Transcript Lecture on Chlorine Demand

Chlorine Demand
WQT 134
Environmental Chemistry II
Lab 9 Objectives
Reading assignment:
AWWA 2003. Water Treatment, Principles and Practices of Water Supply Operation, Third
Edition, American Waterworks Association, ISBN 1-58321-230-2
Chapter 7 Disinfection
•
Understand/Review Chlorination in the disinfection
Process
pH Impact on Free Chlorine
http://ewr.cee.vt.edu/environmental/teach/wtprimer/chlorine/chlorine.html
Chlorine Chemistry in Water
1. Start with chlorine gas Cl2 + H2O
HOCl + HCl
2. If the pH of the water is greater than 8, the hypochlorus
acid (HOCl-) will dissociate (break) to yield hypochlorite
ion.
HOCl
H+ + OCl–
@pH>8
3. pH is less than 7, HOCl will not dissociate.
HOCl
HOCl
@pH<7
Free residual hypochlorous acid (HOCl) is more
1000 times more effective than combined residual
monchloramine
http://ewr.cee.vt.edu/environmental/teach/wtprimer/chlorine/chlorine.html
Which of the following is true
about chlorine chemistry?
1. pH < 7 favors
hypochlorous acid
2. pH >8 hypochlorite ions
form which can still
disinfectant
3. HOCL is 100 times more
powerful then OCL4. All of the above
70%
30%
.
ft
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o
0
10
is
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ab
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tim
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>8
pH
A
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.
...
oc
hy
p
rs
fa
vo
7
<
pH
ov
e
0% 0%
HOCL is 1,000 times more
powerful then monochloramine?
88%
Fa
ls
e
13%
Tr
ue
1. True
2. False
Basic Chlorine Compounds
Chlorine Demand: chlorine reacted with
– Clay, silt, iron, manganese, bacteria
Combined Chlorine Residuals
Chloramines: chlorine reacted with
– Inorganic ammonia compounds (NH3)
– Organic nitrogen proteins (amino acids)
– Less DBP
– Weak against virus and protozoa
Cl:NH3 and pH Impact in Water
1. If ammonia (NH3) is present in water, the
hypochlorus acid (HOCL) will react to form one
kind of chloramine depending on the pH,
temperature, and reaction time.
2. Monochloramine and dichloramine are formed at pH 4.5 8.5
3. Monochloramine is most common @pH>7.5; Cl2:NH3 <5:1
4. pH 4-5-7.5, Cl:NH3 > 5:1 -Monochloramine converts to
dichloramines
5. pH < 4.5, Cl2:NH3 ~15:1 -Trichloramine which produces a
very foul odor.
http://ewr.cee.vt.edu/environmental/teach/wtprimer/chlorine/chlorine.html
Chlorine-Ammonia (chloramines)
Chlorine- Ammonia reactions are governed by:
1.
2.
3.
4.
5.
Rates of formation of mono and dichloramine.
pH
Temperature
Time
Cl:NH3 ratio
Fact: High Cl:NH3 ratio, low temp & pH favor dichloramine
Combined Available Chlorine- chlorine existing in the water in
chemical combination with ammonia-nitrogen or organic
nitrogen
http://ewr.cee.vt.edu/environmental/teach/wtprimer/chlorine/chlorine.html
Chloramines in Water
Chloramines
3. pH 4.5-8.5; pH 8.0-8.5
Monochloramine: NH3 + HOCl
NH2Cl + H2O
2. pH 4.5-8.5; pH 4.5-8.0
Dichloramine: NH2Cl + 2HOCl
NHCl2 + 2H2O
4. pH <4.5
Trichloramine: NHCl2 + 3HOCl
NHCl3 + 3H2O
•Chloramines are an effective disinfectant against bacteria
but not against viruses.
•Add more chlorine to the wastewater to prevent the
formation of chloramines and form stronger disinfectants.
http://ewr.cee.vt.edu/environmental/teach/wtprimer/chlorine/chlorine.html
Add the Extra Free Chlorine
(HOCL)
5. Additional free chlorine (HOCL or OCL-) reacts with
chloramine to produce hydrogen ion, water , and nitrogen gas
which will come out of solution.
