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Combined available chlorine residual is defined as a chlorine
residual in which the chlorine present has combined with ammonia
to form pure chloramines or other chloro-derivatives.
A free available chlorine residual is defined as a chlorine
residual which is present either as hypochlorous acid or hypochlorite
ion, or both depending upon the pH of the water in which
the residual is present.
In water treatment the free available chlorine residual is
composed of hypochlorous acid only below a pH of 6.5 and hypochlorite
ion, almost exclusively above a pH of 9.5. Free available
chlorine residual will therefore normally be composed of a mixture
of hypochlorous acid and hypochlorite ion. At a pH of 7.5 there
is an approximately equal mixture.
Combined available chlorine is a weak oxidising agent, and is
slow in killing bacteria. For residuals of the same magnitude and
normal pH water, a combined available chlorine residual requires
for certain bacteria a contact time one hundred times longer than
a free available chlorine residual.
To kill certain bacteria in water of normal pH in the same
period of time, the magnitude of combined available chlorine
residual must be twenty-five times as great as that of free available
chlorine residual.
Free available chlorine is a strong oxidising agent, and its
action on bacteria is rapid. Certain types commonly present in
drinking water are killed after only a few seconds exposure. The
killing time is, however, materially increased when the pH of the
water reaches 9.5 to 10.0.
A free available chlorine residual is a persistent and stable
chlorine residual provided the chlorine demand of the water, the
plant and the pipelines have been satisfied. This may appear to
be an overstatement, but it is an established fact that the residual
is persistent. Several plants in the country are maintaining a free
available chlorine residual throughout the distributing system.
Colour removal is a particularly interesting field, where a
number of investigators have reported excellent results with free
available chlorination. The effect, however, is somewhat unpredictable,
and the pH value of the water is of importance. Often
where indifferent results have been obtained a slight alteration in