Sewage

SEWAGE
WHAT IS SEWAGE?
 Waste water from houses and community, factories
including solid and liquid excreta.
 Sullage: waste water which does not contain excreta
but water from kitchen, bathroom etc.
 Different amount of flow at different times of the
day depending upon habit of the people.
 Average amount of sewage which flows through the
sewerage system in 24 hours is called “ dry weather
flow”.
COMPOSITION OF SEWAGE
 99.9% water and 0.1% organic + inorganic materials
 Micro-organisms which may be pathogenic
• An adult person excretes 100 gm of feces daily.
• 1 gm of feces contains 1,000 million E coli, 10-100

million Fecal streptococci and 1-10 million spores of
Clostridium perfringens
CONSEQUENCES OF UNMANAGED
SEWAGE
 Looks bad, odour
 Breeding place for mosquitoes and flies
 Water and soil pollution
 Contamination of food
 Increased risk of diarrhoeal diseases

AIM OF SEWAGE TREATMENT
 Stabilize the organic matter in the sewage so that it
can be safely disposed
 Convert sewage water into acceptable standard of
purity before it is disposed in the environement
tests
Biochemical oxygen demand
Chemical oxygen demand
Suspended particles
BIOCHEMICAL OXYGEN DEMAND
(BOD)
 It is an indicator of organic content of the sewage.
 It is the amount of oxygen absorbed by a sample of
sewage during a specified period, generally 5 days at
a specified temperature i.e. 20 deg C for the aerobic
destruction or use of organic matter by living
organisms.
 BOD: 1mg/L for natural water and 300 mg/L for
untreated domestic sewage
CHEMICAL OXYGEN DEMAND (COD)
 Measures the amount of oxygen required to oxidize
inorganic chemicals such as Ammonia and nitrite
present in the sewage.
 It confirms the presence of toxic substance in water.
SUSPENDED SOLIDS
 It is the small solid particles which remain
in suspension in water.
 It is used as one of the indicator of water quality or
strength of sewage
 Domestic sewage may contain 100-500 mg/L
suspended particles
 Weak sewage: suspended particles 100 mg/L
 Strong sewage: suspended particles 500 mg/L

DECOMPOSITION OF ORGANIC MATTER
IN SEWAGE
 Aerobicprocess
 Anaerobic process
AEROBIC PROCESS
 Continuous supply of free dissolved oxygen
 Action of bacterial organisms including fungi and
protozoa.
 Organic matter is broken down into simpler
compounds i.e. CO2, water, ammonia, nitrites,
nitrates and sulphates.
Disadvantage:
More sludge is produced
High aeration required.

ANAEROBIC PROCESS
 Decomposed by anaerobic bacteria, fungi, protozoa
etc. in sludge digestion tank.
 Reaction is slower
 End products are biogas, methane, ammonia, CO2,

H2.
Advantage:
Biogas can be transformed into heat or power
Less sludge is produced.

MODERN SEWAGE TREATMENT
 Done at special sewage treatment plants.
 Aerobic and anaerobic process.
 Primary and secondary treatment

Primary treatment Secondary treatment
Sewage
Primary Final
Screen Grit Biological
sedimentati sedimentatio
chamber treatment
on tank n tank
Methane Sludge
gas digester
Sludge drying beds

