A Chloramine Primer

In my last post, I wrote about using the power of chlorine contained in sodium hypochlorite to inactivate microbial contaminants; give residual protection in the distribution system; and oxidize inorganic contaminants.  This time, we’ll look at using another chlorine containing compound, monochloramine, to do the first two of those three jobs.

The use of monochloramine as a drinking water disinfectant residual is nothing new; Denver, Colorado has been using it since 1917.  Monochloramine has a chemical formula of NH₂Cl.  It’s created by mixing the correct ratios of chlorine and ammonia, usually somewhere in a range from 3:1 to 5:1, chlorine:ammonia by weight.  The chemical reaction, using our old friend sodium hypochlorite for the source of the chlorine, looks like this:

HOCl + NH3 → NH2Cl + H2O

The reaction of monochloramine is also very dependent on the pH level.  Where a typical pH in any free chlorinated system is usually around 7, chloraminated systems usually have a pH closer to 8.

Monochloramine, also referred to as just chloramine, does not have the same inactivation or disinfecting “power” of free chlorine, due primarily to the fact that it is not as strong an oxidizer.  Some sources estimate that monochloramine is 200 times less effective as a disinfectant than free chlorine.  For that reason, chloramine levels need to be higher than free chlorine levels.  Many systems that run free chlorine often maintain a residual level of 0.5 to 1.0 mg/L, but chloraminated systems often run at 2.0 to 3.0 mg/L total chlorine. 

Without very careful monitoring of monochloramine formation, chloraminated systems can also have significant taste and odor issues.  This can result from the formation of di-chloramine (NHCL2) and tri-chloramine (NCL3), compounds similar to monochloramine but with additional chlorine atoms added.  These compounds have a very strong chlorine taste and odor.  Their formation is generally tied to an incorrect chlorine:ammonia ratio.  Specifically, there is too much chlorine for the given amount of ammonia.

Monochloramine

In any chloraminated system, there is also the presence of some level of ammonia.  The more closely controlled the process of chloramine formation, the less ammonia should be present.  But even with low levels of ammonia, under the right conditions, certain bacteria can use that ammonia for a food source and produce as by-products both nitrite and nitrate in a process called nitrification.  Nitrite in particular can become a problem because the Maximum Contaminant Level (MCL) is so low, only 1.0 mg/L.  Once the nitrification process gets established, it will also cause the chloramine residual level to drop, allowing for the formation of hard to treat biofilms in the distribution system.

With all of these issues with the use of chloramines, why would anyone want to use them?  Under the right conditions, a chloramine residual will last longer in the distribution system than a free chlorine residual.  If you have a very large distribution system, or very long transmission lines, using chloramines can provide better protection.  Another common reason for the use of chloramines is the fact that they can result in the formation of fewer disinfection by-products, particularly trihalomethanes (THMs) and haloacetic acids (HAAs).  The implementation of the Stage 2 Disinfectants and Disinfection Byproducts Rule resulted in a great number of utilities that could no longer meet the MCLs for THMs and HAAs like they could under the Stage 1 Disinfectants and Disinfection Byproducts Rule.  As a result, many of these utilities switched from free chlorine to chloramination.  Chloramines can also combat biofilms better than free chlorine, as long as nitrification is kept under control.  Free chlorine can’t penetrate through the outer layer of the biofilm very well, while monochloramine does a better job, resulting in increased inactivation of the organisms within the biofilm.  Since free chlorine and monochloramine each have their own pluses and minuses when it comes to combating biofilms and inactivating microorganisms, many utilities that routinely chloraminate make a yearly switch to free chlorine for several weeks to try and utilize the benefits of both types of disinfectants.

For a more detailed discussion of free chlorine, chloramines, and related disinfection topics, please check out these additional sources:

·          HACH has a very good discussion of all these topics located here - http://www.hach.com/DisinfectionSeries

·          HACH also has a very nice video on You Tube – http://www.youtube.com/watch?v=3rXZg6VDVRQ