5. Optimizing Monochloramine Production
In the first part of this series on disinfection we described a colorimetric reagent available from Hach Company called Monochlor F, which is specific for the determination of monochloramine, the preferred chloramine species in the chloramination treatment process. Maximizing the concentration of monochloramine is important from a water quality and from an economic perspective. The over-feeding of chlorine converts monochloramine into dichloramine, which can lead to taste and odor issues. The over-feeding of chlorine also means that excess chlorine has been used which translates into increased operational expenses.
Maximizing the production of monochloramine is one part of the treatment process. Equally important is the need to minimize the concentration of free ammonia entering the distribution system. Free ammonia is the remaining ammonia concentration that has not reacted with chlorine in the treatment process to form monochloramine. It is important to keep the free ammonia to as low a level as possible as it becomes a food source for nitrifying bacteria in the distribution system. Also, this is the last opportunity that an operator has to minimize its concentration as the level can only increase as the treated water moves through the distribution system.
The free ammonia concentration in the distribution system can increase according to two predominant pathways. First, as the finished water containing monochloramine moves through the system and reacts with biofilm, bacteria, and natural organic matter (NOM) in the water, additional free ammonia is generated. While this is discouraging from a system management perspective, this is a consequence of using chloramines.
NOM + NH2Cl → NH3 + Reaction Products
Monochloramine → Ammonia
The second pathway by which additional free ammonia is formed is the autocatalytic decomposition of monochloramine itself.
3NH2Cl → NH3 + N2 + 3HCl
Monochloramine → Ammonia
Optimizing the production of monochloramine is best understood by taking a look at the standard Chlorine Breakpoint Curve in Figure 1 . This diagram illustrates the three stages in the chloramination process and how chlorine and ammonia concentrations change as increasing amounts of chlorine are added. Plotting the chlorine concentration added versus the residual total chlorine measured generates the curve. In optimizing the formation of monochloramine, one is mainly focused on the reactions taking place and the species formed in Sections I and II.
Chlorine Breakpoint Curve
Section I illustrates the first stage of the process where the chlorine added first satisfies the chlorine demand of the raw source water and then reacts with the ammonia added to form monochloramine. The monochloramine is traditionally measured as total chlorine by the DPD Total Chlorine method or by amperometric titration. The monochloramine can be measured individually using modified DPD methods or by using Monochlor F Reagent. The maximum monochloramine concentration is reached when all available ammonia has reacted with the chlorine added. This point is referred to as the "Monochloramine Hump" and is illustrated on the curve as the dividing line between Sections I and II. This is also the point where chlorine (as Cl2) has reacted with ammonia (as N) at a 5:1 ratio. For clarification, the 5:1 ratio is the ratio of the measured chlorine (not chlorine dose) that has reacted with ammonia nitrogen after the chlorine demand of the water is satisfied. The chlorine-dosing rate is higher than the measured chlorine for waters in Section I due to the chlorine consumed or lost by the demand of the raw water.
Chemical test results on waters located in Section I of the Breakpoint Chlorine Diagram will have the following characteristics:
Free Chlorine is Zero in Section I. No free chlorine will exist for waters located in Section I of the breakpoint diagram. Chlorine reacts with ammonia nitrogen instantaneously at pH 8.3. Within the pH range of 7.5 – 9.0 commonly used in producing chloramines, the reaction will be complete within a few seconds provided good mixing is taking place. It should be noted that depending on sample pH, temperature and contact time there may actually be free chlorine present which will continue to react with ammonia over an extended period of time. The reaction slows appreciably as the temperature drops.
During pilot or plant optimization studies free chlorine measurements are sometimes made to study reaction kinetics and mixing efficiencies for waters in Section I of the treatment process. This is a difficult analysis for a number of reasons, but the data obtained is useful if one understands the limitations of the test results.
The water chemistry is rapidly changing if the reaction kinetics are not complete. Grab samples with on-site analysis or on-line analyzers are recommended for these types of studies. Take a composite sample in a chlorine demand-free bottle or beaker when doing grab sample testing. Do not take sequential or separate samples for repeat testing. When low concentrations of free chlorine (< 0.05 mg/L) are detected using DPD Free Chlorine Reagent, the results should be carefully reviewed to insure that it is actually free chlorine. Trace levels of manganese can give a false positive value and be determined as free chlorine; a slowly increasing free chlorine value indicates chloramine breakthrough in the DPD Free Chlorine test. Read the free chlorine value within 10 seconds after adding the reagent. Repeat the DPD Free Chlorine test on another portion of the composite sample. If the free chlorine concentration is equivalent to the initial analysis, then the reaction kinetics are complete. If the free chlorine level is lower or has disappeared altogether on the replicate sample, then the reaction kinetics were not complete when the initial sample was taken.
