium chloride used to disinfect is
1.0 mg/L HOCl
Cl
2
HOCl

2 35.5()

1.008 +16 +35.5

1.35== = mg/L Cl
2
1.0 mg/L NaOCl
Cl
2
NaOCl

2 35.5()
23 +16 +35.5

0.95== = mg/L Cl
2
1.0 mg/L Ca OCl()
2
2Cl
2
Ca OCl()
2

4 35.5()
40.1 2 16 +35.5()+

0.99== = mg/L Cl
2
2HOCl 2H
+
2Cl
−

O
2
++→
2NaOCl 2Na
+
2Cl
−
O
2
++→
NaCl
NaOCl

50()
23 35.5+
23 16 35.5++

50()39.26== kg Ans
TX249_frame_C17.fm Page 751 Friday, June 14, 2002 4:49 PM
© 2003 by A. P. Sincero and G. A. Sincero
Reactions with inorganics. Reducing substances that could be present in the raw
water and raw wastewater and treated water and treated wastewater are ferrous, man-
ganous, nitrites, and hydrogen sulfide. Thus, these are the major substances that can
interfere with the effectiveness of chlorine as a disinfectant. The interfering reactions
are written as follows:
with ferrous:
(17.22)
(17.23)
––––––––––––––––––––––––––––––––––
(17.24)

with manganous:
(17.25)
(17.26)
–––––––––––––––––––––––––––––––––
(17.27)
with nitrites:
(17.28)
(17.29)
–––––––––––––––––––––––––––
(17.30)
with hydrogen sulfide:
(17.31)
(17.32)
–––––––––––––––––––––––––––––––
(17.33)
For activated sludge plants that produce only partial nitrification, it is a common
complaint of operators that a residual chlorine cannot be obtained at the effluent.
The reason for this is the reaction of nitrites with the chlorine disinfectant producing
nitrates as shown in Reaction (17.30).
Example 17.5 An activated sludge plant of a small development is out of
order, and a decision has been made following approval from a state agency to
discharge raw sewage to a river. The effluent was found to contain 8 mg/L of ferrous,
3 mg/L manganous, 20 mg/L nitrite as nitrogen, and 4 mg/L hydrogen sulfide.
Calculate the mg/L of HTH needed to be dosed before actual disinfection is realized.
What is the chlorine concentration?
2Fe
2+
2Fe
3+
2e

−
+→
HOCl H
+
2e
−
Cl
−
H
2
O+→++
2Fe
2+
HOCl H
+
2Fe
3+
Cl
−
H
2
O++→++
Mn
2+
Mn
4+
2e
−
+→
HOCl H

+
2e
−
Cl
−
H
2
O+→++
Mn
2+
HOCl H
+
Mn
4+
Cl
−
H
2
O++→++
NO
2
−
H
2
O+ NO
3
−
2H
+
2e

−
++→
HOCl H
+
2e
−
Cl
−
H
2
O+→++
NO
2
−
HOCl NO
3
−
Cl
−
H
+
++→+
H
2
S4H
2
O+ SO
4
2−
10H

+
8e
−
++→
4HOCl 4H
+
8e
−
4Cl
−
4H
2
O+→++
H
2
S 4HOCl SO
4
2−
4Cl
−
6H
+
++→+
TX249_frame_C17.fm Page 752 Friday, June 14, 2002 4:49 PM
© 2003 by A. P. Sincero and G. A. Sincero
Solution: Examining Reactions (17.22) to (17.33) reveals that the number of
reference species is equal to two moles of electrons except Reaction (17.33), which
has eight moles of electrons. By considering all the other reactions, the number of
milliequivalents of HOCl needed
Therefore,

From Equation (17.20), the chlorine concentration = 73.69(0.99) = 72.95 mg/L
Ans
Reactions with ammonia and optimum pH range for chloramine formation.
Effluents from sewage treatment plants can contain significant amounts of ammonia
that when disinfected, instead of finding free chlorine, substitution products of ammonia
called chloramines are found. In addition, in water treatment plants, ammonia are
often purposely added to chlorine. This, again, also forms the chloramines. Chloramines
are disinfectants like chlorine, but they are slow reacting, and it is this slow-reacting
property that is the reason why ammonia is used. The purpose is to provide residual
disinfectant in the distribution system. In other words, the formation of chloramines
assures that when the water arrives at the tap of the consumer, a certain amount of
disinfectant still exists.
The formation of chloramines is a stepwise reaction sequence. When ammonia
and chlorine are injected into the water that is to be disinfected, the following reactions
occur, one after the other in a stepwise manner.
(17.34)
(17.35)
(17.36)
8
2Fe/2

