4. Introduction to Chloramination
The term "chloramines" as used in the water industry describes the three compounds that can be formed from the reaction of ammonia with chlorine. These compounds are formed in drinking water facilities practicing chloramination, unknowingly in chlorinated ground water systems containing natural ammonia and in chlorinated wastewater effluents. It is important to note that chloramines refer to a group of compounds and not a single compound. A more technical description for this group would be inorganic chloramines to distinguish them from the organic chloramines that will be discussed later. Chloramines will be used to describe the inorganic chloramines throughout the rest of this discussion.
Inorganic Chloramine Formation
As chlorine (Cl2) is added to water containing ammonia (NH3), one chlorine ion will replace one hydrogen ion on the ammonia molecule. This one (mono) substituted product is called monochloramine.
Cl2+NH3 → NH2Cl + HCl
Ammonia → Monochloramine
It is generally understood that all the available ammonia present, referred to as free ammonia, will react initially in this manner assuming optimum pH, temperature and mixing conditions. This is also the point where the chlorine as Cl2 and ammonia as N are present in the 5:1 weight ratio referenced in the literature as the point where all available free ammonia has been converted to monochloramine.
After all the free ammonia has reacted to form monochloramine, a second chloramine compound will form when additional chlorine is added. The additional chlorine will displace a second hydrogen ion from the monochloramine and replace it with another chlorine ion. The resulting compound now has two chlorines attached to the original ammonia molecule and is called dichloramine. Again, the rate of formation is pH, temperature and mixing dependent.
Cl2+NH2Cl → NHCl2 + HCl
Monochloramine → Dichloramine
Similarly, the third member of the group, trichloramine, is formed. Additional chlorine reacts with the dichloramine to give the three substituted or tri-substituted trichloramine. Trichloramine is also commonly called nitrogen trichloride.
Cl2+NHCl2 → NCl3 + HCl
Dichloramine → Trichloramine
The volatile and unstable trichloramine species is not usually formed under the treatment conditions used in drinking water facilities. The rest of this discussion on chloramines will focus on the production and control of monochloramine and dichloramine; however one should remember that this third species is included in the "chloramines" definition. It is generally understood that the chloramines form sequentially; first monochloramine, then dichloramine and finally trichloramine if excess chlorine is continually added to the system.
Organic Chloramine Formation
Chlorine is not specific in forming only chlorinated compounds with ammonia. In drinking water production, influent waters contain compounds that react in a similar manner to ammonia. These compounds are part of the natural organic matter (NOM) that is present in influent waters. The compounds of interest contain carbon and nitrogen and are called organic nitrogen compounds. They can be generally described by the formula Organic(R)-N where R represents a carbon chain of some undefined length and contains at least one reactive nitrogen group, N. These compounds react with chlorine to various degrees similar to the reaction of chlorine with ammonia.
The reaction products formed are referred to as organic chloramines or organochloramine compounds and often get confused with the inorganic chloramines formed directly from ammonia. These organic chloramines are present at low levels, but their levels can be significant in influent surface waters containing a high organic load or in treated wastewater effluents. While the exact structure is not important to this discussion, it is important to note that they have little or no disinfectant properties. What is important is that these organic chloramines can react as chloramines under the test conditions used in most methods for controlling monochloramine and dichloramine.
Free/Total Chlorine Analysis
One method for monitoring the production of chloramines is to perform a total chlorine analysis and a free chlorine analysis. The theory behind this method of control is that the total chlorine test value is the sum of the free chlorine, monochloramine and dichloramine present in the water while the free chlorine test value gives the free chlorine concentration only. The difference between the two values is referred to as combined chlorine and is theoretically the sum of the monochloramine and dichloramine concentrations present.
With the increased use of chloramines as a strategy to reduce disinfection byproduct levels in the produced waters, operators and analysts started demanding enhanced testing techniques to better control the process in order to reduce chemical usage and to eliminate the nitrification issues and taste and odor problems often associated with chloramines when a system is not optimally controlled.
The total chlorine/free chlorine method has two limitations that make tight control of the chloramination process difficult. One limitation is that the combined chlorine value does not determine the individual concentrations of monochloramine and dichloramine. Monochloramine is the preferred species; dichloramine leads to taste and odor complaints (swimming pool smell) and results when excess chlorine is added. The second limitation of the total chlorine/free chlorine method of control is that free chlorine measurements made in the presence of the high levels of chloramines are not always accurate. The formed chloramines react slowly or "breakthrough" in the free chlorine determination. This is usually noticed as a slow steadily increasing test value in the free chlorine method causing one to question the accuracy of the free chlorine value.
Standard methods to measure monochloramine only are available. The monochloramine can be determined amperometrically or titrated with ferrous ammonium sulfate (FAS) using DPD indicator under controlled conditions. These methods are best used in a lab situation and require a higher degree of skill and care to perform the analysis. Both methods require good control of the iodide added to limit dichloramine interference and can also have interference from organic chloramines.
Hach Company has available a colorimetric method for monochloramine. This single reagent, Monochlor F, is specific for monochloramine, is free from interferences commonly found in chloraminated waters and runs on all Hach spectrophotometers and colorimeters. The method allows a user to easily maximize the monochloramine concentration while keeping dichloramine concentrations to a minimum. The test may be run under lab or field conditions.
In the next issue we will discuss how to use Monochlor F together with the Free Ammonia test to maximize monochloramine production, minimize dichloramine levels and how to control the residual free ammonia levels in the finished water. We will also cover the question of "Why does my total chlorine value decrease when I add additional chlorine in my chloramination treatment process?".