recommendations on the design and
most important.
A chemical agent is
selection
of
water-purification
added to the water to kill pathogenic
equipment.
bacteria.
The chemical is usually
chlorine, but iodine may be used.
(3) Chemical
units.
Chemical
9.
STORAGE
units
have
responsibilities
with
respect to water supply systems which
have
been
contaminated
by
toxic
The first two parts of a water supply
agents.
system are a source and a treatment
plant.
The third is a storage
b. Treatment processes. Four steps
facility.
The storage requirement is
are generally used in the treatment of
simply one half of the total daily
any water.
These are sedimentation,
requirement.
coagulation,
filtration,
and
disinfection.
Both of the last two
steps, filtration and disinfection,
are
necessary
for
the
complete
treatment of water.
The filtering
media, however, will rapidly become
The total daily requirement is divided
clogged
if
the
first
two,
by two to reduce the amount of
sedimentation and coagulation, do not
construction effort for the storage
precede filtration. Thus, all four of
facility while still allowing for
these steps are necessary in practice
enough
to
meet
the
peak
demands
for the complete treatment of water.
without continual operation of pumping
Either
filtration
or
disinfection,
and treatment facilities.
Once the
however, without the other steps, will
total storage requirement is found,
improve
water
found
in
natural
the water should be stored at two
sources.
different
locations
to
reduce
vulnerability and to allow for repair
(1) Sedimentation.
Sedimentation
and cleaning of the tanks without
is no more than allowing water to
interrupting service. Thus storage is
stand so that particles heavier than
usually best accomplished by using two
the water itself will settle out.
tanks, each of whose capacity is one
quarter
of
the
total
daily
(2) Coagulation.
Coagulation
requirement.
takes place when a coagulant (usually
alum) reacts with alkalines in the
10. DISTRIBUTION SYSTEM
water to form a jelly-like mass. This
jelly-like mass, called floc, collects
The last part of a water supply
the smaller, lighter particles and the
system is the distribution system. It
entire mass settles out.
Coagulation
can be classified in two ways by the
is normally considered a separate
type of energy used to distribute the
treatment step.
water, and by the arrangement of the
pipes.
(3) Filtration.
Filtration
through sand, diatomaceous earth, or
a. Classification by energy used.
other
filter
elements
removes
additional
and
some
(1) Gravity.
An
all-gravity
pathogenic
organisms.
Activated
system needs a source above the storage
carbon or charcoal used as a filter
facility so that the water from the
element will remove undesirable tastes
treatment plant may flow by gravity to
and odors from water.
storage.
The storage facility must
again be elevated enough above the
(4) Disinfection.
This step in
users so that gravity may be used
the treatment process is probably the
again to distribute the water.
The
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