6. Dealing with Special Situations when Chloraminating
The first two articles on chloramination in this series discussed chloramines and the methods available for optimizing and controlling the chloramination process. These discussions also covered strategies to identify water characteristics based on test results providing the guidance needed to make treatment adjustments. It also became apparent from the discussions that the water chemistry is not as clear-cut as is often depicted in the diagrams and tables referenced in discussions on these topics. Water temperature, mixing efficiencies, water composition and lack of specificity in test methods used add to the complexity of interpreting test results and defining a water matrix. Fortunately, improvements in testing methods continue to develop that provide a more in-depth understanding of the water composition at any specific point in the treatment process. These test improvements help in making better decisions to provide improved water quality.
The following examples illustrate water treatment situations which can provide unexpected challenges to a treatment operator if not recognized. These special water circumstances sometimes get lost in the simple illustrations on chlorination and chloramine formation. The simplified Chlorine Breakpoint Curve in Figure 1 will be used to illustrate the examples. Figure 2 illustrates how free ammonia, monochloramine, total chlorine and free chlorine concentrations change during the chloramination process.
Source waters containing natural ammonia
Source waters containing natural ammonia should be monitored to determine if the ammonia concentration is constant and to insure that the ammonia is compensated for in maintaining the optimum chlorine to nitrogen ratio for a treatment system. Figure 1 illustrates the formation of monochloramine is Section I. Figure I assumes a constant ammonia concentration with chlorine being the variable that is added. The ammonia naturally present in the source raw becomes part of the total ammonia concentration used to calculate the chlorine to ammonia ratio. The ammonia in the source water prior to the addition of chlorine is best determined by the salicylate colorimetric method or by ion selective electrode. If the ammonia concentration is variable, an on-line analyzer or monitor is recommended. The Hach Free Ammonia method is not recommended for determining ammonia in these unchlorinated source waters. The Nessler method for determining ammonia is not recommended unless the proper disposal procedures are in place to handle the hazardous waste generated by using the method.
Any prechlorination steps in a treatment process used to oxidize manganese and iron and inactivate pathogens will also oxidize the natural ammonia. This can take the water through breakpoint when enough chlorine is added. The water after this preoxidation step can be thought of as being in Section III of Figure 1. The natural ammonia has been destroyed and the chlorine demand has been met. After this point when additional chlorine and ammonia are added to form chloramines, the water can be thought of as being back in Section I of Figure 1.
Groundwater systems required to chlorinate under the Ground Water Rule should be aware that adding free chlorine to waters containing natural ammonia creates an uncontrolled chloramination process. Ammonia is a non-regulated parameter and often is not monitored. Operators using chlorine for the first time may not be aware of the chloramines being formed. This can result in nitrification and taste and odor issues if the natural ammonia levels are high. A single DPD Total Chlorine test will not determine if chloramines are present; nor will a single total chlorine test determine if the finished water is in Section II or III on the breakpoint curve. A free chlorine test will determine if the level of chlorine added was sufficient to take the ammonia and other chlorine demand compounds through breakpoint and into Section III on the breakpoint diagram. DPD Free Chlorine Reagent can be used when manganese and chloramines are not present. A new Indophenol method using Freechlor F and Monochlor F reagents can be used without interference from manganese and chloramines to give the free chlorine level.
The need to increase the chloramine residual
This is a common treatment adjustment that an operator needs to make. Chlorinated systems using free chlorine were simple to adjust; just add more chlorine. However, as discussed in the second chloramination article of this series, chloraminated waters require an operator to know where the water is located on the breakpoint curve in order to know if chlorine, ammonia or both are required to increase the total chlorine residual. Figure illustrates that water with a measured chlorine concentration of 3.0 mg/L could be locate in Section I, II or III. Does one add ammonia or chlorine to move the total chlorine residual to 4.0 mg/L?
Free ammonia, monochloramine and total chlorine testing will properly determine where the water is located in relation to the monochloramine hump and to breakpoint. Either ammonia or chlorine can be added appropriately to form additional monochloramine in order to increase the total chlorine concentration and keep dichloramine and free ammonia concentration levels to a minimum. Figure 4 illustrates where each species is found in the treatment process based on test results.
