a. Chlorine Dosage. Chlorine dosage is the amount of chlorine added to water
to satisfy the chlorine demand (b, below) and to provide a chlorine residual (c, below)
after a specified period of time. The amount required to disinfect the water varies with
the organic content of the water, the pH, the temperature, the contact time, and the
residual required. High pH and low temperatures retard disinfection by chlorination,
thus requiring a higher dosage. Dosage is usually expressed in terms of parts per
million (ppm) or milligrams per liter (mg/1).
b. Chlorine Demand. Chlorine demand is the amount of chlorine dosage that is
consumed by the substances, either organic or inorganic, in the water. Some of these
chlorine-consuming agents in the water are nonpathogenic organisms, but they still
contribute to the total chlorine demand of the water.
c. Chlorine Residual. The chlorine residual is the amount of measurable
unreacted chlorine remaining at a specific time (contact time) after the chlorine
compound has been added. Dosage minus demand equals residual. The total residual
chlorine in water can be chemically divided into three types:
(1) Free available chlorine. The free available chlorine (FAC) is the chlorine
present in the form of hypochlorous acid and hypochlorite ion. These forms are the
most effective disinfectants. Free available chlorine is rapid in its action, destroying
bacteria relatively quickly-20 to 30 times faster reacting than in the combined form.
(2) Combined available chlorine. Combined available chlorine (CAC) is the
chlorine that has combined with ammonia or organic nitrogen in the water to form
chloramines. Chloramines are weak disinfectants and are much less active than FAC.
A free available chlorine residual of 0.05 ppm with a contact time of 10minutes at a pH
of 7.0 will result in the same bacterial kill as a combined available chlorine residual of
0.6 ppm with a contact time of 60 minutes. It is the chloramines in water that cause the
chlorine tastes which many people find objectionable. When the combined chlorine
residual is replaced by free chlorine residual, there is a distinct improvement in the taste
of most waters.
(3) Total available residual chlorine. Total available residual chlorine (TAC)
is the sum of the free available chlorine ((I), above) and the combined available chlorine
d. Contact Time. Chlorine demand in most water is likely to be largely satisfied
after a 10-minute contact time. After the first 10 minutes of chlorination, disinfection
continues, but at a diminished rate. A standard contact time of 30 minutes is used in
Army chlorination to assure that highly resistant or highly pathogenic organisms are