science of chloramination - mwuamanual of water supply practices, m56. 1st ed. maine water utilities...

Science of Chloramination

Maine Water Utilities AssociationMichael Koza, Portland Water District

June 2010

Chloramination

The process of combining chlorine and ammonia to create a “combined” form of chlorine for drinking water disinfection

Maine Water Utilities Association

Combined Compounds

Chloramine is a general term that describes three related compounds:

MONOchloramine, NH2Cl

DIchloramine, NHCl2

TRIchloramine, NCl3

Monochloramine is preferred for drinking water disinfection.


Chloramine structureThe molecular structure of all three chloramine compounds resembles ammonia, NH3A chlorine atom will replace one, two, and three hydrogen atoms respectively for the formation of mono-, di-, and trichloramines.


MonochloramineDichloramineTrichloramine

Why Chloraminate?

Provide Longer Lasting Residual –Driving factor for PWD in 1939Reduce DBP Formation – To meet lower MCL’s of Stage 1 D/DBP RuleReduce T & O – if applicable


Chloramine History

Successful usage in 1920-30’s (16%)

Decreased in popularity in 1940-50’s - scarcity of ammonia - World War II (2.6%)

SDWA in 1974 and 1986. THM regs (23%)

More stringent D/DBP Rules pushing % up ~30%

Utility ConsiderationsFor a drinking water utility, chloramination may or may not be a good alternative to free chlorination.What is the source water type and quality and the overall treatment process required to produce potable drinking water.Even with the full range of chloramine benefits, consideration must be given to secondary impacts associated with chloramination – primarily the potential for nitrification within the distribution system.


Considerations (cont.)With chloramination, an additional chemical must be purchased, stored, and applied to the process.To form the preferred chloramine compound, the appropriate weight ratio of chlorine and ammonia must be determined and then carefully managed.Free ammonia entering the distribution system must be monitored and limited to reduce the potential for nitrification.Beyond that, it may be necessary to incorporate any number of steps to actively control nitrification.


Considerations (cont.)

Consumers must be notified before a water system begins chloramination so that, if necessary, corrective action at point of use can be taken to minimize any risks to sensitive users.Chloramines are more difficult to remove, and ammonia can adversely affect patients receiving kidney dialysisAmmonia is toxic to aquarium fish.


History of Chlorine Usage

First used to disinfect water mains in the UK in 1897 after an outbreak of typhoid Introduced in the United States in the early 1900’s Credited with reducing the incidence of:

-cholera by 90%-typhoid and leptospirosis by 80% -amoebic dysentery by 50%

Most widely used chemical for disinfection in the United States


Adding Chlorine to Water… Meeting the Demand

Reaction 1: Iron, manganese, or hydrogen sulfide presentReaction 2: Next, any un-reacted chlorine will react with organic material, including bacteriaThese reactions satisfy the natural chlorinedemand of the water


Residual Chlorine

Standard procedure to add excess chlorine to allow for a residual amount in the water to retard microbial growth in the distribution system

Cl2 Dose - Demand = Cl2 Residual

2.50 mg/L - 0.2 mg/L = 2.30 mg/LMaine Water Utilities Association

Free Chlorine

Cl2 + H2O HOCl + H+ + Cl-

This reaction produces free chlorine, when measuring a free Cl2 residual….

HOCl = hypochlorous acid

….is being measuredMaine Water Utilities Association

Free Chlorination (Cont.)


Free chlorine is a strong oxidizing agent

Reacts with a wide variety of compounds

Therefore, not very persistent


Chloramination

Ammonia can occur naturally in a body of water or be added to drinking water as aqueous ammonia, ammonia gas or ammonia salts

….either way, the ammonia reacts with chlorine to form chloramines


Chloramination

Adding chlorine to water yields…


NH3

Ammonia reacts with hypochlorous acid, to produce a desirable, combined form of chlorine called monochloramine


Conditions for this ReactionNH3 + HOCl NH2Cl + H2O

The reaction of the hypochlorous acid and ammonia will convert practically all the free chlorine to monochloramine in under 1 second when pH is near 8 , temperature is near 25 degrees C and the chlorine to ammonia weight ratio is near 5 : 1


Ratio? What’s the Big Deal

The weight ratio of Cl2 to NH3 directly affectsthe type of chloramine formed. Under normal chloramination conditions, the Cl2 : NH3 ratio can range from 3 : 1 to 5 : 1, causing monochloramine to be the dominant species formed. Ratios greater than 5 : 1 favor the formation of di-chloramine due to the reaction of excess hypochlorous acid with monochloramine


Ratio Calculation

The standardized method for determining the chlorine to ammonia ratio is:

Chloramine residual (mg/L as Cl2) divided by…

Ammonia residual (mg/L as N)

PWD (typically) 2.30 mg/L as Cl2 / 0.50 mg/L NH3-N = 4.6


Ratio

Ideally, 1 molecule of Cl2 combines with 1 molecule of NH3

All Cl2 and NH3 molecules are combinedNothing left over, no free Cl2 or NH3

A 1 : 1 ratio, based on # of molecules ….but what about weight


Where Does the Weight Ratio Come From?

