2-4.
CHLORINATION METHODS
a. Marginal Chlorination. This term refers to the application of chlorine to
produce the desired total chlorine residual without reference to the relative amounts of
free or combined chlorine present. In marginal chlorination, the initial chlorine demand
has been satisfied but some oxidizable substances remain. This type of chlorination is
the most common type of chlorination practiced at fixed facilities in CONUS and their
overseas equivalent.
b. Breakpoint Chlorination. If chlorine is added to water that has no chlorine
demand, the chlorine residual is the same as the chlorine dosage, as shown by line A in
figure 2-1. However, in water that has a chlorine demand, the chlorine residual appears
graphically as a curve, as shown by line B. The initial dosage (line B-1) is used up
satisfying the chlorine demand, and no residual is obtained. As the dosage is increased,
a combined residual (CAC) in the form of nitrogen compounds, mostly chloramines, is
produced (line B-2). As the dosage is further increased, the increasing concentration of
chlorine breaks down the chloramines (line B-3). When this reaction is complete, any
additional chlorine added will be present as free available chlorine, a very active
disinfecting agent (line B-4). The point at which the breakdown of chloramines is
completed, and the addition of chlorine results in free available chlorine at a one-to-one
ratio, is known as the "breakpoint." Breakpoint chlorination results in a residue that is in
excess of 90 percent free available chlorine. Advantages of chlorinating to and beyond
the breakpoint to obtain free available chlorine residuals are that most odors and tastes
normal to water are destroyed and rigorous disinfection is assured. For field water
supplies, Army procedures require the provision of breakpoint chlorination in obtaining
the minimum residual requirement of free available chlorine.
c. Super chlorination. Super chlorination is the process of adding an amount
of chlorine greater than that required to achieve a chlorine residual. It is used to remove
contamination by various poisons, including some CBR agents (see Table 2-1), to
disinfect distribution systems following repairs, or to remove undesirable tastes and/or
odors. After super chlorination, excess chlorine must be removed by dechlorination
(see d, below).
d. Dechlorination. Dechlorination is the process of removing excess chlorine
from water in order to avoid chlorinous taste before distribution to the consumer. In
addition to the activated carbon process, water may be dechlorinated by the addition of
sodium thiosulfate (Na2S2O3.), sodium metabisulfate (Na2S2O5.), sodium sulfite
(Na2SO3), sodium bisulphite (NaHSO3), ammonia (NH4OH), or sulfur dioxide (SO2).
Aeration is also a means of dechlorination.
MD0160
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