Analysis of Free Ammonia in Chloramination Applications Using Lab Method and the APA6000

Transcription

1 Analysis of Free Ammonia in Chloramination Applications Using Lab Method and the APA6000 Chloramination Chemistry Knowledge of chloramination chemistry is required to understand the rationale behind the need to test for free ammonia in chloramination applications. Free ammonia is defined as the chemical species present in the water either as ammonia, NH 3, or as the ammonium ion, NH 4 +, depending upon the ph of the sample matrix. With the promulgation of the D/DBP rule, the use of chloramines as a drinking water disinfectant has increased. Plants needing to reduce their levels of disinfection byproducts (DBPs) may choose chloramines as an alternative disinfectant. Chloramines form fewer DBPs than free chlorine. These DBPs are formed when natural organic matter, usually referred to as NOM, reacts with chlorine. Chloroform is the most commonly produced DBP. Roughly 35% of drinking water facilities in the United States now use chloramines as a disinfectant. Chloramines are formed when chlorine and ammonia are added to water and allowed to react. This process of forming chloramines is called chloramination. Chlorine and ammonia are added to the water to maintain a Cl 2 :N ratio of approximately 5:1 by weight. This is the theoretical ratio to optimize the formation of monochloramine, the preferred chloramine species. Cl 2 + NH 3 NH 2 Cl + HCl 2 x MW of Cl = 2 x = 70.9; MW of N = 14; Cl 2 :N = 70.9:14 = 5:1 ratio However, the feed ratio of chlorine to ammonia is seldom at a ratio of 5:1 as the chlorine demand of the water and the ammonia already present in the source water all must be accounted for in setting the chlorine and ammonia feed rates. In practice, the chlorine and ammonia ratio is adjusted until the required monochloramine concentration is reached. This will vary from facility to facility according to the individual water characteristics; hence, the need for good analytical tools is required to optimize the process. Initially, chlorine reacts with the ammonia to form monochloramine. As chlorine addition continues, monochloramine continues to form until all of the ammonia present has been converted to monochloramine. At this point, additional chlorine added reacts with the monochloramine to form dichloramine, nitrogen trichloride, and various chlorinated organic nitrogen compounds. As dichloramine and nitrogen trichloride form, the addition of chlorine continues to oxidize these compounds to nitrogen gases. The point at which all chloramines are converted to nitrogen gas is the breakpoint (9:1 Cl 2 :N ratio). After the breakpoint, additional chlorine added to the water persists only as free chlorine.

2 The disinfection properties of dichloramine are similar to those of monochloramine, which suggests that either the monochloramine or the dichloramine form would be acceptable for disinfection. While this is true, the formation of dichloramine should be avoided in order to control taste and odor problems and to reduce the costs associated with the over-feeding of chlorine. For optimal disinfection, most drinking water facilities try to remain just shy of the first peak on the chloramination curve. See Figure 2. At this point on the curve, there is maximum formation of monochloramine with a low concentration of ammonia remaining in solution. It is important to minimize the ammonia level to as close to zero as possible in order to maintain nitrification control throughout the distribution system. It is important to note that different species only exist at specific points throughout the chloramination curve. The two major species of importance in chloramination monitoring and control are ammonia and monochloramine. A graph of which chemical species are present at the various stages throughout the chloramination process is also attached. See Figure 2. Ammonia begins at a given concentration. As ammonia reacts with chlorine to form monochloramine, the free ammonia concentration decreases. At the top of the first peak, all of the ammonia has been converted to monochloramine, and ammonia is no longer present in the system. At the beginning of the chlorine addition, no monochloramine exists in solution. As chlorine is added, it reacts with the free ammonia to form monochloramine. The monochloramine concentration increases until the top of the first peak, where all ammonia has been converted to monochloramine. At this point, the monochloramine concentration decreases as additional chlorine reacts with monochloramine to form dichloramine. Chloramination Control Monitoring the various chemical species present throughout the process can help optimize the chloramination process. Typically monitored species include total chlorine, monochloramine, and ammonia. Monitoring methods for each of these species are discussed below. Chlorine Alone, a total chlorine value does not provide meaningful information regarding chloramination control or guidance for optimization. For example, a 1 mg/l total chlorine value could indicate one of three possible locations on the chloramination curve in Figure 1: sub-optimal chlorine feed, overfeed of chlorine resulting in the formation of dichloramine and nitrogen trichloride, or breakpoint chlorination. The total chlorine value would not guide an operator as to whether he should add more chlorine or more ammonia to increase to his target disinfectant level of 2 mg/l. However, a total chlorine measurement used in conjunction with free ammonia and monochloramine measurements provides a more complete picture of the chloramination process and where he is at on the chlorine breakpoint curve. It is the ability to monitor free ammonia that gives an operator the information to determine where he is at on the breakpoint curve.

