of turbidity (that is foreign suspended matter) by this
natural settling process is known as SEDIMENTA-
TION. Sedimentation is accomplished in settling tanks
where the water is held for a time to allow the floc to
form and settle out turbidity. In conventional
treatment, settling immediately follows flocculation.
The ideal detention period for settling after slow
mixing is about 1 1/2 hours.
Not all suspended matter is removed by
sedimentation. Therefore, another process known as
FILTRATION is used. An effective type of filter used
in the filtration process is the diatomite. Because it is
lightweight, this filter is widely used at overseas bases.
It removes suspended matter from water by passing it
through a porous mat of diatomaceous silica.
Diatomaceous silica is the skeletal remains of tiny
algae, called diatoms, found in marine deposits that
have been lifted above sea level. The diatomite (also
called diatomaceous earth or filter aid) is supported by
a filter element. This supporting base is porous endugh
to permit maximum flow. It is also fine enough to
support the filter cake that coats the element.
Diatomite filters are backwashed by reversing the flow
of water and drawing filtered water through the filter to
keep the filter output from falling off. The turbidity of
the water is largely determined by the frequency of
Except in rare instances, all water supplies require
disinfection. Disinfection is the chemical destruction
of bacteria. Because of its economy, dependability,
efficiency, and ease of handling, chlorine is almost
always used for this purpose. For this reason, the term
chlorination generally means the same as
Disinfection is a necessary step in ensuring a safe
water supply. Ali new, altered, or repaired water-
supply facilities must be disinfected before they are
placed in service. Water from surface supplies may be
disinfected before filtration or before coagulation and
sedimentation to prevent the growth of organisms.
This procedure is known as precholorination. The
water must also be disinfected after filtration to destroy
organisms that still remain and to provide a safeguard
against recontamination. This procedure is known as
Chlorine is the disinfectant specified for Navy use.
Inthe form of chlorine gas or of hypochlorites that
yield chlorine in water, chlorine is presently the only
widely accepted agent that destroys organisms in the
water and leaves an easily detectable residual that
serves as an indicator of the completeness of treatment.
The sudden disappearance of residual chlorine may
signal contamination in the system. Under ordinary
temperatures and pressures, chlorine gas is greenish
yellow and is heavier than air. Its effectiveness as a
disinfectant depends on the temperature and the
hydrogen-ion concentration (pH) of the water to which
it is added. Disinfecting action is faster at higher
temperatures, but is retarded by pH. When the pH is
above 8.4, the rate of disinfection decreases sharply.
Ozone, potassium permanganate, bromine, and.
iodine are also used to a limited extent as disinfectants.
If excess lime is used for softening water, it makes the
water alkaline and disinfects after about 10 hours of
contact. However, the general applicability and
economic advantage of chlorine have established it as
the preferred disinfectant.
disinfectants are available in a number of different
forms as described in the following paragraphs
LIQUID CHLORINE is liquefied gas under
pressure and is shipped in seamless steel cylinders
under the regulations established by the Interstate
Commerce Commission. The standard sizes of
shipping containers are 150-pound cylinders, 1-ton
containers, and 30-ton tank trailers.
Each pound of liquid chlorine produces about 5
cubic feet of chlorine gas at atmospheric pressure and
at a temperature of 68°F. A standard Chlorine Institute
valve and a protective valve hood are screwed into the
neck of each cylinder. The valve has a safety plug
containing fusible metal that softens between 157°F
and 162°F to protect the cylinder from bursting in case
of fire. All cylinders must be factory tested every 5
years; 150-pound cylinders are tested at 500 pounds of
pressure; and I-ton containers are tested at 800 pounds
HIGH-TEST CALCIUM HYPOCHLORITE is a
relatively stable, dry granular solid or powder that is
readily soluble to form a chlorine solution. Prepared
under a number of trade names, including HTH,
Perchloron, and Pittchlor, it is furnished in 3- to 100-
pound containers and has 65 to 70 percent available
chlorine by weight. Because of its concentrated form
and ease of handling, calcium hypochlorite is preferred
over other hypochlorites.