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OXIDATION AND DISINFECTION 1D. 15

perature, pH mixing regimen, and ClffNH3 weight ratio all influence both the rate and the
products of the reaction. When a small amount of chlorine is added (ClffNH3 < 4), mono-
chloramine is the dominant species formed. As more chlorine is added, dichloramine and
trichloramine are formed, along with other products such as NO3 and N2 gas. The fol-
lowing are some of the typical reactions that may occur:

Monochloramine: NH4 + + HOC1 = NH2C1 + H20 + H +
Dichloramine: NH2C1 + HOC1 = NHC12 + H20
Trichloramine: NHC12 + HOC1 = NC13 + H20
Nitrogen: 2NH4 + + 3HOC1 = N2 + 5H + + 3C1 + 3H20
The relationships between these reactions are shown in Figure 10.7. Monochloramine is
the major compound formed until the mg Cl2/mg NH3 ratio exceeds 4. This is the com-
pound desired when chloramines are used for disinfection in potable water. The recom-
mended C12/NH3 weight ratio for the formation of monochloramine is 3 to 4 because this
will minimize the concentration of unreacted ammonia remaining in the water.
Weight ratios of CI2/NH3 greater than 4 but less than 8 should be avoided because
dichloramine is formed in this region. Dichloramine is a disinfectant, but it also produces
undesirable tastes and odors. Formation of nitrogen gas is not shown in Figure 10.7, but
it also starts to occur in this region.
The low point in the total chlorine residual curve shown at a CI2/NH3 weight ratio of
8 in Figure 10.7 is called the breakpoint. Increasing the CI2/NH3 ratio above this value
will produce a free-chlorine residual. Nitrogen gas is also produced in the breakpoint
reaction. A dose of 9 to 10 mg Clz/mg NH3 should be used for designing breakpoint
facilities.
Increasing the free-chlorine residual to values well in excess of the breakpoint is counter
productive because nitrogen trichloride will start to form. This is a sparingly soluble, foul-
smelling gas that will cause consumer complaints. Excessive chlorine doses should be
avoided.
Saunier (1976) has extensively modeled the chloramine reactions in both potable wa-
ter and wastewater. Additional insights to the reactions between these compounds are
available in his paper.
Combined residual chlorination was first used at Ottawa, Ontario, Canada, in 1916.
The process enjoyed modest success as a way to eliminate tastes and odors produced by
the use of free chlorine until 1939, but was widely discontinued during World War II be-
cause ammonia was not available for civilian use. Combined residual chlorination did not
return to widespread use until the concentrations of chlorinated disinfection by-products
(TTHMs and HAA5) in the water delivered to consumers were regulated by the USEPA.
Use of a monochloramine residual in the distribution system became common at that time
because combined chlorine did not produce these by-products. However, the use of this
compound is not a panacea. Its germicidal action is substantially less than that of free
chlorine, and the ammonia released during decay of this compound is a food source for
nitrifying bacteria.
The USEPA permits the use of chloramine as a primary disinfectant if the CT re-
quirements published by this agency are met. However, chloramine is much more com-
monly used as a secondary disinfectant in the distribution system after primary disinfec-
tion is achieved by the use of free chlorine, ozone, chlorine dioxide, or UV in the treatment
plant. The concentration of chloramine required will depend upon the size of the distri-
bution system and the decay rate of the residual. The maximum chioramine concentration
allowed by USEPA is 4 mg/L as C12. Most facilities provide an initial residual from 2.5
to 3.5 mg/L to minimize taste and odor complaints from consumers. The minimum chlo-