In the case of the monochloramine, the following reaction
occurs:
2NH2Cl + HOCl
N2 + 6HCl + H2O
Added free chlorine reduces the concentration of chloramines
in the disinfection process. Instead the chlorine that is added
is allowed to form the stronger disinfectant, hypochlorus acid.
http://ewr.cee.vt.edu/environmental/teach/wtprimer/chlorine/chlorine.html
If abundant (15 times) Cl is
around and the pH < 4.5
Trichloramine is the most likely
chloramine to form?
82%
Fa
ls
e
18%
Tr
ue
1. True
2. False
Which of the following impact
chloramine disinfection?
pH
Temperature
Time
All of the above
100%
m
e
ov
e
0%
ft
he
ll
o
A
Te
m
pe
ab
Ti
0%
ra
tu
re
0%
pH
1.
2.
3.
4.
Ammonium nitrogen and
chlorine
Taste and Odor Problems
• Free (HOCL)
20 mg/L
• Monochloramine
5 mg/L
• Dichloramine
.8 mg/L
• Trichloramine
.02 mg/L
Chlorine Chemistry Summary
Cl2 + H2O
Chlorine
HCl
+
Hydrochloric
HOCl
Hypochlorous
Ca(OCl)2 (aqueous) Ca2+ +2 OClCalcium Hypochlorite Hypochlorite
NaOCl2 (aqueous)
Na+ + OClSodium Hypochlorite
Hypochlorite
HOCl
Hypochlorous
HOCl +
Hypochlorous
H+ + OClHypochlorite
NH3
Ammonia
NH2Cl + H2O
Chloramine
Chlorine Residual Analysis
• Starch Iodide Titration: common in wastewater
• Amperometric Titration: common in wastewater
with cloudy or turbidity problems
• DPD colorometric- common in water and
wastewater. Measures free residual or combined
residual.
Breakpoint Chlorination
Breakpoint chlorination: The point at which near complete oxidation of
nitrogen compounds are reached . Any point beyond breakpoint is
mostly free chlorine (HOCL and OCL-)
A. Amount of chlorine required
Theory: 7.6 to 15 times the ammonia nitrogen content of the water
Practice: up to 25 times the ammonia nitrogen content
B. Beyond breakpoint
90% free residual chlorine (HOCL and OCL-)
10% combined chlorine
C. Why must breakpoint chlorination be reached?
• Necessary for the production of free residual chlorine (HOCL and OCL-)
• Reduces taste and odors
• Reduces chloramines
Breakpoint Chlorination
Zone I: Chlorine is destroyed by reducing agents such as
iron, manganese, clay and silt. Chlorine reduced to
chloride
Zone II: Chlorine comes into contact with organics and
ammonia. Chloroorganics and chloramines are formed.
Zone III: Chloroorganics and chloramines are partially
destroyed. Chloramines are broken down and converted to
nitrogen gas which leaves the system
Zone IV: Breakpoint. Beyond this point, free available
residual is formed. Some chloroorganics still remain as
combined residual.
Chlorine demand is difference between applied chlorine and the free chlorine
residual at any two points on the breakpoint curve.
In Zone 1 most of the chlorine
dose gets used by chlorine
demand ?
100%
Fa
ls
e
0%
Tr
ue
1. True
2. False
In Zone II chlorine reacts with
ammonia to form combined
chlorine ?
70%
Fa
ls
e
30%
Tr
ue
1. True
2. False
In Zone III some combined
chlorine gets broken down to
nitrogen gas ?
100%
Fa
ls
e
0%
Tr
ue
1. True
2. False
At Breakpoint chlorine added
becomes free available chlorine?
100%
Fa
ls
e
0%
Tr
ue
1. True
2. False
Application of Chlorine in
Water Treatment Distribution
• Free Chlorine Residual- chlorine dosed in excess of the demand and
beyond the breakpoint will produce free chlorine residual
HOC (hypochlorous acid) and OCL- (hypochlorite ion).