Chlorine
Final sedimentation
tank Effluent disposal
A) PRIMARY TREATMENT
1. Screening: Steel bars which are set 5 cm apart
screen for large floating objects like wood, rags,
masses of garbage and dead animals. The screened
wastes are removed from time to time and disposed
off.
 Grit chamber: Long narrow chamber, 10-20 m in length
through which sewage is passed. The flow is maintained
at the velocity of 1 foot per second so as to allow
settlement of heavier solids like sand and gravel. The
settled particles are removed from time to time and
disposed off.
3. PRIMARY SEDIMENTATION:
 Sewage is passed into a huge tank called the primary
sedimentation tank usually rectangular in shape.
 Sewage is made to flow very slowly across the tank
at a velocity of 1-2 feet per minute.
 The sewage remains in the tank for 6-8 hours during
which sedimentation of most of the suspended
particles take place.
 The organic matter which settles down is called
sludge which is removed by mechanically operated
devices.
PRIMARY SEDIMENTATION
 Biological action takes place in the tank in which
micro-organisms present in the sewage
decomposes complex organic solids into simpler
compounds and ammonia.
 Certain amount of fat and grease rise to the surface
i.e. scum which is also removed from time to time.
 If organic waste from factories is present then,
chemical treatment is done with lime, aluminium
sulphate and ferrous sulphate.
PRIMARY SEDIMENTATION TANK
B) SECONDARY TREATMENT
 The sewage which has undergone primary treatment
still contains organic matters and living organisms
thus secondary treatment is done.
 Aerobic oxidation is done by i) Trickling filter
method or ii) Activated sludge process
I) TRICKLING FILTER METHOD
 A bed of crushed stones or clinker, 1-2 m (4-8 ft) deep
and 2-30 m (6-100 ft) diameter is used.
 The effluent from the primary sedimentation tank is
sprinkled uniformly on the surface of the bed by a
revolving device.
 Over the surface of the filter bed, numerous micro-
organisms grow (bacteria, fungi, algae, protozoa)
which is known as “zoogleal layer”.
 As the effluent passes through the zoogleal layer, it
gets oxidized by the bacterial flora in the zoogleal
layer.
II) ACTIVATED SLUDGE PROCESS
 Effluent from the primary sedimentation tank is mixed
with sludge taken from final settling tank (activated
sludge) in the proportion of 20-30% which is rich in
aerobic bacteria.
 The mixture is left for aeration in the aeration tank for 6-
8 hours.
 Mechanical agitation done or compressed air forced
continuously from the bottom of the tank also known as
diffuse aeration.
 Organic matter of the sewage gets oxidized into carbon
dioxide, nitrates and water with the help of aerobic
bacteria in the activated sludge.
 Modern method of purifying sewage.
 Aeration tank is the main part.
SECONDARY SEDIMENTATION
 The oxidized sewage after secondary treatment is
passed to the secondary sedimentation tank and kept
for 2-3 hours.
 Sludge collected in the secondary sedimentation
tank is called activated sludge or aerated sludge.
 Some part of the activated sludge is pumped back to
the aeration tank for “the active sludge process” in
secondary treatment while rest is pumped into
sludge digestion tank for treatment and disposal.
SLUDGE DIGESTION
1. Digestion
2. Sea disposal and Land disposal
1. DIGESTION
 Sludge is incubated after which it undergoes anaerobic
digestion.
 Complex solids are broken down into water, ammonia,
methane and carbon dioxide.
 Volume is also reduced.
 3-4 weeks time
 Done in sludge digestion tank
 Can be used as manure after drying

2. SEA AND LAND DISPOSAL
 Pumped into the sea
 Composting along with waste from cities
DISPOSAL OF EFFLUENT
a) Disposal by dilution: Effluent is disposed into
water bodies where it gets diluted and remaining
impurities are oxidized by dissolved oxygen in
water.
b) Disposal on land: can be used for irrigation
purpose.
OTHER METHODS OF SEWAGE
DISPOSAL
 Sea outfall: disposed to the sea directly through
sewerage pipes
 River outfall: disposed to the river
 Land treatment: also known as sewage farming. Sewage

is used to grow specific type of crops after screening,
grit removal and sedimentation.
OXIDATION POND:
 Open, shallow pool 1-1.5 m deep with an inlet and

outlet.
 There must be algae, specific bacteria which feed upon
decaying organic matter and presence of adequate
sunlight.
 Algae liberates oxygen under the influence of sunlight.
 Organic matter in the sewage is oxidized by bacteria

into simpler compounds like CO2, ammonia and
water.
 Algae utilize the CO2, water and inorganic minerals
with the help of sunlight for their growth.
 Anaerobic digestion also takes place at the bottom of
the pond.
 Effluent can be used for irrigation or disposed into
water sources after appropriate treatment.
OXIDATION DITCHES
 Mechanical rotors are used for extended aeration.
 Requires less area thus low cost method
Thank you!

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SEWAGE

• An adult person excretes 100 gm of feces daily.

• 1 gm of feces contains 1,000 million E coli, 10-100

 Water and soil pollution

 Increased risk of diarrhoeal diseases

Chemical oxygen demand

 Strong sewage: suspended particles 500 mg/L

High aeration required.

 End products are biogas, methane, ammonia, CO2,

Less sludge is produced.

 Primary and secondary treatment

Sludge drying beds

 3-4 weeks time

 Done in sludge digestion tank

 Can be used as manure after drying

 Land treatment: also known as sewage farming. Sewage

 Open, shallow pool 1-1.5 m deep with an inlet and

 Organic matter in the sewage is oxidized by bacteria

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