Individual atoms have weight or massTherefore, groups of atoms, or molecules, can be weighed


Chemistry of chloraminationTo fully understand the chemistry involved with chloramination, it is helpful to start with the basics:


Elements Have Mass


Weight Ratio

Cl weighs ~ 35. Since chlorine atoms occur in pairs as Cl2, the total weight is ~ 70Ammonia is measured as N (which is why it is expressed as NH3-N). Nitrogen, N, weighs 14The ratio between the weights is:

70 / 14 = 5.0

Cl2 and NH3 at 5:1

Dosing 3 chlorines into the water followed by 3 ammonias. Each chlorine will react with one ammonia to form a monochloramine molecule.

Cl2 ⇒ HOCl + NH3 = NH2ClWater flow ⇒ Cl2 ⇒ HOCl + NH3 = NH2Cl

Cl2 ⇒ HOCl + NH3 = NH2Cl

Perfect

Di and Trichloramine compounds

During monochloramine formation, an excess of chlorine can result in the unintended formation of di and trichloramine compounds…

NH2Cl + HOCl → NHCl2 (dichloramine) + H2O

NHCl2 + HOCl → NCl3 (trichloramine) + H2O


Overfeeding Chlorine Ratio Too High

What if too much chlorine is added (or too little ammonia)? Ratio greater than 5 : 1


HOCl NH3 NH2ClWater flow ⇒ HOCl + NH3 = NH2Cl

HOCl NH3 NH2Cl

HOCl ⇑HOCl ⇒ ⇒ ⇒ ⇒HOCl

di-chloramine; undesirable T & O

NHCl2

= NHCl2

NHCl2

Underfeeding Chlorine Ratio Too Low

What if too little chlorine is dosed (or too much ammonia? Ratio less than 5 : 1


HOCl NH3 NH2ClWater flow ⇒ HOCl + NH3 = NH2Cl

HOCl NH3 NH2Cl

NH3 NH3 ⇒NH3

Free ammonia; > 0.10 mg/L fuels nitrification in DS

Chloramination Trade offBENEFITS

Less potential for DBP’s (THM’s HAA’s)Longer lasting residualLower taste/odor threshold than free Cl2

CHALLENGESMonochloramine 4x weaker biocide than free Cl2

… takes more of it (2.0 vs 0.5 mg/L)More to think about, i.e feed pumps, ratioNitrification


Nitrification A Biochemical Process

Free ammonia fuels the nitrification process.Bacteria (AOB) oxidize the ammonia and produce nitrite, NO2.This process eliminates ammonia, as it creates the nitrite necessary for the next step.Bacteria (NOB) then oxidize nitrite into nitrate, NO3 and nitrogen gas N2


Free AmmoniaNH3

NitriteNO2

NitrateNO3

Conditions for Nitrification

The main contributing factors include:Excess ammonia in the distribution systemPresence of nitrifying bacteriaWarm water temperatures (15 C and higher)Long detention times (excessive water age)Low disinfectant residuals to combat nitrifiers


Impacts of Nitrification

Rapid chloramine lossDecrease in pH Elevation of nitrite, nitrate and HPC

For example, a 0.40 mg/L nitrite concentration will consume 2.0 mg/L of the chloramine residual.

The depletion of chloramine residuals could leave a system vulnerable to bacteriological contamination.


Nitrification Control Measures

Monitor ratio and free ammonia at POEFlushing programExercising tanks/reservoirsMoving water through system quicklyFree chlorine boost to recombine free NH3Periodic switch to free Cl2, or breakpoint chlorinating


Breakpoint phenomena


Long Term ControlWhat Does AWWA Say?

“…limiting the amount of free ammonia available in the treatment plant finished water is a valid practice and one that utilities should pursue.”


Wrap up

Valid reasons to switch to chloramination, must consider many things

Process must be monitored at the POE as well as the DS for effectiveness

Nitrification is a real concern but can be managed successfully


References

Connell, G. F., 1996. The Chlorination/ Chloramination Handbook. American Water Works Association. Water Disinfection Series.

Fundamentals and Control of Nitrification in Chloraminated Drinking Water Systems. 2006.American Water Works Association. Manual of Water Supply Practices, M56. 1st ed.


Questions

Contact info:

Michael KozaRegulatory/Security Advisor

Portland Water District(207) 523-5402

[email protected]

science of chloramination - mwuamanual of water supply practices, m56. 1st ed. maine water utilities...

Documents