3
Mn/2

20
NO
2
/2

4

H
2
S/8

++ +=
8
2 55.8()/2

3
54.9/2

20
14+32()/2

++=
4
2 1.008()32.1+[]/8

+
2.06= meq/L 2.06
HOCl
2



⇒ 2.06
1.008 16 35.5++
2




54.08 mg/L==
Ca OCl()
2
Ca
2+
2OCl
−
+→
2OCl
−
2H
2
O+  2HOCl 2OH
−
+
mg/L of HTH 54.08
Ca OCl()
2
2HOCl



54.08
40.1 2 16 35.5+()+
2 1.008 16 35.5++()

==
73.69= mg/L Ans
NH

3
HOCl+ NH
2
Cl monochloramine()H
2
O+→
NH
2
Cl HOCl+ NHCl
2
dichloramine()H
2
O+→
NHCl
2
HOCl+ NCl
3
trichloramine()H
2
O+→
TX249_frame_C17.fm Page 753 Friday, June 14, 2002 4:49 PM
© 2003 by A. P. Sincero and G. A. Sincero
First, it is to be noted that the reaction is expressed in terms of HOCl. By the
equivalence of reactions, however, the above reactions can be manipulated if the
equivalent amount of the other two species is desired to be known. In monochloram-
ines and dichloramines, therefore, the chlorine is combined in ammonia; they are
called combined chlorines. As will be shown in subsequent discussions, the concen-
tration of trichloramine is practically zero during disinfection; thus, it is not included
in the definition of combined chlorine.
Reaction (17.34) indicates that at the time when one mole of HOCl is added to

one mole of NH
3
, the conversion into monochloramine is essentially complete. In
view of the relationship of HOCl and OCl
−
as a function of pH, however, this
statement is not exactly correct. From previous discussions, at pH 7.5, hypochlorous
acid and the hypochlorite ion exist in equal mole concentrations, but beyond pH 7.5,
the hypochlorite ion predominates. OCl
−
does not directly react with NH
3
to form
the monochloramine, but must first hydrolyze to produce the HOCl before Reaction
(17.34) proceeds. Thus, when the pH is above 7.5, addition of one mole of HOCl
to one mole of ammonia does not guarantee complete conversion into NH
2
Cl. At
these pH values, the one mole of HOCl added becomes lesser, because of the
predominance of the hypochlorite ion. HOCl, however, exists at practically 100
concentrations at pH’s below 7.0; hence, at this range, a mole for mole addition
would essentially guarantee the aforementioned conversion into monochloramine.
Now, Reactions (17.35) and (17.36) indicate that by adding two moles of HOCl
and three moles of HOCl, the conversion into dichloramine and the trichloramine are,
respectively, essentially complete. For the same reason as in the case of the conversion
into monochloramine, these two and three moles are not really these values, because
the resulting concentrations depend upon the pH of the solution. Above pH 7.5, the
conversions are not complete.
Let us have more discussion regarding the formation of dichloramine. The oxida-
tion state of nitrogen in NH

2
Cl from where the dichloramine comes from is −1. The
oxidation state of the nitrogen in NHCl
2
, itself, is +1. This means that in order to form
the dichloramine, two electrons must be abstracted from the nitrogen atom. Now, the
other substances that have been observed to occur, as the amount of hypochlorous acid
added is increased, are the nitrogen gas and nitrates. Take the case of the nitrogen gas.
The oxidation state of the N atom in the N
2
molecule is zero. This means that in order
to form the nitrogen gas from NH
2
Cl only one electron needs to be abstracted from
the nitrogen atom; this is an easier process than abstracting two electrons. It must then
be concluded that before the dichloramine is formed, the gas must have already been
forming, and that for the dichloramine to be formed, more HOCl is needed than is
needed for the formation of the gas.
The reaction for the formation of the nitrogen gas may be written as follows:
(17.37)
(17.38)
––––––––––––––––––––––––––––––––––––––
(17.39)
2NH
2
Cl N
2 g()
2Cl
−
4H