3 Total chlorine can be monitored using any of Hach s DPD chemistries, amperometric titration, or the CL17 on-line chlorine analyzer. The primary value for determining the total chlorine concentration is that this value is used to report residual disinfectant levels for regulatory reporting purposes. Free chlorine monitoring is not ordinarily recommended for chloramination control. Free chlorine will only be present after the breakpoint in the chloramination curve. This is well beyond the optimum control point. Also, the high levels of chloramines used in chloramination can interfere with the DPD Free Chlorine chemistry and will cause steadily increasing values in the free chlorine reading. This can partially be avoided by reading the free chlorine results within ten seconds of reagent addition. It should be noted that free chlorine residuals occurring prior to the breakpoint have been reported in the literature. These occurrences can be attributed to ph and temperature conditions that slow the chlorine/ammonia reaction, insufficient mixing that causes stratifying or dead zones, and erroneous interpretation of free chlorine values in not recognizing the chloramine breakthrough in the free chlorine test method. Free chlorine measurements are useful when one is purposely trying to reach breakpoint conditions. Breakpoint chlorination is used to treat nitrification episodes in distribution system lines, storage reservoirs and at booster stations where chloraminated water concentrations are boosted with free chlorine only. Free chlorine measurements are also used when utilities periodically switch back to free chlorine for a limited number of weeks to burn-out the entire distribution system that may have nitrification issues. The free chlorine value will be generally stable when read before one minute in these types of sample waters., in conjunction with free ammonia measurements, is another useful value in chloramination monitoring. In the lab method, Hach s Monochlor-F chemistry was developed specifically for monitoring monochloramine in chloramination applications. This chemistry is specific for the monochloramine species, and utilizes the Berthelot reaction sequence, which results in the formation of a colored indophenol complex. The APA6000 and Free Ammonia analyzer provides on-line monitoring capability for monochloramine. The APA6000 utilizes an all-liquid form of the chemistry, which is chemically equivalent to the indophenol laboratory chemistry. The APA6000 s analysis method will be described in detail later in this document. Ammonia If only one measurement could be used to monitor chloramination, that measurement would be ammonia. Ammonia is a very useful measurement in controlling chloramination. This is because complete formation of monochloramine results when all of the free ammonia in the system has been converted to monochloramine. Optimal feed

