1. WQT 134 Environmental Chemistry II
Chlorine Demand
WQT 134
Environmental Chemistry II

2. Lab 9 Objectives
Reading assignment:
AWWA Water Treatment, Principles and Practices of Water Supply Operation, Third Edition, American Waterworks Association, ISBN
Chapter 7 Disinfection
Understand/Review Chlorination in the disinfection Process

3. pH Impact on Free Chlorine

4. Chlorine Chemistry in Water
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

5. Which of the following is true about chlorine chemistry?
pH < 7 favors hypochlorous acid
pH >8 hypochlorite ions form which can still disinfectant
HOCL is 100 times more powerful then OCL-
All of the above

6. HOCL is 1,000 times more powerful then monochloramine?
True
False

7. 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

8. Cl:NH3 and pH Impact in Water
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
3. Monochloramine is most Cl2:NH3 <5:1
4. pH , Cl:NH3 > 5:1 -Monochloramine converts to dichloramines
5. pH < 4.5, Cl2:NH3 ~15:1 -Trichloramine which produces a very foul odor.

9. Chlorine-Ammonia (chloramines)
Chlorine- Ammonia reactions are governed by:
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

10. Chloramines in Water 3. pH 4.5-8.5; pH 8.0-8.5
Monochloramine: NH3 + HOCl NH2Cl + H2O
2. pH ; pH
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.

11. 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.

12. If abundant (15 times) Cl is around and the pH < 4
If abundant (15 times) Cl is around and the pH < 4.5 Trichloramine is the most likely chloramine to form?
True
False

13. Which of the following impact chloramine disinfection?pH
Temperature
Time
All of the above

14. Ammonium nitrogen and chlorine

15. Taste and Odor Problems
Free (HOCL) mg/L
Monochloramine 5 mg/L
Dichloramine .8 mg/L
Trichloramine mg/L

16. Chlorine Chemistry Summary
Cl2 + H2O HCl + HOCl
Chlorine Hydrochloric Hypochlorous
Ca(OCl)2 (aqueous) Ca2+ +2 OCl-
Calcium Hypochlorite Hypochlorite
NaOCl2 (aqueous) Na+ + OCl-
Sodium Hypochlorite Hypochlorite
HOCl H+ + OCl-
Hypochlorous Hypochlorite
HOCl NH NH2Cl + H2O
Hypochlorous Ammonia Chloramine

17. 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.

18. 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

20. 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.

21. In Zone 1 most of the chlorine dose gets used by chlorine demand ?
True
False

22. In Zone II chlorine reacts with ammonia to form combined chlorine ?
True
False

23. In Zone III some combined chlorine gets broken down to nitrogen gas ?
True
False

24. At Breakpoint chlorine added becomes free available chlorine?
True
False

25. 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

26. 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

27. Which of the following compounds is considered to be "free" chlorine in its present state?
HCl
Ca(OCl)2
NaOCl
HOCl

28. The amount of chlorine used by organic matter in water is a portion of the:
chlorine demand
chlorine dioxide
free available chlorine
chlorine residual

29. When feeding chlorine gas and ammonia, it is noticed that you have gone over the "hump", this means that you could be
feeding "free" chlorine
past the breakpoint toward the "free" point on the curve
forming chloramines
destroying chloramines and building DBP's

30. 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?
NCl3
NHCl2
NH2Cl
N3HCl2

31. Breakpoint chlorination is achieved when
no chlorine residual is detected
the strong chlorine taste at the plant is not found in the distribution system
the ammonia level in the water decreases
a chlorine dose increase results in an increase of chlorine residual

32. 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.
disinfection-by-product (DBP)
chloramine destruction
free-chlorine
monochloramine buildup

33. In water disinfection, hypochlorination refers to the use of _________ hypochlorite
Sodium
Calcium
Magnesium
Potassium

34. Chlorine residual may be determined using the reagent:
Diethyl-p-phenylene diamine
Ethylene diamine tetraacetic acid
Polychlorinated biphenyls
Sodium thiosulfate

35. A chlorine demand test will show the:
Safe amount of chlorine that may be fed without killing people
Number of lbs required to kill 100% of coliforms
Amount of chlorine required to give a desired residual after a given time
Amount of chlorine required to satisfy the biochemical oxygen demand

36. Which of the following methods is not used to determine chlorine residual?
Photometric
Iodometric
Titrimetric
Amperometric

37. Today's Lecture Objectives: To understand basic chlorine demand chemistry has been met?
Strongly Agree
Agree
Neutral
Disagree
Strongly Disagree