Another difficulty with analyzing for free chlorine in Section I is that test results are difficult to interpret if the water chemistry is still changing. The tendency is to first blame the method chemistry, analyst technique or faulty test equipment for the non-reproducible results. However, understanding that the free chlorine may still be reacting due to reaction kinetics, insufficient mixing or improper sampling may help interpret the free chlorine value.
Hach Company has recently introduced a new free chlorine application method that is designed for determining free chlorine residuals in these types of studies without interference from manganese or chloramines.
Total chlorine concentration equals the monochloramine concentration in Section I. It is important to note that the measured total chlorine residual is not specific for monochloramine. A single DPD Total Chlorine residual determination alone cannot accurately predict if the treated water is in Section I, Section II or Section III of the breakpoint curve diagram. Total chlorine measurements by DPD or amperometric titration do not distinguish between free chlorine, monochloramine, dichloramine or interference from organic chloramines. Use Monochlor F Reagent to determine the specific monochloramine concentration.
Free ammonia equals the concentration of residual ammonia that has not reacted with chlorine. A residual free ammonia value indicates the treated water has not reached the 5:1 chlorine to nitrogen ratio and indicates the water is definitely in Section I on the breakpoint curve. As noted for free chlorine above, depending on sample pH, temperature and contact time, residual free ammonia may continue to slowly react with any available free chlorine over an extended period of time. Free ammonia is determined with an ammonia ion selective electrode or the Hach Free Ammonia colorimetric method. Two points of importance need to be noted here. The ammonia ion selective electrode method requires that a sample be made alkaline, (pH >10), for the ammonia to exist as NH3 gas in order to permeate through the ammonia ISE membrane and be detected. Use only strong inorganic bases to adjust the sample pH. Organic bases, colored buffers or buffers containing pH indicators used to visually signal the correct sample pH will react with the monochloramine present in the water and generate additional ammonia. This will cause a biased high free ammonia value to be determined.
The Hach colorimetric Free Ammonia method reacts directly with the free residual ammonia and does not suffer from monochloramine interference. Free Ammonia cannot be determined by traditional total ammonia methods. Traditional colorimetric methods for ammonia such as the phenate, salicylate, and the Nessler methods suffer to various degrees from interference due to monochloramine, dichloramine or organic chloramines. The level of interference in these methods depends on the chloramine concentration, the form of the organic chloramines present and the unique characteristics of the method being used.
Section II of Figure 1 illustrates that portion of the breakpoint curve where additional chlorine is added beyond the 5:1 ratio after the maximum monochloramine concentration has been formed. This is the section where the continued addition of chlorine converts monochloramine into dichloramine.
One should notice something unusual on the breakpoint curve at this point. The continued addition of chlorine results in a lower measured total chlorine residual. This can be confusing when one does not recognize where the treated water is at on the breakpoint curve. Continued addition of chlorine forms trichloramine , continues to "break" down the nitrogen species present in the water and drives the reaction to "breakpoint" after which only free chlorine will exist in the water (Section III).
Test results for treated waters that are located in Section II of the Chlorine Breakpoint Curve diagram will have the following characteristics:
Total Chlorine equals the sum of monochloramine, dichloramine, trichloramine and any interfering organic chloramines present. The total chlorine test results alone cannot predict if the treated water is in Section I, II or III. However, if the total chlorine residual keeps decreasing even when additional chlorine has been added to the water, one can conclude that the water is very likely in Section II of the diagram and the water is heading towards breakpoint. Increasing the chlorine feedrate at this point is going in the opposite direction if one wants to increase the maximum monochloramine resdiual in the finished water.
Free Chlorine equals Zero for waters in Section II. No free chlorine will be present. DPD Free Chlorine Reagents may detect "apparent" free chlorine, but this is likely from the breakthrough of the chloramines in the free chlorine test methods.
Monochloramine concentrations will decrease in Section II. Monochloramine concentrations will decrease from the highest level determined at the "monochloramine hump" as additional chlorine is added. Use Monochlor F Reagent to determine monochloramine residuals without interference from unreacted free chlorine, dichloramine or organochloramines.
Free Ammonia equals Zero for water in Section II. Free ammonia can only be present in waters (Section I) that have not had sufficient chlorine added in order to form monochloramine.
Chemical testing is required to fully characterize the treated water and to better understand the process adjustments that can be made to maximize the monochloramine concentration while keeping free ammonia levels and operating expenses to a minimum.
In the next issue on chloramination we will discuss the special challenges that chloraminated waters present and how chemical testing is used to control these special situations.