Science of Water : Concepts and Applications

Water Treatment 317
The selection of one form of chlorine over the others for a given water system depends on the
amount of water to be treated, confi guration of the water system, the local availability of the chemicals,
and the skill of the operator.
One of the major advantages of using chlorine is the effective residual that it produces. A
residual indicates that disinfection is completed, and the system has an acceptable bacteriological
quality. Maintaining a residual in the distribution system provides another line of defense against
pathogenic organisms that could enter the distribution system and helps prevent regrowth of those
microorganisms that were injured but not killed during the initial disinfection stage.
Common chlorination terms include the following:
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Chlorine reaction—Regardless of the form of chlorine used for disinfection, the reaction
in water is basically the same. The same amount of disinfection can be expected, provided
the same amount of available chlorine is added to the water. The standard term for the
concentration of chlorine in water is milligrams per liter (mg/L) or parts per million (ppm);
these terms indicate the same quantity.
Chlorine dose—The amount of chlorine added to the system. It can be determined by
adding the desired residual for the fi nished water to the chlorine demand of the untreated
water. Dosage can be either milligrams per liter (mg/L) or pounds per day. The most common
is mg/L.
Chlorine demand—The amount of chlorine used by iron, manganese, turbidity, algae, and
microorganisms in the water. Because the reaction between chlorine and microorganisms
is not instantaneous, demand is relative to time. For instance, the demand 5 min after
applying chlorine will be less than the demand after 20 min. Demand, like dosage, is
expressed in mg/L. The chlorine demand is as follows:
Cl 2 demand Cl 2 dose Cl 2 residual
(9.7)
Chlorine residual—The amount of chlorine (determined by testing) that remains after the
demand is satisfi ed. Residual, like demand, is based on time. The longer the time after dosage,
the lower the residual will be, until all of the demand has been satisfi ed. Residual, like
dosage and demand, is expressed in milligrams per liter. The presence of a free residual of
at least 0.2–0.4 ppm usually provides a high degree of assurance that the disinfection of the
water is complete. Combined residual is the result of combining free chlorine with nitrogen
compounds. Combined residuals are also called chloramines. Total chlorine residual is the
mathematical combination of free and combined residuals. Total residual can be determined
directly with standard chlorine residual test kits.
Chlorine contact time (CT)—One of the key items in predicting the effectiveness of chlorine
on microorganisms. It is the interval (usually only a few minutes) between the time
when chlorine is added to the water and the time the water passes by the sampling point,
contact time is the “T ” in CT. CT is calculated based on the free chlorine residual prior to
the fi rst customer times the contact time in minutes.
CT concentrationcontact time milligram per literminutes √ Note: A certain minimum time period is required for the disinfecting action to be completed.
The contact time is usually a fi xed condition determined by the rate of fl ow of the water and the
distance from the chlorination point to the fi rst consumer connection. Ideally, the contact time
should not be