Disposal of wastewater.pptx

Environmental Engineering (3160611) KEYUR NAGECHA
Keyur Nagecha
Department of Civil Engineering
V. V. P. Engineering College: Rajkot
Environmental Engineering(3160611)

Disposal of treated sewage or sewage effluents:
• Convey sewage through sewers
• Next step disposal
► After treatment
► Or before treatment.
• Disposed off in natural water courses
disposal
Natural method
By dilution
By land treatment
Artificial method
Primary treatment
Secondary treatment

Disposal by Dilution:
• Dilution:
► Disposal of sewage by discharging it into a river stream, or a large
body of water such as lake or sea.
• Possible only when required quantity of water is available.
• Care
► Should not pollute receiving water

Disposal by Dilution:
• Conditions favoring disposal by dilution:
► Location near sea, river or lake.
► Sewage reaching is fresh
► Receiving water has high DO content.
► Sufficient depth of water
► Large quantity
► During flood no backward flow
► When wastewater does not contain industrial wastewater having toxic
substance
► When diluting water is not used for drinking or navigation.

Disposal by Dilution: Standards of dilution:
• Disposal by dilution is based on dilution factor
• Dilution factor.
► Ratio of quantity of receiving water to that of wastewater or effluent
discharge
• Royal Commission Report on sewage disposal set various criteria
for disposal through dilution.

Dilution
factor
Standard of purification required based on Royal
Commission report
> 500 No treatment required. Direct discharge of raw sewage is
allowed.
300 to
500
Primary treatment like plain sedimentation
Effluent should not contain suspended solids more than 150
ppm.
150 to
300
Various treatments required. Sedimentation, screening,
chemical precipitation etc. Effluent should not contain
suspended solids more than 60 ppm.
< 150 Complete and thorough treatment required.
Suspended solids not more than 30 ppm. BOD5 not more
than 20 ppm.

• Indian standards has given various tolerance limits and limits for
various factors like pH, BOD, COD, chemical compounds like
sulphide, fluorides etc.
► IS:4764  sewage effluent discharged in to inland surface waters.
► IS:2490  industrial effluents discharged into inland surface waters
► IS:3306  effluents discharged into sewers
► IS: 2296  inland surface waters subjected to pollution.

Types of receiving water for dilution:
• Receiving waters
► Perennial rivers or streams.
► Lakes
► Ocean or sea
► Estuaries
► creeks

• Perennial rivers or streams.
► Best type
► Flow throughout the year
► Balance between plant and animal life
► Different discharge during summer and winter
► Summer low dilution ratio compare to winter

• Lakes:
► Enclosed water space
► Critical Factors
• Size, shape, volume of fresh water flowing into it
► Self purifying capacity must be checked.
• Ocean or sea
► Abundant water
► Unlimited dilution factor
► Any sewage can be diluted.

► However
• 20% less DO than river
• Turbid water due to dissolved impurities
• Less penetration of sun’s rays
• Results in anaerobic conditions
• Formation of sludge banks and emission of foul odour.

• Estuaries:
► Wide lower tidal part of a river.
► Affected by both sea water as well as river water.
• Creeks:
► Inlet on sea coast
► May not have dry weather flow during some part of the year.
► Great care is required.

Self purification of natural streams:
• Sewage discharged in to a natural stream
• Organic matter breaks down by bacteria to ammonia, nitrates,
nitrites, sulphates, carbon dioxide etc.
• In this process of oxidation
► DO of water is utilized.
► Creates deficiency of DO
• This condition do not remain forever.

► Natural forces of purification such as
• Dilution
• Sedimentation
• Oxidation
• Reduction in sun light etc.
► Replenish the DO
► Brings water to its original conditions.
• This automatic purification of polluted water in due course is called
self purification phenomena.

• Actions (forces) involved in self purification.
► Dilution
► Dispersion due to currents
► Sedimentation
► Oxidation
► Reduction
► Temperature
► sunlight

Actions involved in self purification
• Dilution:
► Wastewater discharged into a large volume of
receiving water  dilution takes place.
► Reduction in concentration of organic matter
► Potential nuisance of sewage is also reduced
► Concentration ‘C’ of resulting mixture.
• Sewage of concentration Cs flows at a rate of Qs in
a river stream with concentration CR flowing at a
rate of QR.
• CS . QS + CR . QR = C (QS + QR)
• C = (CS . QS + CR . QR ) / (QS + QR)

► When dilution ratio is high, large quantities of DO
are always available
► Reduces the chances of putrefaction and pollution
effects.
► Aerobic conditions will always exist because of
dilution.

• Dispersion due to currents:
► Self purification largely depends upon currents.
► Currents disperse the wastewater in the stream
► Prevents high concentration of pollutants
► High velocity improve re-aeration
• Reduces the time of recovery.

• Sedimentation:
► If the stream velocity is less than scouring velocity
of particles
• The settleable solids will settle down near the
outfall of sewage
► Helping self purification process.

• Oxidation:
► The oxidation of the organic matter present in
sewage effluents will start as soon as the sewage
outfalls into the river water containing dissolved
oxygen.
► Creates oxygen deficiency
► Filled up by atmospheric oxygen.
► The process of oxidation will continue till the
organic matter has been completely oxidized.

