-
Unit 13
Text: Water Treatment
Processes
One
of
the
great
achievements
of
modern
technology
has
been
to
drastically
reduce
the
incidence
of
waterborne
of
diseases
such
as
cholera
and
typhoid
fever.
These
diseases
are
no
longer
the
great
risks
to
pubic
health
that
they
once
were.
The
key
to
this
advance
was
the
recognition that contamination of pubic
water supplies by human wastes was the main source
ci
infection,
and
that
it
could
be
eliminated
by
more
effective
water
treatment
and
better
waste
disposal.
Tod
ay’s
water
treatment
plants
are
designed
to
provide
water
continuously
that
meets
drinking water
standards at the tap. There are four main
considerations involved in accomplishing
this1 source selection, protection of
water quality, treatment methods to be used, and
prevention of
recontamination. Common
precautions to prevent groundwater and surface
water pollution include
prohibiting
the
discharge
of
sanitary
and
storm
sewers
close
to
the
water
reservoir,
installing
fences to prevent
pollution from recreational uses of water, and
restrictions on the application of
fertilizers and pesticides in areas
that drain to the reservoir.
Screening,
coagulation/flocculation,
sedimentation,
filtration.
and
disinfection
are
the
main
unit operations involved in the
treatment of surface water. Water treatment
operationa fulfill one or
more of three
key tasks1 removal of particulate substances such
aa sand and clay, organic matter,
bacteria, and algae; removal of
dissolved substances such as those causing color
and hardness; and
removal
or
destruction
of
pathogenic
bacteria
and
viruses.
The
actual
selection
of
treatment
processes depends on the type of water
source and the desired water quality.
Occasionally,
raw
water
with
low
turbidity
can
be
treated
by
plain
sedimentation
(no
chemicals) to remove larger particles
and then filtration to remove the few particles
that failed to
settle out. Usually,
however, particles in the raw water are too small
to be removed in a reasonably
short
time through sedimentation and simple filtration
alone. To remedy this, a chemical is added
to
coagulate/flocculate
the
small
particles,
called
colloids,
into
large
ones,
which
can
then
be
settled out in sedimentation tanks or
removed directly in filters.
Removal of Particulate Matter
The
unit
operations
employed
for
the
removal
of
particulate
matter
from
water
include
screening,
sedimentation, coagulation/flocculation, and
filtration.
Screening
to
remove large solids such as logs, branches, rags,
and small fish is the first
stage in
the treatment of water. Allowing such debris into
the treatment plant could damage pumps
and clog pipes and channels. For the
same reasons, water intakes are located below the
surface of
the lake or river in order
to exclude floating objects and minimize physical
damage from ice.
Sedimentation
,
the
oldest
and
moat
widely
used
form
of
water
and
wastewater
treatment,
uses gravity
settling to remove particles from water. It is
relatively simple and inexpensive and can
be implemented in basins that are
round, square, or rectangular. As noted earlier,
sedimentation
may
follow
coagulation
and
flocculation
(for
highly
turbid
water)
or
be
omitted
entirely
(with
moderately turbid
water) .Particulates suspended in surface water
can range in size from 10
-1
to
10
-7
mm
in
diameter,
the
size
of
fine
sand
and
small
clay
particles,
respectively.
Turbidity
or
cloudiness in water is caused by those
particles larger than 10 mm, while particles
smaller than 10
mm contribute to the
color and taste.