McDonald Group International, Inc

Water, Wastewater, Environmental Engineering 

 

Disinfection ByProducts and Chloramination

Disinfection by products are chemicals that are formed when chlorine is used as a disinfectant and 
it reacts with naturally occurring organic compounds in the water, producing mainly two chemicals with long 
complicated names, but for short, TTHMs and HAA5s, which in recent years have been deemed to be health hazards.

The formation of disinfection byproducts takes three things, the presence of organic compounds,
which are called disinfection byproduct precursors,  chlorine, and sufficient time for the two to mix
together and react.

As such, disinfection by products can genjerally reduced through one of three  ways:

Line Flushing

This helps to reduce the time in contact; this is generally done by regularly flushing alot of water out of your water
distribution system.  Its cheap  to do, easy,  but, does not always work and it may have to be done for years.
Still, it is the first and best approach.  Florida operators would be advised to get with the
Florida Rural Water Association and have them work with the operator on how best to flush your system,

Removal of Precursors
It is possible to remove the precursors, however it requires the design, permitting and installation of appropriate treatment equipment. 
It is necessary to test to see what precursors are in the water first.
Generally, Total Organic Carbon is usually selected as a test parameter to indicate if disinfection byproduct formation is likely.
 
One study indicates there is a correlation between DBBP level and TOC level below 1.5 mg/L and above 3.5 mg/L. In between, the correlation is indefinite.
That does not mean that DBBP might not form, only that it cannot be said for certain how much
will form. Another FRWA paper  indicates DBBP formation in water containing TOC of 2 and TOC of 6 mg/L.
The treatment technology is likely to be what are called anion exchanagers. As with most treatment
systems, they require chemicals and do not make  the precursors dissappear, they are simply removed and as a result create a waste stream that
has to be dealt with   Usually this is not  pursued unless  there are other problems with the
water like it s too hard, has color in it, etc
Chloramination
What is commonly done today is treat with chloramines, which is a disinfection compound formed 
by the reaction,i n specific doses, of chlorine and ammonia.   It is not expensive,  does take a permit to do,
and requires more testing of the water to show that it is not causing other problems like nitrates to spike.
According to the USEPA, “Chloramines are formed by the reaction of ammonia with aqueous chlorine 
(i.e., HOCl). Initially,chloramines were used for taste and odor control. However, it was soon
recognized that chloramines were more stable than free chlorine in the distribution system and
consequently were found to be effective for controlling bacterial regrowth. As a result, chloramines
were used regularly during the 1930s and 1940s for disinfection. Due to an ammonia shortage during
World War II, however, the popularity of chloramination declined. Concern during the past
two decades over chlorinated  organics (e.g., THM and HAA formation) in water treatment and
distribution systems, increased interest in chloramines because they form very few disinfection
byproducts (DBPs).” (From Alternative Disinfectants and Oxidants Guidance Manual, USPEA).
Monochloramine is the preferred chloramine species for use in  disinfecting drinking water because 
of taste and odor problems associated with dichloramine and nitrogen trichloride. To ensure that
these compounds are not formed, common practice was to limit the chlorine to ammonia ratio to 3:1.
However, because of problems such as nitrification and biofilm  growth, which can be caused by
excess ammonia, current practice is to use a Cl2:N ratio in the range of 3:1 to 5:1,
with a typical value of 4:1.
For smaller water plants, an ammonia solution is often used. 
Commercially mixed and supplied liquid aqua ammonia in a solution strength of less than 19% is used
in some cases.  On site mixing or use of gaseous ammonia or anhydrous ammonia for smaller facilities should
be avoided. As the percentage strength increases, the boiling point of the liquid lowers.  At 19%
strength, the boiling point is 134 degrees F.  At higher strength or at temperatures above ambient, the vapor pressure of the liquid
merits storage in pressure vessels, but at less than 19%, the solution may be stored in conventional
chemical storage tanks.
However, even 134 degrees is too low for many places in the US and the tropics.  For this reason, ammonium sulfate solution is preferred.
Liquid Ammonium Sulfate ( or LAS)  is typically a 38-40% aqueous solution of ammonium sulfate,
(NH4)2SO4. Each gallon of LAS has a specific gravity from 1.216 to 1.228 or 10.15 to 10.25 #/gallon
 
 
.

For more information, Contact Us