Waste Water Treatment
(Sewage Treatment)
Outline
•
Objectives
•
Outline of treatment process (flow diagram)
•
Preliminary treatment
•
Primary treatment (sedimentation)
•
Secondary treatment (activated sludge, filtration)
•
Sludge treatment
•
Disinfection
•
Tertiary treatment
O
bjective of Sewage Treatment
To prevent pollution of the receiving water
To prevent offensive odour in the water
To prevent the destruction aquatic life
If the sewage has to be disposed of on land,
the soil will become sewage sick after some
time and cannot take any more sewage
Outline of Sewage Treatment
Preliminary treatment
Primary treatment
Secondary treatment
Disinfecting
Tertiary Treatment
Preliminary Treatment
•
Waster water contains floating suspended
solids such as rags, wood, metal, plastic, etc.
•
these suspended impurities have to be
removed as they interfere with the treatment
processes or mechanical equipment.
Primary Treatment
•
consists of mainly the sedimentation
process to remove suspended organic solids
•
Chemicals are sometimes added in primary
clarifier to assist in the removal of finely
divided and colloidal solids or to precipitate
phosphorous
Secondary or Biological
Treatment
•
Activated sludge process
•
Filtration
Activated sludge process
•
the sewage is biologically treated.
Filtration
•
is done in contact beds or intermittent sand
filters or tricking filters.
•
It removes finely divided suspended matter.
Sludge & Effluent
•
The semi
-
solid that settles down at the
bottom after treatment,is called sludge
•
liquid to be discharged is called effluent.
Disinfecting
•
Disinfecting
is
carried
out
if
necessary
by
chlorination
to
kill
the
bacteria
which
remain
in
the
effluent
of
sewage
Tertiary Treatment
•
If
the
receiving
water
provides
only
little
dilution
or
there
is
a
need
to
preserve
a
particularly
high
quality
of
effluent,
tertiary
treatment
can
be
used
.
PRELIMINARY TREATMENT
Objectives
Remove
coarse
suspended
and
floating
matter
such
as
rags,
plastic,
pieces
of
wood,
etc
.
,
by
means
of
screening
Remove
grit,
sand,
broken
glass,
etc
.
,
in
grit
chambers
Remove
grease,
and
oil
substances
by
floatation
in
skimming
tanks
Processes
Screening
Silt
and
grit
removal
Oil
and
grease
removal
PRIMARY TREATMENT
PRIMARY TREATMENT
Primary
sedimentation
tank
(Secondary
sedimentation
tank)
-
sedimentation
after
activated
sludge
process
Advantages
•
The
advantages
of
coagulation
over
plain
sedimentation
are
:
Sedimentation
by
coagulation
is
more
effective
BOD,
colour
and
turbidity
are
reduced
Less
capacity
of
sedimentation
tanks
The
process
is
simple
Disadvantages
•
The
disadvantages
are
:
Chemicals
destroy
the
bacteria
that
digest
the
sludge
Chemicals
increase
the
cost
of
sedimentation
Skilled
supervision
is
required
Large
quantity
of
sludge
is
produced
SECONDARY TREATMENT
SECONDARY TREATMENT
•
The secondary treatment converts the
remaining organic matter into stable form
by putting aerobic and anaerobic bacteria
into action
Processes
Filtration
Activated
sludge
process
Filtration
•
Aerobic bacteria need a contact surface to
live and carry out their activities
Filtration
Contact
beds
Intermittent
sand
filters
Trickling
filters
Contact Beds
•
A
contact
bed
consists
of
filtering
medium
.
•
When
the
sewage
is
passed
over
the
medium,
a
thin
film
is
produced
around
the
particles
.
•
The
bacteria
present
in
the
film
oxidise
the
organic
matter
.
Intermittent Sand Filter
•
the
filtering
medium
consists
of
sand
.
•
The
action
of
the
filter
is
due
to
:
Mechanical
straining
and
–
Bacterial action
Trickling Filters
•
A trickling filter is an artificial bed of stone
or broken brick material
•
waste water is distributed an applied in
drops, films or spray
•
A zoogleal film is formed on the surface
media.
ACTIVATED SLUDGE
PROCESS
•
is
a
biological
sewage
treatment
process
•
a
mixture
of
sewage
and
activated
sludge
is
agitated
and
aerated
.
•
The
activated
sludge
is
subsequently
separated
from
the
treated
sewage
by
sedimentation
What is ACTIVATED SLUDGE
•
is
that
sludge
which
settles
down
after
the
sewage
has
been
freely
aerated
and
agitated
for
a
certain
time
.
How Activated Sludge works
•
Activated
sludge
contains
numerous
bacteria
and
other
microorganisms
.
When
it
is
mixed
with
raw
sewage
saturated
with
oxygen,
the
bacteria
perform
:
–
Oxidise
the
organic
solids
Promote
coagulation
and
flocculation
and
convert
the
colloidal
and
suspended
solids
into
settable
solids
Steps in Activated Sludge
process
•
Treat
raw
sewage
in
the
primary
settling
tank
•
mix
sewage
with
the
activated
sludge
and
aerated
for
4
-
10
hours
•
final
(secondary)
settling
tank
•
dispose
of
effluent
•
return
some
sludge
to
the
aeration
tanks
•
dispose
of
the
rest
of
the
sludge
Advantages
•
Gives
clear
sparkling
treated
effluent
•
Effluent
free
from
offensive
odour
•
Degree
of
purity
can
be
varied
as
desired
•
Cost
of
installation
lesser
than
for
a
trickling
filter
•
Small
loss
of
head
in
the
process
Disadvantages
Skilled
supervision
and
constant
check
on
the
return
sludge
is
necessary
When
there
is
change
in
the
quality
or
quantity
of
sewage,
the
process
is
upset
The
process
does
not
work
for
some
industrial
wastes
Large
volume
of
sludge
increases
difficulty
in
disposal
Methods of Aeration
•
The
success
of
the
activated
sludge
process
depends
on
the
aeration
provided
.
Diffused
air
aeration
Mechanical
aeration
Combination
of
the
above
two
methods
TERTIARY TREATMENT
TERTIARY TREATMENT
•
MICROSTRAINERS
•
RAPID GRAVITY SAND FILTERS
•
UPWARD
-
FLOW “MEDUIM” SAND
FILTERS
•
SLOW SAND FILTERS
•
PEBBLE
-
BED CLARIFIERS
•
SETTLEMENT
•
GRASS PLOTS
•
LAGOONS
SLUDGE TREATMENT AND
DISPOSAL
Sludge
•
Sludge
is a semi
-
liquid
•
Sludge produced by plain sedimentation
•
Sludge produced by chemical
precipitation
•
Trickling filter
•
Activated sludge
Methods of Disposal
•
Disposal
on
land
•
Drying
on
drying
beds
•
Dumping
into
the
sea
•
Heat
-
drying
•
Incineration
•
Lagooning
or
ponding
•
Sludge
Digestion
Sludge Digestion
•
It transforms a portion of solids into
liquids and gases, thereby reducing the
sludge volume to be dealt with
•
It breaks the organic matter of sludge
into simpler compounds by the action of
anaerobic bacteria.
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