Chloramine Production & Monitoring in Florida's Water Supply Systems
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Chloramine Production & Monitoring in Florida’s Water Supply Systems Robert D. McVay ource waters in many areas of the state weeks, during which superchlorinating with contain elevated levels of total organic car - free chlorine is used to destroy problematic or - Robert McVay, P.E., is the drinking water bon (TOC) which combines with free ganisms and unstable conditions in the water trainer for the Florida Rural Water Associa - Schlorine to produce disinfection byproducts system. When this happens, DBPs often exceed tion. His duties include assisting water (DBPs). These compounds are regulated at 80 regulatory limits, sloughing of pipeline growth treatment systems that experience disinfec - parts per billion and 60 parts per billion for total and sediments can occur, and customers re - tion byproduct problems and troubleshoot - trihalomethanes and haloacetic acids. Meeting ceive high concentrations of chlorine, often re - ing small- and medium-size water these low regulatory limits can be difficult when sulting in taste and odor complaints. treatment plants in Florida. This article was free chlorine is used as a disinfectant. Fortunately, the use of chloramines and the presented as a technical paper at the With the increase in regulatory attention control of problems in the distribution system FSAWWA Fall Conference in November such as the new Stage 2, Initial Distribution follow predictable patterns that can be identified 2007. System Evaluation, rules that require all pub - and corrected before conditions deteriorate to a lic water supply systems to report elevated point where problems occur. This article exam - DBP values, and given the problems meeting ines the concepts in the formation and the use of current rules, many water treatment systems chloramines and suggests pre-emptive tech - distribution system. The purpose of second - have elected to switch to chloramines as their niques to identify the status of deteriorating con - ary disinfection is to provide enough disinfec - secondary disinfectant. ditions and implement corrective actions to keep tant to prevent bacteriological growth. Chloramination, or producing chlorine the chloramination process under control. The It is useful to review the mechanisms of by adding ammonia in the presence of free work supporting these recommendations was how free chlorine and chloramines are made chlorine, has been used by many water treat - performed in a Florida Department of Environ - in the disinfection process. When free chlorine ment systems for a number of years for con - mental Protection-sponsored study for the Lake (Cl 2) is initially added to water, it goes through trolling the production of DBPs. The process City Florida Water System conducted in 2008. a series of chemical actions and eventually of forming chloramines is well understood splits into two components: hypochlorous acid and consists of adding ammonia in the pres - Understanding Chlorine Addition (the active compound that forms the DBPs) ence of free chlorine in a ratio of about 3-5 for Primary Disinfection and the hypochlorite ion. milligrams per liter (mg/l) free chlorine to 1 Although both hypochlorous acid and mg/l of ammonia. DBPs are formed when chlorine, in the the hypochorite ion provide disinfection, the Unfortunately, switching water systems form of hypochlorous acid formed in the dis - hypochorous acid that is formed is much from free chlorine to chloramine creates new infection process, is allowed to contact natu - stronger than the ion. The amount of and unexpected problems. Attempting to find rally occurring organic material. To control hypochlorous acid or the hypochlorite ion an explanation for what is occurring is not the production of DBPs effectively, three formed is completely dependent on pH. These easy, and when problems grow worse, the sys - methods can be employed: relationships are shown in Figure 1. tem operator is left with no other option than 1) Remove the organic precursors that are re - As can be seen from the curve, when the to switch back to free chlorine disinfection and active with the chlorine. pH of the water is around 7.5, a value near most performing a “burn.” 2) Decrease the amount of hypochlorous acid Florida groundwaters, the relative concentra - A burn is a process that lasts several available for the reaction. tions of hypochlorous acid is around 50 per - 3) Decrease the time of contact between the cent. If pH is lowered, more hypochlorous acid is available and more DBPs form at a faster rate. Figure 1 organic material and the acid. Generally, operators will accomplish all Conversely, when pH is increased, the 100 these objectives by moving chlorine dosing available hypochlorous acid is lower and DBP 90 points to locations that allow some precursor formation will be slower. As the pH ap - 80 removal to occur within the plant and then proaches 8, the relative portion of the hypochlorous acid is about 20 percent. 