Science of Chloramination Maine Water Utilities Association Michael 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. Maine Water Utilities Association Chloramine structure The molecular structure MonochloramineTrichloramineDichloramine of all three chloramine compounds resembles ammonia, NH3 A chlorine atom will replace one, two, and three hydrogen atoms respectively for the formation of mono-, di-, and trichloramines. Maine Water Utilities Association Why Chloraminate? Provide Longer Lasting Residual – Driving factor for PWD in 1939 Reduce DBP Formation – To meet lower MCL’s of Stage 1 D/DBP Rule Reduce T & O – if applicable Maine Water Utilities Association 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 Considerations For 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. Maine Water Utilities Association 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. Maine Water Utilities Association 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 dialysis Ammonia is toxic to aquarium fish. Maine Water Utilities Association 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 Maine Water Utilities Association Adding Chlorine to Water… Meeting the Demand Reaction 1: Iron, manganese, or hydrogen sulfide present Reaction 2: Next, any un-reacted chlorine will react with organic material, including bacteria These reactions satisfy the natural chlorine demand of the water Maine Water Utilities Association 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/L Maine 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 measured Maine Water Utilities Association Free Chlorination (Cont.) + - Cl2 + H2O ' HOCl + H + Cl Free chlorine is a strong oxidizing agent Reacts with a wide variety of compounds Therefore, not very persistent Maine Water Utilities Association 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 Maine Water Utilities Association Chloramination Adding chlorine to water yields… + - Cl2 + H2O ' HOCl + H + Cl NH3 Ammonia reacts with hypochlorous acid, to produce a desirable, combined form of chlorine called monochloramine Maine Water Utilities Association Conditions for this Reaction NH3 + 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 Maine Water Utilities Association Ratio? What’s the Big Deal The weight ratio of Cl2 to NH3 directly affects the 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 Maine Water Utilities Association 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 Maine Water Utilities Association Ratio Ideally, 1 molecule of Cl2 combines with 1 molecule of NH3 All Cl2 and NH3 molecules are combined Nothing left over, no free Cl2 or NH3 A 1 : 1 ratio, based on # of molecules ….but what about weight Maine Water Utilities Association Where Does the Weight Ratio Come From? Individual atoms have weight or mass Therefore, groups of atoms, or molecules, can be weighed Maine Water Utilities Association Chemistry of chloramination To fully understand the chemistry involved with chloramination, it is helpful to start with the basics: Maine Water Utilities Association Elements Have Mass Maine Water Utilities Association Weight Ratio Cl weighs ~ 35. Since chlorine atoms occur in pairs as Cl2, the total weight is ~ 70 Ammonia is measured as N (which is why it is expressed as NH3-N). Nitrogen, N, weighs 14 The 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 = NH2Cl Water 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 Maine Water Utilities Association Overfeeding Chlorine Ratio Too High What if too much chlorine is added (or too little ammonia)? Ratio greater than 5 : 1 HOCl NH3 NH2Cl NHCl2 Water flow ⇒ HOCl + NH3 = NH2Cl = NHCl2 HOCl NH3 NH2Cl NHCl2 HOCl ⇑ di-chloramine; HOCl ⇒⇒⇒ ⇒ undesirable T & O HOCl Maine Water Utilities Association Underfeeding Chlorine Ratio Too Low What if too little chlorine is dosed (or too much ammonia? Ratio less than 5 : 1 HOCl NH3 NH2Cl Water flow ⇒ HOCl + NH3 = NH2Cl HOCl NH3 NH2Cl NH3 NH3 ⇒ Free ammonia; NH3 > 0.10 mg/L fuels nitrification in DS Maine Water Utilities Association Chloramination Trade off BENEFITS Less potential for DBP’s (THM’s HAA’s) Longer lasting residual Lower taste/odor threshold than free Cl2 CHALLENGES Monochloramine 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, ratio Nitrification Maine Water Utilities Association 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 Ammonia Nitrite Nitrate NH3 NO2 NO3 Maine Water Utilities Association Conditions for Nitrification The main contributing factors include: Excess ammonia in the distribution system Presence of nitrifying bacteria Warm water temperatures (15 C and higher) Long detention times (excessive water age) Low disinfectant residuals to combat nitrifiers Maine Water Utilities Association Impacts of Nitrification Rapid chloramine loss Decrease 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. Maine Water Utilities Association Nitrification Control Measures Monitor ratio and free ammonia at POE Flushing program Exercising tanks/reservoirs Moving water through system quickly Free chlorine boost to recombine free NH3 Periodic switch to free Cl2, or breakpoint chlorinating Maine Water Utilities Association Breakpoint phenomena Maine Water Utilities Association Long Term Control What 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.” Maine Water Utilities Association 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 Maine Water Utilities Association 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. Maine Water Utilities Association Questions Contact info: Michael Koza Regulatory/Security Advisor Portland Water District (207) 523-5402 [email protected].