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

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

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?

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

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)

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

Disinfection Performance Comparison: Free Chlorine vs. Chloramines

Disinfection Performance Comparison: Free Chlorine vs. Chloramines

How to Achieve Free Chlorine

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

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

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

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

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

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

How Much Chlorine is Required? Reaction Review: NH4+ + HOCl → NH2Cl + H2O + H+ NH2Cl + HOCl → NHCl2 + H2O 0.5NHCl2 + 0.5 H2O → NOH + H+ + Cl- 0.5NHCl2 + 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 Cl-

How Much Chlorine is Required? NH4+ + 1.5 HOCL → 0.5 N2 + 1.5H2O + 2.5 H+ + 1.5 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

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?

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

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

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

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

City of Munnsville Example 6.5:1 10.6 9:1

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

Disinfection Performance Comparison: Free Chlorine vs. Chloramines

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