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
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.
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| 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