+
2e
−
+++→
HOCl H
+
2e
−
→ Cl
−
H
2
O++ +
2NHCl
2
HOCl+ N
2 g()
3Cl
−
3H
+
H
2
O+++→
TX249_frame_C17.fm Page 754 Friday, June 14, 2002 4:49 PM
© 2003 by A. P. Sincero and G. A. Sincero
Let us discuss the formation of the monochloramine versus the formation of the
nitrogen gas. The oxidation state of the nitrogen atom in ammonia is −3. And, again,
its oxidation state in NH
2

Cl is −1. Thus, forming the monochloramine from ammonia
needs the abstraction of two electrons from the nitrogen atom. Now, again, the
oxidation state of nitrogen in the nitrogen gas is zero, which means that to form the
nitrogen gas from ammonia needs the abstraction of three electrons; this is harder
than abstracting two electrons. Thus, in the reaction of HOCl and NH
3
, the
monochloramine is formed rather than the nitrogen gas, and the gas is formed only
when the conversion into monochloramine is complete by more additions of HOCl.
Consider the formation of the nitrate ion. The oxidation state of nitrogen in the
nitrate ion is +5. Thus, this ion would not be formed from ammonia, because this
would need the abstraction of eight electrons. If it is formed from the monochloram-
ine, it would need the abstraction of six electrons, and if formed from the dichloram-
ine, it would need the abstraction of four electrons. Thus, in the chloramine reactions
with HOCl, the nitrate is formed from the dichloramine. We will, however, compare
which formation forms first from the dichloramine: trichloramine or the nitrate ion.
The oxidation state of the nitrogen atom in trichloramine is +3. Thus, to form the
trichloramine, two electrons need to be abstracted from the nitrogen atom. This
may be compared to the abstraction of four electrons from the nitrogen atom to
form the nitrate ion. Therefore, the trichloramine forms first before the nitrate ion
does.
The reaction for the formation of the nitrate ion may be written as follows:
(17.40)
(17.41)
––––––––––––––––––––––––––––––––––––––
(17.42)
Now, let us discuss the final fate of trichloramine during disinfection. In accor-
dance with the chloramine reactions [Reactions (17.34) to (17.36)], by the time three
moles of HOCl have been added, a mole of trichloramine would have been formed.
This, however, is not the case. As mentioned, while the monochloramine decomposes

in a stepwise fashion to convert into the dichloramine, its destruction into the nitrogen
gas intervenes. Thus, the eventual formation of the dichloramine would be less; in
fact, much, much less, since, as we have found, formation of the gas is favored over
the formation of the dichloramine. In addition, monochloramine and dichloramine,
themselves, react with each other along with HOCl to form another gas N
2
O [NH
2
Cl +
NHCl
2
+ HOCl → N
2
O + 4H
+
+ 4Cl
−
]. Also, there may be more other side reactions
that could occur before the eventual formation of the dichloramine from mono-
chloramine. Overall, as soon as the step for the conversion of the dichloramine to the
trichloramine is reached, the concentration of dichloramine is already very low and
the amount of trichloramine produced is also very low. Thus, if, indeed, trichloramine
has a disinfecting power, this disinfectant property is useless, since the concentra-
tion is already very low in the first place. This is the reason why combined chlorine
is only composed of the monochloramine and the dichloramine. Also, it follows
NHCl
2
3H
2
O → NO

3
−
2Cl
−
7H
+
4e
−
++++
2HOCl 2H
+
4e
−
→ 2Cl
−
2H
2
O++ +
NHCl
2
2HOCl H
2
O++NO
3
−
2Cl
−
5H
+
++→

TX249_frame_C17.fm Page 755 Friday, June 14, 2002 4:49 PM
© 2003 by A. P. Sincero and G. A. Sincero
that since dichloramine is, itself, simply decomposed, it is not the important combined
chlorine disinfectant; the monochloramine is. If the objective is the formation of the
disinfecting chloramines, it is only necessary to add chlorine to a level of a little
more than one mole of chlorine to one mole of ammonia in order to simply form
monochloramine. Beyond this is a waste of chlorine.
Now, let us determine the optimum pH range for the formation of the mono-
chloramine. The key to the determination of this range is the predominance of HOCl.
Hypochlorous acid predominates over the pH range of less the 7.0; therefore, the
optimum pH range for the formation of monochloramine is also less than 7.0.
Example 17.6 In order to provide residual disinfectant in the distribution
system, chloramination is applied to the treated water. If two moles of HOCl have
been added per mole of ammonia, calculate the moles of nitrogen gas produced.
Solution: So many intervening reactions may be occurring during chlorami-
nation that it is not possible to determine exactly the amount of resulting species.
Experimentally, a sample may be put in a closed jar and chloramination performed.
The liberated nitrogen gas may then be measured; but in this problem the moles of
nitrogen produced simply cannot be calculated. Ans
Reactions with organic nitrogen. Chlorine reacts with organic amines to form
organic chloramines. Examples of the organic amines are those with the groups
–
NH
2
, −NH−, and −N =. Parallel to its reaction with ammonia, HOCl also reacts
with organic amines to form organic monochloramines and organic dichloramines
by the chloride atom simply attaching to the nitrogen atom in the organic molecule.
For example, methyl amine reacts with HOCl as follows:
(17.43)
As in the conversion of monochloramine to dichloramine, monochloromethyl amine