70 OCl - lower free chlorine concentrations to only those 60 levels needed to meet regulatory requirements. Above a pH of 9, most of the chlorine is 50 When in-plant strategies are not success - in the form of the less effective hypochlorite 40 ful, many systems have chosen to reduce the ion, and disinfectant ability will be about 1 HOCl 30 amount of hypochlorous acid available for re - percent of the disinfectant power compared to 20 action within the distribution system by the acid form. Raising pH does have a signifi - cant drawback: As the amount of hypochlor - 10 switching to chloramine as a secondary disin - ous acid is reduced, disinfection ability is 0 fectant. Free chlorine or other disinfectant is used to provide primary disinfection and meet reduced accordingly. 4 5 6 7 8 9 10 11 initial disinfectant demand, and chloramine is The practical significance of this fact in Continued on page 18 pH used as the secondary disinfectant, i.e., in the 16 • APRIL 2009 • FLORIDA WATER RESOURCES JOURNAL be naturally present. FREE CHLORINE This oxidation-reduction reaction will proceed to completion until all inorganic sub - 0.2 0.6 1.0 2 3 5 7 10 PPM 8.5 strate is consumed. Within this range, free chlorine may be added to the water without producing any significant DBPs. This concept is important because many Florida water 8 sources contain iron, hydrogen sulfide, and background ammonia—all inorganic-de - manding substances that can be oxidized with p chlorine without producing significant DBPs. 7.5 H Between segments 2 and 3, there are no Temper ature 25o C longer any unreacted inorganic compounds in Alkalinity100 mg/l the water, and oxidation of organic contami - 7 TDS 500 mg/l nants that produce DBPs will now occur. To prevent this undesirable situation, many water treatment plants with DBP problems have switched to the use of chloramines. Chlo - 6.5 ramine is produced by adding 1 mg/l ammo - 550 600 650 700 750 800 850 900 nia to a ratio of between 3 mg/l to 5 mg/l measured free chlorine. ORP mV Note that when the chlorine to ammonia ratio exceeds about 5 mg/l chlorine to 1 mg/l ammonia, t he production of other chloramine Figure 2: Breakpoint Chlorination Curve and Chloramine Development products besides monochloramine occurs si - multaneously with a decrease in the measured total chlorine residual concentration. These re - actions result in additional chlorine demand. Continued from page 16 they follow the general pattern illustrated in Note that when the 5:1 ratio is exceeded, water treatment is that free chlorine residual Figure 3. Note that between the segments la - monochloramine (NH 2Cl) is being converted does not indicate microbial inactivation un - beled 1 and 2, free chlorine has been added but into dichloramine (NHCl 2), and finally as less pH is accounted for. This limitation can be no chlorine residual of any kind is shown on more chlorine is added, the dichloramine is avoided when the hypochlorous acid compo - the left Y-axis because within this range, the converted to nitrogen trichloride before off nent is measured directly, which can be ac - chlorine is reacting with inorganic con - gassing as nitrogen. complished by the use of an ORP meter. stituents in the water such as iron, manganese, The most desirable form of chloramine is An ORP meter will provide an estimate hydrogen sulfide, and any ammonia that may Continued on page 20 of the inactivation power of chlorine at any given pH. An ORP value of 650 mV has been used since mid-1980 for municipal drinking water in Europe to maintain high oxidative ) E F D S S S S S D D S conditions in water distribution systems. N L I E E E O C E C E D C I I I B A N Y N Y N M I I N N N N Figure 2 illustrates how this value relates A M O U O R A A O M M L A I E I R R O O G A A G T G T T R A F to current Florida Department of Environ - P R R R R A S S V O M L O O O O A E E M R A mental Protection residual requirements for O L L R R D D R O E U C H H O O L O E Y E D C C L free chlorine in a water distribution system. At L I F G L H R N I H H S T N F C D D I R E C C R ORP values between 500 and 600 mV, bacter - N N E C R O L A A A U M L P ial inactivation will occur but it will require A D O H E S C U ( much longer contact time to be effective.