Considering Chloramines and Free Chlorine - David

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Chlorination Chemistry;
Considering Chloramines and Free Chlorine
Harris County Wastewater Symposium
Wastewater Treatment Plants & Bacteria: Strategies for Compliance
David Munn, PE
April 26, 2011
Wastewater
Treatment Plants & Bacteria:
Strategies for Compliance
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
Wastewater Treatment Plants & Bacteria:
Strategies for Compliance
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?
Wastewater Treatment Plants & Bacteria:
Strategies for Compliance
Introduction of the Cast
• Chlorine Compounds:
– Chlorine = Cl2
– Through Hydrolysis:
Cl2 + H2O ↔ HOCl + H+ + ClHOCl ↔ H+ + OCl– Hyprochlorous acid = HOCl
– Hypochlorite ion = OClWastewater Treatment Plants & Bacteria:
Strategies for Compliance
Introduction of the Cast
• Ammonia Compounds:
– Ammonia = NH3
• Often referenced as ammonia-nitrogen = NH3-N
• Typical wastewater concentration: 25-45 mg/l NH3-N
– Chloramine Compounds:
• Monochloramine = NH2Cl
• Dichloramine = NHCl2
• Tri-chloramine = NCl3 (aka nitrogen tri-chloride)
Wastewater Treatment Plants & Bacteria:
Strategies for Compliance
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
Wastewater Treatment Plants & Bacteria:
Strategies for Compliance
Disinfection
Performance
Comparison:
Free Chlorine
vs. Chloramines
Wastewater Treatment Plants & Bacteria:
Strategies for Compliance
Disinfection
Performance
Comparison:
Free Chlorine
vs. Chloramines
Wastewater Treatment Plants & Bacteria:
Strategies for Compliance
How to Achieve Free Chlorine
Wastewater Treatment Plants & Bacteria:
Strategies for Compliance
Steps to Free Chlorine
• Step 1 – Competing Reactions
– Organics, Fe2+, Mn2+, and H2S
– Result = No Chlorine Residual, No Disinfection
Capability
Wastewater Treatment Plants & Bacteria:
Strategies for Compliance
Steps to Free Chlorine
• Step 2 – Formation of Mono-chloramine
– NH3 + HOCl → NH2Cl (monochloramine) + H2
– Result = Increasing Chlorine Residual, Weak
Disinfection Capability
Wastewater Treatment Plants & Bacteria:
Strategies for Compliance
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
Wastewater Treatment Plants & Bacteria:
Strategies for Compliance
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
Wastewater Treatment Plants & Bacteria:
Strategies for Compliance
Steps to Free Chlorine
• Step 4 – Breakpoint
– Result= Oxidation of all ammonia and competing
substances
Wastewater Treatment Plants & Bacteria:
Strategies for Compliance
Steps to Free Chlorine
• Step 5 – Formation of Free Chlorine
– Result = Begin to Form Free Chlorine; Much
Stronger Disinfection Capability
Wastewater Treatment Plants & Bacteria:
Strategies for Compliance
How Much Chlorine is Required?
• Reaction Review:
–
–
–
–
NH4+ + HOCl → NH2Cl + H2O + H+
NH2Cl + HOCl → NHCl2 + H2O
0.5NHCl2 + 0.5 H2O → NOH + H+ + Cl0.5NHCl2 + 0.5 NOH → 0.5 N2 + 0.5 HOCl + 0.5H+ + 0.5Cl-
Summary of Reactions:
NH4+ + 1.5 HOCL → 0.5 N2 1.5 H2O + 2.5 H+ + 1.5 ClWastewater Treatment Plants & Bacteria:
Strategies for Compliance
How Much Chlorine is Required?
NH4+ + 1.5 HOCL → 0.5 N2 + 1.5H2O + 2.5 H+ + 1.5 ClExpress ammonia as N; Express Chlorine as CL2
Cl2 1.5  (2  35.45)
Weight Ratio  
 7.6
N
1  14
Theoretical Weight Ratio = 7.6 mg Cl2 to 1 mg N
“Real World” Weight Ratio = 8:1 to 10:1
Wastewater Treatment Plants & Bacteria:
Strategies for Compliance
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?
Wastewater Treatment Plants & Bacteria:
Strategies for Compliance
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
Wastewater Treatment Plants & Bacteria:
Strategies for Compliance
City of Munnsville Example
6.5:1
5.3:1
9:1
Wastewater Treatment Plants & Bacteria:
Strategies for Compliance
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
Wastewater Treatment Plants & Bacteria:
Strategies for Compliance
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 =
9.5 mg Cl2 / 0.9 mg N = 10.6
– 6.5:1 < 9:1 < 10.6:1
Wastewater Treatment Plants & Bacteria:
Strategies for Compliance
City of Munnsville Example
6.5:1
10.6
9:1
Wastewater Treatment Plants & Bacteria:
Strategies for Compliance
City of Munnsville Example
• Munnsville WWTP
– What is Free Chlorine Residual:
9.0 Cl2
Combined Chlorine  0.9 mg/l N 
 8.1mg/l Cl2
N
Residual Free Chlorine  9.5 mg/l Cl2 - 8.1 mg/l Cl2  1.4 mg/l Cl2
– Free Chlorine = 1.4 mg/l Cl2
Wastewater Treatment Plants & Bacteria:
Strategies for Compliance
Disinfection
Performance
Comparison:
Free Chlorine
vs. Chloramines
Wastewater Treatment Plants & Bacteria:
Strategies for Compliance
Disadvantages to Breakpoint
Chlorination
• Higher chlorine usage and chemical costs
• Possibly higher chlorine residual in effluent
• Greater potential for creating disinfectant byproducts
Wastewater Treatment Plants & Bacteria:
Strategies for Compliance

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