Free chlorine – 10 to 100 times more effective then
combined residual
•Oxidizes certain taste and odor compounds
like H2S
•Oxidizes Fe+2, Mn+2
•Controls biofilms on filters and mains
Which of the following waters
would have the highest
concentration of
hypochlorous acid?
a. pH of 7
b. pH of 8
c. pH of 9
d. pH of 10
82%
18%
10
9
d.
pH
of
of
pH
c.
b.
pH
of
of
pH
a.
0%
8
7
0%
Which of the following
compounds is considered to be
"free" chlorine in its
present state?
HCl
Ca(OCl)2
NaOCl
HOCl
91%
9%
H
O
C
l
l
aO
C
a(
O
C
0%
N
C
Cl
)2
l
0%
H
1.
2.
3.
4.
The amount of chlorine used by
organic matter in water is a
portion of the:
chlorine demand
chlorine dioxide
free available chlorine
chlorine residual
89%
11%
al
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ch
1.
2.
3.
4.
When feeding chlorine gas and ammonia,
it is noticed that you have gone over the
"hump", this means that you could be
36%
1. feeding "free" chlorine
2. past the breakpoint
toward the "free" point
on the curve
3. forming chloramines
4. destroying chloramines
and building DBP's
36%
18%
de
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9%
Chlorine can react with nitrogen in several ways to
form chloramine products. Which of the following is
the trichloramine (nitrogen trichloride) compound
mostly responsible for taste & odor development,
when using ammonia and chlorine as a disinfectant?
91%
NCl3
NHCl2
NH2Cl
N3HCl2
9%
N
3H
C
l2
l
2C
H
N
N
H
l3
C
0%
Cl
2
0%
N
1.
2.
3.
4.
Breakpoint chlorination is
achieved when
1. no chlorine residual is
detected
2. the strong chlorine
taste at the plant is not
found in the
distribution system
3. the ammonia level in
the water decreases
4. a chlorine dose
increase results in an
increase of chlorine
residual
88%
13%
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0% 0%
The chlorine curve has three locations
where a measurable residual can be
detected. The strongest, most aggressive
form of chlorine is found on the _______
portion of the curve.
1. disinfection-by-product
(DBP)
2. chloramine destruction
3. free-chlorine
4. monochloramine buildup
91%
9%
...
bu
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-c
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-p
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n
0% 0%
In water disinfection,
hypochlorination refers to the
use of _________ hypochlorite
0%
M
ta
s
si
um
si
um
0%
ag
ne
um
al
ci
C
di
um
0%
Po
Sodium
Calcium
Magnesium
Potassium
So
1.
2.
3.
4.
100%
Chlorine residual may be
determined using the reagent:
1. Diethyl-p-phenylene diamine
2. Ethylene diamine tetraacetic
acid
3. Polychlorinated biphenyls
4. Sodium thiosulfate
100%
te
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So
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0% 0% 0%
A chlorine demand test will show
the:
1. Safe amount of chlorine that
may be fed without killing
people
2. Number of lbs required to
kill 100% of coliforms
3. Amount of chlorine required
to give a desired residual
after a given time
4. Amount of chlorine required
to satisfy the biochemical
oxygen demand
75%
13% 13%
A
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bs
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N
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fe
am
ou
nt
of
ch
lo
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.
0%
Which of the following methods
is not used to determine chlorine
residual?
Photometric
Iodometric
Titrimetric
Amperometric
50%
25%
17%
A
m
pe
ro
m
et
r ic
tri
m
et
ric
Ti
ric
do
m
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ot
om
et
ric
8%
Ph
1.
2.
3.
4.
Today's Lecture Objectives: To
understand basic chlorine
demand chemistry has been
met?
91%
9%
gr
e
St
ro
ng
ly
Di
sa
Di
sa
0%
gr
ee
e
0%
l
ee
Ag
r
Ne
ut
ra
ng
ly
Ag
re
e
0%
St
ro
1. Strongly Agree
2. Agree
3. Neutral
4. Disagree
5. Strongly Disagree