• Reduction:
► Occurs in the streams due to hydrolysis of the
organic matter biologically or chemically.
► Anaerobic bacteria splits the organic matter into
liquid and gases
► Helps stabilization by oxidation.

• Temperature:
► Low temperature organisms activities are slow down.
• Rate of decomposition slowdown.
► Reverse in warm temperature.
► Summer  less time for self purification
► Winter  more time but DO is more in cold water.

• Sunlight:
► Kills pathogens
► Helps self purification
► Algae grows in sunlight
• Production of oxygen
• Indirect help in oxidation and stabilisation.

Zones of pollution in a river stream
• Polluted stream undergoing self purification can be divided in to
four zones
► Zone of degradation
► Zone of active decomposition
► Zone of recovery
► Zone of clear water

Fig. 20-5, p. 536
Point source

• Zone of degradation:
► Certain length just below the point of sewage discharge.
► Water is dark, turbid
► Formation of sludge deposits at the bottom
► DO is reduced to 40% of the saturation value
► Increase in CO2 content

► Re-aeration is slower
► Unfavorable conditions for aquatic life.
► Algae dies out
► Some fish life may be present feeding on fresh organic matter
► Decomposition of solid matter takes place in this zone
► Anaerobic decomposition prevails.

• Zone of active decomposition:
► Just after degradation zone
► Heavy pollution
► Greyish and darker water
► DO concentration falls down to zero.
► Active anaerobic organic decomposition takes place
► Absent fish life

► Bacteria flora will flourish with the presence of anaerobic bacteria at
upper end and aerobic bacteria at the lower end.
► At the end of this zone
• decomposition slackens ,
• Re-aeration sets in and
• DO rises again to its original state.

• Zone of recovery
► Process of recovery starts
► Stabilization of organic matter takes place
► Most of the stabilized organic matter settles as sludge
► BOD falls and DO rises above 40%
► Organic matter gets mineralized to form nitrates, sulphates, carbonates
etc.
► Microscopic aquatic life reappears,
► fungi decreases and algae reappears.

• Zone of clear water:
► The river attains its original condition
► DO rising up to the saturation value
► Water becomes attractive in appearance
► Usual aquatic life prevails
► However
• Some pathogenic organisms may be present in this zone.

DO sag curve / Oxygen sag curve /
oxygen deficit curve

• Oxygen sag or oxygen deficit
in the stream at any point of
time during self purification
process is the difference
between the saturation DO
content and the actual DO
content at that time.
• Oxygen deficit (D) =
saturation DO – actual DO

• The normal saturation DO value for fresh water
depends upon temperature
► Varies from 14.62 mg/l at 0 C to 7.63 mg/l at
30 C
• In order to maintain clean conditions in a river
stream,
► The oxygen deficit (D) must be nil
► This can be found out by knowing the rates of
de-oxygenation and re-oxygenation.

• De-oxygenation curve:
► When pollution load is discharged in to stream, the DO content of the
stream goes on reducing due to decomposition of volatile organic
matter.
► This depletion of DO content is known as de-oxygenation.
► The rate of de-oxygenation depends upon the amount of organic
matter remaining (Lt) to be oxidized at any time (t) and temperature
(T) of the reaction.

• De-oxygenation curve:
► At a given temperature the curve showing depletion of
DO with time is known as de-oxygenation curve.

• Re-oxygenation curve:
► To Counter balance the consumption of DO due to de-oxygenation
• Atmosphere supplies oxygen to the water
► Process known as
• Re-oxygenation or re-aeration.
► Rate of re-oxygenation depends on
• Depth of receiving water (higher in shallow water)
• Condition of the water body (higher in running water)
• Saturation deficit or oxygen deficit
• Temperature of water

• Oxygen Deficit Curve
► In a running polluted stream exposed to the atmosphere,
► The de-oxygenation as well as re-oxygenation go hand in hand.
► If de-oxygenation is more rapid than the re-oxygenation
• An oxygen deficit results.

► If the DO content becomes zero,
• aerobic conditions will no longer be maintained and putrefaction will set
in.
► The amount of resultant oxygen deficit can be obtained by
algebraically adding the de-oxygenation and re-oxygenation curves.
► The resultant curve so obtained is called the DO curve or Oxygen
deficit curve or oxygen sag curve.

► When the de-oxygenation
rates exceeds re-
oxygenation rate, the
oxygen curve shows
increasing deficit of oxygen,
but when both the rates
become equal the critical
point is reached.
► Finally when the rate of
decomposition falls below
that of re-oxygenation, the
oxygen deficit goes on
decreasing till becoming
zero.

Important Equations for the Self purification of the stream
• BOD of diluted mixture (concentration)
• 𝐶 =
𝐶𝑆𝑄𝑆+𝐶𝑅𝑄𝑅
𝑄𝑆+𝑄𝑅
• Where
► C = BOD of diluted mixture
► Cs = BOD of sewage (mg/l)
► CR = BOD of river (mg/l)
► QS = Discharge of the sewage (l/sec)
► QR = Discharge of the river (l/sec)

End of just another session……..

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