1. Chlorination Chemistry; Considering Chloramines and Free Chlorine
Harris County Wastewater Symposium
Wastewater Treatment Plants & Bacteria: Strategies for Compliance
David Munn, PE
April 26, 2011

2. Chlorination Chemistry; Considering Chloramines and Free Chlorine
Purpose of this Section:
Explain how ammonia affects your wastewater disinfection capabilities
Provide another tool for reducing bacteria levels in wastewater effluent

3. Chlorination Chemistry; Considering Chloramines and Free Chlorine
Goals of this Section:
Introduction of Basic Concepts & Chemical Compounds
Discuss the Formation of Chloramines
Disinfection Strength Comparison: Chloramines vs. Free Chlorine
How to Achieve a Free Chlorine Residual?

4. Introduction of the Cast
Chlorine Compounds:
Chlorine = Cl2
Through Hydrolysis:
Cl2 + H2O ↔ HOCl + H+ + Cl-
HOCl ↔ H+ + OCl-
Hyprochlorous acid = HOCl
Hypochlorite ion = OCl-

5. Introduction of the Cast
Ammonia Compounds:
Ammonia = NH3
Often referenced as ammonia-nitrogen = NH3-N
Typical wastewater concentration: mg/l NH3-N
Chloramine Compounds:
Monochloramine = NH2Cl
Dichloramine = NHCl2
Tri-chloramine = NCl3 (aka nitrogen tri-chloride)

6. Is My WWTP Forming Chloramines?
Typical Influent Ammonia Concentration = 30 mg/l NH3-N
Simultaneous Nitrification / Denitrification > 90% removal
3.0 mg NH3-N requires 27 mg/l Cl2 to achieve breakpoint
Lower NH3-N concentrations at WWTP outfall is a result of NH3-N oxidation by Cl2

7. Disinfection Performance Comparison: Free Chlorine vs. Chloramines

8. Disinfection Performance Comparison: Free Chlorine vs. Chloramines

9. How to Achieve Free Chlorine

10. Steps to Free Chlorine Step 1 – Competing Reactions
Organics, Fe2+, Mn2+, and H2S
Result = No Chlorine Residual, No Disinfection Capability

11. Steps to Free Chlorine Step 2 – Formation of Mono-chloramine
NH3 + HOCl → NH2Cl (monochloramine) + H2
Result = Increasing Chlorine Residual, Weak Disinfection Capability

12. Steps to Free Chlorine Step 2 – Formation of Mono-chloramine
Theoretical Weight Ratio: 71 mg Cl2 to 14 mg N = 5.1:1
“Realworld” Weight Ratio: ≈ 6.5:1

13. Steps to Free Chlorine Step 3 – Formation of Di- and Tri-chloramines
NH2Cl + HOCl → NHCl2 (dichloramine) + H2O
NHCl2 + HOCl → NCl3 (nitrogen tri-chloride ) + H2O
Result = Lower Chlorine Residual, Lower Disinfection Capability

14. Steps to Free Chlorine Step 4 – Breakpoint
Result= Oxidation of all ammonia and competing substances

15. Steps to Free Chlorine Step 5 – Formation of Free Chlorine
Result = Begin to Form Free Chlorine; Much Stronger Disinfection Capability

16. How Much Chlorine is Required?
Reaction Review:
NH4+ + HOCl → NH2Cl + H2O + H+
NH2Cl + HOCl → NHCl2 + H2O
0.5NHCl H2O → NOH + H+ + Cl-
0.5NHCl NOH → 0.5 N HOCl + 0.5H Cl-
Summary of Reactions:
NH HOCL → 0.5 N2 1.5 H2O H Cl-

17. How Much Chlorine is Required?
NH HOCL → 0.5 N H2O H Cl-
Express ammonia as N; Express Chlorine as CL2
Theoretical Weight Ratio = 7.6 mg Cl2 to 1 mg N
“Real World” Weight Ratio = 8:1 to 10:1

18. City of Munnsville Example
Munnsville WWTP
Flow = 1.0 MGD
Influent Ammonia = 30 mg/l NH3-N
Biological Nitrification/Denitrification Capability= 95% NH3-N removal
Chlorination injection rate = 8 mg/l
What is primary chlorine compound?

19. City of Munnsville Example
Munnsville WWTP
Clarifier Effluent NH3-N Concentration = 30 mg/l * (1 – 95%) = 1.5 mg/l
Chlorine Injection Rate = 8 mg/l
Chlorine : Nitrogen Ratio = 8 mg Cl2 / 1.5 mg N = 5.3
5.3:1 < 6.5:1 < 9:1
Walk through example
What is primary chlorine compound?
chloramine
If mixing is good and pH is in a reasonable range, then probably monochloramine.
Certainly not free chlorine
Recall what 6.5:1 and 9:1 reference
6.5:1 = realworld weight ratio necessary to consume all free ammonia into chloramine compounds
9:1 = realworld weight ratio necessary to oxidize all ammonia and achieve breakpoint chlorination

20. City of Munnsville Example
6.5:1
5.3:1
How to get free chlorine?
Introduce road map
Go over in more detail in next 4 or 5 slides
As an overview
X and y axis
As chlorine dosage is added
Chlorine residual
What compound you have
Notice there is not a 1:1 ratio
Walk through steps
Step 1 consuming chlorine through competing reactions: no chlorine residual
9:1

21. City of Munnsville Example
Munnsville WWTP
Break-point Chlorination Strategy:
Flow = 1.0 MGD
Influent Ammonia = 30 mg/l
Biological Nitrification/Denitrification Capability= 97% NH3 removal
Chlorination injection rate = 9.5 mg/l

22. City of Munnsville Example
Munnsville WWTP
Clarifier Effluent NH3 Concentration = 30 mg/l * (1 – 97%) = 0.9 mg/l N
Chlorine Injection Rate = 9.5 mg/l Cl2
Chlorine : Ammonia Ratio = mg Cl2 / 0.9 mg N = 10.6
6.5:1 < 9:1 < 10.6:1

23. City of Munnsville Example
6.5:1
10.6
9:1

24. City of Munnsville Example
Munnsville WWTP
What is Free Chlorine Residual:
Free Chlorine = 1.4 mg/l Cl2

25. Disinfection Performance Comparison: Free Chlorine vs. Chloramines

26. Disadvantages to Breakpoint Chlorination
Higher chlorine usage and chemical costs
Possibly higher chlorine residual in effluent
Greater potential for creating disinfectant by-products