4 rates and production of monochloramine result in a small amount of residual ammonia, usually less than 0.1 mg/l NH 3 -N. Higher concentrations of ammonia present in the system may indicate sub-optimal chlorine feed and a propensity for biofilm growth and nitrification in the distribution system. Free ammonia is the food source for bacterial regrowth in the distribution system. This is the main driver for keeping the concentration as close to zero as possible. Absence of ammonia indicates a possible overfeed of chlorine and the presence of dichloramine. For laboratory determination in a chloramination application, free ammonia can be monitored using the new Free Ammonia colorimetric method or by using an ammonia ion selective electrode (ISE). The Free Ammonia colorimetric method is used in combination with the Monochlor F test for monochloramine. A sample is taken and split. Hypochlorite is added to one sample. This converts the free ammonia present into monochloramine. Both samples are then analyzed for monochloramine using the Monochlor F reagent. Free ammonia is determined by difference between the two samples. In the ISE method, potassium hydroxide is added to a sample to convert all ammonia present as ammonia, NH 3, or as the ammonium ion, NH 4 +, into ammonia. The ammonia then diffuses through a membrane on the ISE probe and is measured. It should be noted that the Hach ISE method for ammonia using the Hach ionic strength adjuster with the colorimetric indicator should not be used. The indicator reacts with the monochloramine to produce additional ammonia and will give high results. The ISE procedure should be run using potassium hydroxide only to adjust the sample ph. These are the only direct ammonia methods with which chloramines do not interfere. Chloramines interfere directly with both the Nessler, salicylate and phenate chemistries for ammonia and cannot be used to test for free ammonia. The APA6000 /Free Ammonia analyzer provides continuous free ammonia monitoring capability. The analyzer measures monochloramine present in the sample directly using a liquid reagent similar in composition to the lab method. To measure free ammonia, the sample is spiked with hypochlorite. The addition of hypochlorite converts the free ammonia to additional monochloramine. All of the monochloramine in the spiked sample is determined, effectively giving a total monochloramine value. The initial monochloramine is subtracted from the total monochloramine value. The free ammonia is the difference between the two. This is very similar to what is happening in the laboratory Monochlor F and Free Ammonia chemistry. What can we expect when comparing the laboratory method vs. the on-line analyzer method? Monochlor F and Free Ammonia Lab Method vs. APA6000 The reaction in the lab method for monochloramine and free ammonia is chemically equivalent to the method used on the APA6000. The major difference is the physical form of the reagent. is determined directly using the indophenol chemistry. A water sample is taken and split into two samples. One sample is spiked

5 with hypochlorite to convert the free ammonia present into monochloramine. This sample will have a monochloramine concentration equal to the sum of the free ammonia concentration plus the original monochloramine concentration in the sample. The second sample will have only the original monochloramine concentration. Both samples are then analyzed for monochloramine using the Monochlor F Reagent supplied in powder form. The difference between the two samples is the free ammonia concentration. Although the theory is quite simple, conversion of free ammonia into monochloramine is a challenge. A complete conversion of the free ammonia into monochloramine is extremely important in order to obtain an accurate free ammonia concentration. As observed on the chloramination curve in Figure 2, a narrow ratio of chlorine to ammonia (approximately 5:1 Cl 2 :N) is required to completely convert all of the ammonia in a sample to monochloramine. To theoretically optimize this conversion, implies that one already knows the chlorine and ammonia nitrogen concentrations. Since ammonia nitrogen is the target analyte and the concentration is unknown, the strategy is to target a concentration range and optimize the chlorine dose for that range. The mg/l ammonia nitrogen is the targeted optimum range of the lab method. Chlorine must be added reproducibly and at the optimum concentration for best results. The chlorine solution is packaged in a high-density poly bottle and dispensed by adding drops from the dispenser tip. Inaccurate measurements can result due to overchlorination or underchlorination caused by addition of the hypochlorite solution. Slight variations in drop size and changes in chlorine concentration can contribute to variable results. The bottle should be held in a completely vertical position to give a reproducible drop size. This chlorine solution, called the Free Ammonia Reagent Solution, should always be kept capped, stored in a cool environment and reagent expiration dates should be closely monitored. The indicator in the Monochlor F is specific to monochloramine. If overchlorination occurs (chlorine reacting with the monochloramine already present) and dichloramine or nitrogen trichloride are formed, these species will not be measured colorimetrically. The total monochloramine value will be low, and an inaccurate free ammonia measurement will result. Also, an incomplete conversion of ammonia to monochloramine caused by underchlorination will result in a low total monochloramine value and inaccurate free ammonia measurement. Additional sources of error may occur due to the temperature sensitivity of the chemical reaction. The temperature compensation chart should be closely observed when running the test. or ammonia loss can also occur in sample handling and in sampling. It is important that a composite sample be taken and then split as directed in the procedure. Hach Company has observed situations when the sampling site was at a location where the ammonia/chlorine reaction was not complete and taking two separate samples gave variable results in test reproducibility studies. Best results are also obtained when one sample is analyzed at a time instead of trying to do a series of samples. The sample should be capped and shaken immediately after the addition of the Monochlor F reagent. This sometimes does not occur when a series of samples are being run. Also, a

