Presentation on theme: "Chlorination Chemistry; Considering Chloramines and Free Chlorine"— Presentation transcript:
1Chlorination Chemistry; Considering Chloramines and Free Chlorine Harris County Wastewater SymposiumWastewater Treatment Plants & Bacteria: Strategies for ComplianceDavid Munn, PEApril 26, 2011
2Chlorination Chemistry; Considering Chloramines and Free Chlorine Purpose of this Section:Explain how ammonia affects your wastewater disinfection capabilitiesProvide another tool for reducing bacteria levels in wastewater effluent
3Chlorination Chemistry; Considering Chloramines and Free Chlorine Goals of this Section:Introduction of Basic Concepts & Chemical CompoundsDiscuss the Formation of ChloraminesDisinfection Strength Comparison: Chloramines vs. Free ChlorineHow to Achieve a Free Chlorine Residual?
4Introduction of the Cast Chlorine Compounds:Chlorine = Cl2Through Hydrolysis:Cl2 + H2O ↔ HOCl + H+ + Cl-HOCl ↔ H+ + OCl-Hyprochlorous acid = HOClHypochlorite ion = OCl-
5Introduction of the Cast Ammonia Compounds:Ammonia = NH3Often referenced as ammonia-nitrogen = NH3-NTypical wastewater concentration: mg/l NH3-NChloramine Compounds:Monochloramine = NH2ClDichloramine = NHCl2Tri-chloramine = NCl3 (aka nitrogen tri-chloride)
6Is My WWTP Forming Chloramines? Typical Influent Ammonia Concentration = 30 mg/l NH3-NSimultaneous Nitrification / Denitrification > 90% removal3.0 mg NH3-N requires 27 mg/l Cl2 to achieve breakpointLower NH3-N concentrations at WWTP outfall is a result of NH3-N oxidation by Cl2
7Disinfection Performance Comparison: Free Chlorine vs. Chloramines
8Disinfection Performance Comparison: Free Chlorine vs. Chloramines
14Steps to Free Chlorine Step 4 – Breakpoint Result= Oxidation of all ammonia and competing substances
15Steps to Free Chlorine Step 5 – Formation of Free Chlorine Result = Begin to Form Free Chlorine; Much Stronger Disinfection Capability
16How Much Chlorine is Required? Reaction Review:NH4+ + HOCl → NH2Cl + H2O + H+NH2Cl + HOCl → NHCl2 + H2O0.5NHCl 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-
17How Much Chlorine is Required? NH HOCL → 0.5 N H2O H Cl-Express ammonia as N; Express Chlorine as CL2Theoretical Weight Ratio = 7.6 mg Cl2 to 1 mg N“Real World” Weight Ratio = 8:1 to 10:1
18City of Munnsville Example Munnsville WWTPFlow = 1.0 MGDInfluent Ammonia = 30 mg/l NH3-NBiological Nitrification/Denitrification Capability= 95% NH3-N removalChlorination injection rate = 8 mg/lWhat is primary chlorine compound?
19City of Munnsville Example Munnsville WWTPClarifier Effluent NH3-N Concentration = 30 mg/l * (1 – 95%) = 1.5 mg/lChlorine Injection Rate = 8 mg/lChlorine : Nitrogen Ratio = 8 mg Cl2 / 1.5 mg N = 5.35.3:1 < 6.5:1 < 9:1Walk through exampleWhat is primary chlorine compound?chloramineIf mixing is good and pH is in a reasonable range, then probably monochloramine.Certainly not free chlorineRecall what 6.5:1 and 9:1 reference6.5:1 = realworld weight ratio necessary to consume all free ammonia into chloramine compounds9:1 = realworld weight ratio necessary to oxidize all ammonia and achieve breakpoint chlorination
20City of Munnsville Example 6.5:15.3:1How to get free chlorine?Introduce road mapGo over in more detail in next 4 or 5 slidesAs an overviewX and y axisAs chlorine dosage is addedChlorine residualWhat compound you haveNotice there is not a 1:1 ratioWalk through stepsStep 1 consuming chlorine through competing reactions: no chlorine residual9:1