converts to dichloromethyl amine in the second step reaction as follows:
(17.44)
Other nitrogen-containing organic compounds are the amides which contain the
group −OCNH
2
and −CNH−. The ammonia and organic amine molecules have basic
properties. They react readily with HOCl, which is acidic. The organic amides, on
the hand, are less basic than the amines are; thus, they do not react as readily to
form organic chloramides with hypochlorous acid. They consume chlorine, however,
so organic amides as well as organic amines are important in chloramination. Although
the organic chloramides and organic chloramines have some disinfecting power, they
are not as potent as the ammonia chloramines; thus, their formation is not beneficial.
Organic chloramides and organic chloramines are also combined chlorines.
Example 17.7 Show the half reaction that will exhibit the property of organic
chloramines as disinfectants.
CH
3
Cl HOCl CH
3
NHCl (an organic monochloramine, →+
monochloromethyl amine) H
2
O+
CH
3
NHCl HOCl CH
3
NCl
2
an organic dichloramine, (→+

dichloromethyl amine) H+
2
O
TX249_frame_C17.fm Page 756 Friday, June 14, 2002 4:49 PM
© 2003 by A. P. Sincero and G. A. Sincero
Solution: A characteristic property of chlorine disinfectant is the conversion of
the chlorine atom in the disinfectant into the chloride ion. Thus, in portraying the chemical
reaction, the formation of the chloride should always be shown. Let CH
3
NHCl represent
the organic chloramines. Therefore, its half reaction as a disinfectant is as follows:
As this half reaction shows, the disinfectant grabs four mole electrons from the
organism disinfected per mole of the disinfectant, disabling the organism. Ans
Breakpoint reactions. Figure 17.1 shows the status of chlorine residual as a
function of chlorine dosage. From zero chlorine applied at the beginning to point A,
the applied chlorine is immediately consumed. This consumption is caused by reduc-
ing species such as Fe
2+
, Mn
2+
, H
2
S, and . The reactions of these substances on
HOCl have been discussed previously. As shown, no chlorine residual is produced
before point A.
In waters and wastewaters, organic amines and their decomposition products such
as ammonia may be present. In addition, ammonia may be purposely added for
chloramine formation to produce chlorine residuals in distribution systems. Also, other
organic substances such as organic amides may be present as well. Thus, from point
A to B, chloro-organic compounds and organic chloramines are formed. Ammonia will

be converted to monochloramine at this range of chlorine dosage.
Beyond point B, the chloro-organic compounds and organic chloramines break
down. Also, at this range of chlorine dosage, the monochloramine starts to convert
to the dichloramine, but, at the same time, it also decomposes into the nitrogen gas and,
possibly, other gases as well. These decomposition reactions were addressed previously.
FIGURE 17.1 Chlorine residual versus applied chlorine.
CH
3
NHCl 4H
+
4e
−
from organism disinfected()Cl
−
NH
4
+
CH
4
++→++
NO
2
−
Destruction of
chlorine residual
by reducing
compounds
Formation of chloro-organic
compounds and chloramines
Chloramines and

chloro-organic
compounds
Presence of
chloro-organic
compounds not
destroyed
Combined residual
Free and combined residual
Free residual
Combined residual
Breakpoint
Chlorine added (mg/L)
Chlorine residual (mg/L)
0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
0.5
0.4
0.3
0.2
0.1
0
A
B
TX249_frame_C17.fm Page 757 Friday, June 14, 2002 4:49 PM
© 2003 by A. P. Sincero and G. A. Sincero
As the curve continues to go “downhill” from point B, the dichloramine converts to
the trichloramine, the conversion being complete at the lowest point indicated by
“breakpoint.” As shown, this lowest point is called the breakpoint. In addition,
nitrates will also be formed from the dichloramine before reaching the breakpoint.
In fact, other substances would have been formed as decomposition products from
monochloramine and dichloramine, as well as other substances would have been

