Water Treatment - The Drinking Water - page 244

that can be damaged by corrosion. Sodium hypochlorite solution is more
costly per pound of available chlorine and does not provide the same
level of protection of chlorine gas.
Calcium hypochlorite is a white solid in granular, powdered, or tablet form
containing 65% available chlorine. In packaged form, calcium hypochlorite
is stable—more stable than solutions of sodium hypochlorite, which deterio-
rate over time; however, calcium hypochlorite is hygroscopic , which means
it readily absorbs moisture. It reacts slowly with moisture in the air to form
chlorine gas. It is a corrosive material with a strong odor and requires proper
handling. Some practical difficulty is involved in dissolving calcium hypo-
chlorite. It must be kept away from organic materials such as wood, cloth,
and petroleum products. Reactions between it and organic materials can
generate enough heat to cause a fire or explosion.
Chlorine Use
Whatever form of chlorine is used for disinfection (elemental chlorine,
sodium hypochlorite, or calcium hypochlorite), it may be added to the
incoming flow (prechlorination) to assist with the oxidation of inorganics or
to arrest biological action that may produce undesirable gases in the bottom
of clarifiers. More often, however, chlorine is added just prior to filtration to
keep algae from growing at the medium surface and to prevent large popula-
tions of bacteria from developing within the filter medium. Safe and effective
application of chlorine requires specialized equipment and considerable care
and skill on the part of the plant operator. Various means of feeding chlorine
have been developed, but probably one of the widest used and safest types
of chlorine feed devices is the all-vacuum chlorinator . Mounted directly on the
chlorine cylinder, the gaseous chlorine is always under a partial vacuum in
the line that carries it to the point of application. In a typical vacuum chlorine
feed system, the vacuum is formed by water flowing through the ejector unit
at high velocity.
Hypochlorites are usually applied to water in liquid form by means of
positive displacement-type pumps, which deliver a specific amount of liquid
on each stroke of a piston or flexible diaphragm. Chlorine, when added to
water, reacts with various substances or impurities in the water (e.g., organic
materials, sulfides, ferrous iron, and nitrites), which creates a chlorine demand .
Chlorine demand is a measure of the amount of chlorine that will combine
with impurities and is therefore available to act as a disinfectant. Chlorine
combines with ammonia or other nitrogen compounds to form chlorine com-
pounds that have some disinfectant properties. These compounds are called
combined available chlorine residual . In the context used here, “available” means
available to act as a disinfectant. The uncombined chlorine that remains in the
water after combined residual is formed is called free available chlorine residual .
Free chlorine is a much more effective disinfectant than combined chlorine.