The need to boost chloramine residuals in distribution systems
Sometimes the chlorine residual concentration needs to be increased or "boosted" to maintain a safe disinfectant level throughout the remainder of the distribution system. This is done at storage reservoirs, entrances to consecutive systems or at selected points in low residual or troublesome sections in a distribution system. Feeding free chlorine and ammonia in the specified ratio forms additional chloramines. This is analogous to Section I in the breakpoint curve diagram. Free ammonia, monochloramine and total chlorine test methods are used to optimize the process. This process can also be used to take advantage of the excess free ammonia levels in the water to form additional monochloramine. This is further discussed in the next section.
The need to reduce excess free ammonia levels
One strategy to reduce free ammonia levels in distribution system waters is to add additional free chlorine to convert the free ammonia into monochloramine, which raises the overall disinfection residual. Use the Hach Free Ammonia method to determine the free ammonia levels. Based on the free ammonia level determined, chlorine is then added at a ratio of 5:1 or less as Cl2:N to form monochloramine and minimize the free ammonia levels. This is equivalent to treating water located in Section I of the breakpoint diagram.
Blending chlorinated and chloraminated waters
This is one of the most common complex situations that exists. Consecutive systems often buy wholesale water to supplement their current supplies. Often one of the waters is chlorinated and the second water is chloraminated. Adding chlorinated water to an already chloraminated water containing chloramines is similar to a water that is at or near the monochloramine hump in Sections I & II in the breakpoint diagram. As indicated in Figure 2, the additional free chlorine from the chlorinated water drives the chloramines from the chloraminated water further into Section II resulting in the formation of additional dichloramine and a drop in overall total residual disinfectant from the blended waters. The question is how to best handle these waters to maintain or increase the total residual disinfectant, yet maintain a system that does not have nitrification or taste and odor problems.
There are a number of strategies used to control these blending operations. One strategy is to add additional free chlorine to the blended water taking the blended water through breakpoint. This moves the finished water into Section III on the breakpoint curve and produces a free chlorine residual throughout the remainder of the distribution system. This strategy is best used when the blended waters have low levels of disinfection byproducts (DBPs). One runs the risk of forming additional DBPs with the free chlorine residual, which may result in finished water that is not in compliance with the Disinfection/Disinfection Byproduct Stage 2 Rule.
A more practical concern in using this free chlorine treatment conversion strategy is to accurately determine when one is at breakpoint. The breakpoint is illustrated in the Figure 1 diagram as the dividing line between Sections II and III. One should note that at breakpoint the apparent total residual chlorine residual measured does not go to zero concentration. This minimum concentration determined at the "breakpoint" by a total chlorine test is an irreducible minimum of the total chlorine residual present. These are called the "nuisance residuals" and are a mixture of free chlorine, monochloramine, dichchloramine and organochloramines. The nuisance residual level varies between water matrices and is a function of contact time, pH, temperature, chlorine and ammonia levels plus and organic chloramines present. This is an area of complex water chemistry with reactions still slowly taking place between the free chlorine being added and the residual chloramines. Use DPD Free Chlorine Reagent to determine the free chlorine concentration carefully. A false free chlorine value can be caused by this mixture of nuisance residuals and the breakthrough of chloramines into the free chlorine reading. An alternative is to test with the new Indophenol method for free chlorine using Freechlor F and Monochlor F reagents. Most diagrams illustrate the free chlorine concentration equal to the total chlorine concentration in Section III. However in all likelihood, the water will contain some traces levels of chloramines and slow to react organic chloramines and will give a total chlorine concentration that is greater than the free chlorine value because of these nuisance residuals. Most references indicate that maintaining a free chlorine residual that is at least 85% of the total chlorine residual gives a taste free water with the optimum germicidal level.
A second treatment option for blending chlorinated and chloraminated waters is to first convert the chlorinated water into a chloraminated water by adding ammonia at a rate to give the 5:1 ratio or less of chlorine to nitrogen. DPD chlorine chemistries can be used to determine the incoming chlorine concentration in the chlorinated water. Use the Free Ammonia and Monochlor F reagents to control the chloramine production. This again is similar to treating water in Section I on the breakpoint curve and optimizing the formation of the monochloramine. This produced water is then blended with the chloraminated water to produce a combined chloraminated water.
In the next discussion on disinfection we will take a more in-depth look at the new indophenol method for free chlorine and how it can be best used to supplement your testing program.