formed as decomposition products from the chloro-organic compounds and organic
chloramines.
As shown by the downward swing of the curve, the reactions that occur between
point B and the breakpoint are all breakdown reactions. Substances that have been
formed before reaching point B are destroyed in this range of dosage of chlorine.
In other words, the chloro-organics that have been formed, the organic chloramines
that have been formed, the ammonia chloramines that have formed, and all other
substances that have been formed from reactions with compounds such as phenols
and fulvic acids are all broken down within this range. These breakdown reactions
have been collectively called breakpoint reactions.
The breakpoint reactions only break down the decomposable fractions of the
respective substances. All the nondecomposables will remain after the breakpoint.
This will include, among other nondecomposables, the residual organic chloramines,
residual chloro-organic compounds, and residual ammonia chloramines. As we have
learned, the trichloramine fraction that comes from ammonia chloramines has to be
very small at this point to be of value as a disinfectant. All the substances that could
interfere with disinfection and all decomposables would have already been destroyed,
therefore, any amount of chlorine applied beyond the breakpoint will appear as free
chlorine residual.
Important knowledge is gained from this “chlorine residual versus applied chlorine”
curve. We have learned that all the ammonia chloramines practically disappear at the
breakpoint. We have also learned that the organic chloramines are not good disinfec-
tants. Therefore, as far as providing residual disinfectant in the distribution system is
concerned, chlorination up to the breakpoint should not be practiced. Since the maxi-
mum point corresponds to the maximum formation of the ammonia monochloramine,
the ideal practice would be to chlorinate with a dosage at this point. Note that, in
Figure 17.1, appreciable amounts of combined residuals still exists beyond the break-
point; however, these combined residuals mainly consist of combined chloro-organics,
which have little or no disinfecting properties, and combined organic chloramines,
which have, again, little or no disinfecting properties. Trichloramine, as we have

mentioned, will be present at a very minuscule concentration.
The practice of chlorinating up to and beyond the breakpoint is called super-
chlorination. Superchlorination ensures complete disinfection; however, it will only
leave free chlorine residuals in the distribution system, which can simply disappear
very quickly.
Note: If superchlorination is to be practiced to ensure complete disinfection and
it is also desired to have long-lasting chlorine residuals, then ammonia
should be added after superchlorination to bring back the chlorine dosage
to the point of maximum monochloramine formation.
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Example 17.8 Referring to Figure 17.1, if a dosage of 1.8 mg/L is administered,
determine the amount free chlorine residual that results, the amount of combined
residual that results, and the amount of combined ammonia chloramine residual that
results. Also, determine the amount of organic chloramine residual that results.
Solution: From the figure, the concentration of residual chlorine at a dosage of
1.8 mg/L = 0.38 mg/L. The concentration of the residual at the breakpoint = 0.20 mg/L.
Therefore,
free chlorine residual = 0.38 − 0.20 = 0.18 mg/L Ans
amount of combined residual = 0.20 mg/L Ans
amount of combined ammonia chloramine Ӎ 0 Ans
amount of organic chloramine cannot be determined Ans
Reactions with phenols. Chlorine reacts readily with phenol and organic com-
pounds containing the phenol group by substituting the hydrogen atom in the phenol
ring with the chlorine atom. These chloride substitution products are extremely odorous.
Because phenols and phenolic groups of compounds can be present in raw water
supplies as a result of discharges from industries and from natural decay of organic
materials, the formation of these odorous substances is a major concern of water
treatment plant operators.
Figure 17.2 shows the threshold odor as a function of pH and the concentration