6 Hardness Treatment Reagent is available for lab samples giving a slight turbidity in certain matrices containing high hardness and alkalinity. This has only been observed in a limited number of samples. No similar reagent exists for the APA6000 nor has it ever been documented that a turbidity problem exists with the APA6000 reagents. The lab method can be used to optimize plant chloramination operations. The method is much simpler than the lab ISE method and can be used to check free ammonia levels at various sites within the plant and to track nitrification issues in distribution system and at storage reservoirs. The method can also serve as a back-up method for the APA6000 analyzer. It is a lower cost alternative for users that do not have the resources to purchase or maintain the APA6000. Because the APA6000 /Free Ammonia analyzer is automated, is temperature controlled and has reproducible reagent dispensation, it can be expect to have improved precision as compared to the bench method for chloramination monitoring. Further experiments have shown that accurate free ammonia results can be obtained if the amount of hypochlorite added can be optimized and controlled. This is what the APA6000 analysis method entails. Although the APA6000 analysis method also involves spiking a sample with hypochlorite, what differentiates the APA6000 from the lab method is that the strength of the hypochlorite solution is constantly monitored. The ammonia standards are periodically spiked with hypochlorite to ensure complete formation of monochloramine. If it is determined that the hypochlorite solution has degraded to the point that complete formation of monochloramine does not occur, the analyzer displays an error message. Also, the amount of hypochlorite added to the sample is optimized for the free ammonia measurement, and the ph of the reaction is tightly controlled to minimize formation of dichloramine. The analyzer is an excellent choice for monitoring monochloramine and free ammonia at a fixed site. The analyzer provides continuous results that can be used to identify trends and concentration changes that are occurring at the analysis site under controlled reproducible analysis conditions. The analyzer can be used to signal feed pumps as necessary to keep the monochloramine and free ammonia values within the desired operating ranges. As has been found with the lab method, it is important to insure that the fixed site chosen for the analyzer be a site where the chlorine/ammonia reaction is complete and where sample is well mixed and completely blended. The performance of the APA relative to the new lab method is very comparable for the monochloramine values over the range of both products. The APA is optimized for a wider range of free ammonia values (0-2 mg/l NH 3 -N) and the lab method is optimized for a tighter range (0-0.5 mg/l NH 3 -N). The new lab method has superior accuracy at the low end of the range. The APA and the new lab method read within ppb NH 3 -N at 100 ppb N on laboratory samples (e.g. lab method reads 100 ppb NH 3 -N versus APA value of ppb NH 3 -N). The new lab method is a much simpler method than the available bench method for the APA if customers would like to verify performance of the APA using the new lab method.

7 At higher levels of free ammonia ( ppb NH 3 -N) the APA and new lab method are comparable in terms of accuracy. The precision of the APA is generally superior to the new lab method. Most customers are interested in trending the free ammonia values both products are very adequate for trending the free ammonia values. The table below summarizes the analysis methods that can be used for the determination of free ammonia, monochloramine, and chlorine in the chloramination process. Analyte Method Chloramination Suitability? Free Ammonia Nessler Method 8038 NO interference Salicylate Method NO interference Free Ammonia Method and Method YES ISE Method YES Monochlor-F Methods & and Free Ammonia Method YES NO (Obsolete) Chlorine Total DPD Methods 8167 & YES Chlorine Free DPD Methods 8021 & NO interference; use for breakpoint chlorination only.

8 Three Equivalent Chlorine Concentrations on Breakpoint Curve Figure 1.

9 Chlorine Measured Chlorine Measured Figure 2. Chloramination Ratios and Chloramination Species Chloramination I II Free Chlorination III 5:1 Cl 2 :N Ratio Breakpoint 9:1 Cl 2 :N Ratio Chlorine Added Chloramination I II Free Chlorination III Total Chlorine Free Chlorine Free Ammonia Chlorine Added