Science of Water : Concepts and Applications

Water Treatment 319
Total residual
Demand
Chloramines
formed
FIGURE 9.9 Breakpoint chlorination curve.
Breakpoint Chlorination
To produce a free chlorine residual, enough chlorine must be added to the water to produce what is
referred to as breakpoint chlorination (i.e., the point at which near complete oxidation of nitrogen
compounds is reached; any residual beyond breakpoint is mostly free chlorine [see Figure 9.9]).
When chlorine is added to natural waters, the chlorine begins combining with and oxidizing the
chemicals in the water before it begins disinfecting. Although residual chlorine will be detectable
in the water, the chlorine will be in the combined form with a weak disinfecting power. As we see
in Figure 9.9, adding more chlorine to the water at this point actually decreases the chlorine residual
as the additional chlorine destroys the combined chlorine compounds. At this stage, water may
have a strong swimming pool or medicinal taste and odor. To avoid this taste and odor, add still
more chlorine to produce a free residual chlorine. Free chlorine has the highest disinfecting power.
The point at which most of the combined chlorine compounds have been destroyed and the free
chlorine starts to form is the breakpoint.
The chlorine breakpoint of water can only be determined by experimentation. This simple
experiment requires 20 1000-mL beakers and a solution of chlorine. Place the raw water in the
beakers and dose with progressively larger amounts of chlorine. For instance, you might start with
0 in the fi rst beaker, then 0.5 mg/L and 1.0 mg/L, and so on. After a period of time, say 20 min, test
each beaker for total chlorine residual and plot the results.
Breakpoint Chlorination Curve *
When the curve starts, no residual exists, even though there was a dosage. This is called the initial
demand and is microorganisms and interfering agents using the result of the chlorine.
After the initial demand, the curve slopes upward. Chlorine combining to form chloramines
produces this part of the curve. All of the residual measured on this part of the curve is combined
residual.
At some point, the curve begins to drop back toward zero. This portion of the curve results from
a reduction in combined residual, which occurs because enough chlorine has been added to destroy
(oxidize) the nitrogen compounds used to form combined residuals.
The breakpoint is the point where the downward slope of the curve breaks upward. At this
point, all of the nitrogen compounds that could be destroyed have been destroyed.
* Refer to Figure 9.9 for the following explanation.
Chloramines
destroyed
Chlorine added to water
Free residual
formed
Combined chlorine residual
Free
available
chlorine
residual