of chlorine dosage. Figure 17.2a uses a concentration of 0.2 mg/L and, at a pH of
9.0, the maximum threshold odor concentration is around 28
µ
g/L. When the pH is
reduced to 8.0 this threshold worsens to around 20
µ
g/L, and when the pH is further
reduced to 7.0, the threshold concentration becomes worst at around 13
µ
g/L. Thus,
chlorination at acidic conditions would produce very bad odors compared to chlo-
rination at high pH values. This is very unfortunate, because HOCl predominates at
the lower pH range, which is the effective range of disinfection.
In Figure 17.2b the concentration of chlorine has been increased to 1.0 mg/L.
For the same adjustments of pH, the maximum threshold concentrations are about
the same as in Figure 17.2a; however, in the cases of pH’s 7.0 and 8.0, the threshold
odors practically vanish at approximately 3 to 5 h after contact as opposed to greater
than 60 h when the dosage was only 0.2 mg/L. Thus, increasing the dosage produces
the worst nightmare for odor production.
Figure 17.3 shows the reaction scheme for the breakdown of phenol to odorless
low molecular weight decomposition products using HOCl. The threshold odor
concentrations of the various chloride substituted phenolic compounds are also
indicated in brackets. Note that the worst offenders are 2-monochlorophenol and
2,4-dichlorophenol, which have an odor threshold of 2.0
µ
g/L. In order to effect
these breakdown reactions, superchlorination would be necessary, which would also
mean that the odor had increased before it disappeared.
Example 17.9 In the reaction scheme of Figure 17.3, what atom has been
displaced in ortho chlorophenol by the chlorine atom to form 2,6-dichlorophenol?

Solution: The hydrogen atom in the phenol ring has been displaced by the
chlorine. Ans
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Formation of trihalomethanes. Reactions of chlorine with organic compounds
such as fulvic and humic acids and humin produce undesirable by-products. These
by-products are known as disinfection by-products, DBPs. Examples of DBPs are
chloroform and bromochloromethane; these DBPs are suspected carcinogens. Snoeyink
and Jenkins (1980) wrote a series of reactions that demonstrate the basic steps by which
chloroform may be formed from an acetyl-group containing organic compounds.
These reactions are shown in Figure 17.4.
Note that the initial reaction involves the splitting of the hydrogen atom from the
methyl group using the hydroxyl ion. The hydroxyl ion is again used in (3), (5), and
(7). Because the hydroxyl is involved, this would mean that chloroform formation is
enchanced at high pH. To prevent formation of the chloroform, all that is necessary
FIGURE 17.2 Threshold odor from chlorination of phenol: (a) chlorine 0.2 mg/L, initial
phenol 5.0 mg/L; (b) chlorine 1.0 mg/L, initial phenol 5.0 mg/L; all at 25°C and threshold
odors are concentrations in
µ
g/L. (From Lee, G. F. (1967). Principles and Applications of
Water Chemistry. S. D. Faust and J. V. Hunters (Eds.). John Wiley & Sons, New York. With
permission.)
Threshold doorThreshold door
28
24
20
16
12
8
4

0
28
24
20
16
12
8
4
0
pH 9
pH 8
pH 7
pH 9
pH 8
pH 7
(a)
(b)
4 8 12 16 20 24 28 32 36 40 44 48 52 56 60
1 2 3 4 5 6 7 8
Time, hr
Time, hr
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then is simply to chlorinate at low pH, and this in fact, would be fortunate, since HOCl
predominates at low pH values instead of at high pH values. Based on this fact, if
superchlorination is to be conducted, it should be done at low pH values. Further
research, however, should be performed to establish the accuracy of this assumption.
FIGURE 17.3 Chlorination for the breakdown of phenol; numbers in brackets are odor
threshold concentrations in
µ

g/L.
FIGURE 17.4 Proposed scheme for chloroform formation.
oxidation low molecular
weight product:
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Chlorinated waters and wastewaters can contain not only chloroform and bro-
mochloromethane but also other brominated compounds. In addition, iodinated com-
pounds may also be produced; that is, this is the case if iodine (or bromine in the
case of brominated compounds) is present, in the first place. In general, the products
formed from the halogen family to produce the derivative products of methane are
called trihalomethanes. The formula is normally represented by CHX
3
, where X can
be Cl, Br, or I. Examples of other brominated species are bromodichloromethane,
chlorodibromomethane, and bromoform. The most commonly observed iodinated
trihalomethane is iododichloromethane. The reason why brominated and iodinated
trihalomethanes can be formed is that bromine and iodine are below chlorine in the
halogen family of the periodic table. It is an observed fact in chemistry that stronger
acids drive the weaker acids. The acid precursor of stronger acids (Cl in HOCl) are
higher in the series than those of the precursor of the weaker acids (Br and I in
HOBr and HOI, respectively). For this reason, HOCl drives the weaker acids HOBr
and HOI. These two acids then react in the same way as HOCl when it produces
the brominated and iodinated trihalomethanes.
Example 17.10 In Figure 17.4, when hydrogen is abstracted from the methyl
group, what happens to the double between carbon and oxygen?
Solution: As shown, the double bond is ruptured making the oxygen end neg-
ative and single bonded. The double bond switches to become a carbon-to-carbon
double bond. As indicated in subsequent reactions, this flip-flopping of the double
bond continues until the formation of chloroform. Ans

Acid generation. Whether or not acid will be produced depends upon the form of
chlorine disinfectant used. Using chlorine gas will definitely produce hydrochloric acid.
Sodium hypochlorite and calcium hypochlorite will not produce any acid; on the
contrary, it can result in the production of alkalinity. Superchlorination using HOCl
will definitely produce acids.
As shown in Equation (17.9), a mole of hydrochloric acid is produced per mole
of chlorine gas that reacts. Chlorination uses up the disinfectant, so this reaction would
be driven to the right and any mole of chlorine gas added will be consumed. Thus, if
a mmol/L of the gas is dosed, this will produce a mmol/L of HCl. This is equivalent
to one mgeq of the acid, which must also be equivalent to a mgeq of alkalinity. The
analytical equivalent mass of alkalinity in terms of CaCO
3
is 50 mg CaCO
3
per mgeq.
Thus, the mmol/L of hydrochloric acid produced will need 50 mg/L of alkalinity
expressed as CaCO
3
for its neutralization. Or, simply, one mmol of hydrochloric acid
requires 50 mg of alkalinity expressed as CaCO
3
for its neutralization.
In superchlorination, breakpoint reactions like Eqs. (17.37) to (17.42) will
transpire. A host of other reactions may also occur such that all these, as shown
by the preceding reactions, produce acids. The number of reactions are many,
therefore, it is not possible to predict the amount of acids produced by using simple
stoichiometry. The only way to determine this amount is to run a jar test as is done
in the determination of the optimum alum dose. Metcalf & Eddy, Inc. wrote that
in practice it is found that 15.0 mg/L of alkalinity is needed per mg/L of ammonia
nitrogen (Metcalf & Eddy, Inc., 1972). But, again, the best method would be to

run the jar test.
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Example 17.11 A flow of 25,000 m
3
/d of treated water is to be disinfected
using chlorine in pressurized steel cylinders. The raw water comes from a reservoir
where the water from the watershed has a very low alkalinity. With this low raw-
water alkalinity, coupled with the use of alum in the coagulation process, the alka-
linity of the treated water when it finally arrives at the chlorination tank is practically
zero. Calculate the amount of alkalinity required to neutralize the acid produced
during the addition of the chlorine gas.
Solution: Because the dose of the chlorine is not given, assume it to be
1.0 mg/L, which is equal to:
= 0.0142 millimol/L of Cl
2
Therefore,
Available chlorine. The strength of a chlorine disinfectant is measured in terms
of available chlorine. Available chlorine is defined as the ratio of the mass of chlorine
to the mass of the disinfectant that has the same unit of oxidizing power as chlorine.
The unit of disinfecting power of chlorine may be found as follows in terms of one
mole of electrons:
(17.45)
From this equation, the unit of oxidizing power of Cl
2
is Cl
2
/2 = 35.5. Consider
another chlorine disinfectant such as NaOCl. To find its available chlorine, its unit
of disinfecting power must also, first, be determined.

(17.46)
From this equation, the unit of disinfecting power of NaOCl is NaOCl/2 = 37.24.
Therefore, the available chlorine of NaOCl is the ratio of the mass of chlorine to
the mass of NaOCl that has the same unit of oxidizing power as chlorine, or available
chlorine of NaOCl = 35.5/37.24 = 0.95 or 95%. In other words, NaOCl is 95%
effective compared with chlorine.
Example 17.12 What is the available chlorine in dichloramine?
Solution: When dichloramine oxidizes a substance, its chlorine atom is reduced
to chloride; And, as gleaned from its formula, the nitrogen must be converted to the
ion. Thus, the oxidation-reduction reaction using only half the reaction is
20
2 35.5()

alkalinity needed 0.0142 50()= 0.71 mg/L as CaCO
3
Ans=
Cl
2
2e
−
2Cl
−
→+
NaOCl 2e
−
2H
+
Cl
−
Na

+
H
2
O++→++
NH
4
+
NHCl
2
4e
−
3H
+
2Cl
−
NH
4
+
+→++
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