Environmental Engineering -II Unit I

Disposal of The Sewage Effluents
Environment Engineering -II

Syllabus
• Sewage Disposal Discharge of raw and treated
sewage on land and water, standards for
disposal, raw and treated sewage on land and
water, limit of dilution, self -purification of
streams, oxygen economy, sewage farming

Disposal of the Treated Sewage or
Sewage Effluent
• The disposal of sewage effluent is the last stage of getting rid of
sewage after subjecting it to various steps of processes (i.e.)
treatment of transforming the sewage into the harmless liquid
which fulfils the minimum standard of health and sanitation. The
main object of controlling disposal of sewage are
1) To render the sewage inoffensive
2) To save the aquatic life in streams
• To eliminate the danger of contamination of water supplies
• The amount or degree of treatment that should be given to the
sewage depends upon the source of its disposal as well as its
capacity to assimilate the impurities present in the sewage without
itself getting polluted or less useful. So before designing the
treatment plant first the source of disposal has to be selected.
• The best method of disposal is always difficult to decide. The
methods of disposal are classified into main groups as:

Natural Methods
• Natural methods- by which the treated on
untreated sewage is disposed off by:
• (i) dilution or disposal into water i.e. into sea,
lakes or rivers
• (ii) Disposal on land or land treatment i.e.
sewage farming and irrigation.

Artificial Methods
• Artificial methods by which the sewage is disposed off
only after subjecting it to various treatments ( primary
and secondary) such as:
• Screening and detritus removal
• Sedimentation with or without chemicals
• Biological treatment (trickling filter, oxidation pond or
activated sludge process)
• Now a days the actual practice is to use both the
methods, the sewage is first given the treatment and
then it is disposed off by any of the natural method. If
full treatment is not given at least the primary
treatment is given before disposal.

Disposal By Dilution
• Dilution is a predominant natural method of disposal. Disposal by
dilution is the process whereby the raw or treated (most treated)
sewage is discharged into water bodies such as rivers, lakes, sea
etc.. The discharge sewage, in due course of time, is purified by the
self purification process of the natural waters. The sewage is
mainly purified due to oxidation of organic matter by bacteria
using the DO present in the water bodies. The degree of the
treatment given to the raw sewage before disposing it in to any
river will depend not only upon thee quality of raw sewage but
also on the self purification capacity of river and the intended use
of its water at the down stream side.
• This method can be used only for the town or city which are
located near the river or sea or have a large lake which can be used
as a source of disposal.

Condition Favoring Disposal by
Dilution
• The Dilution method for disposing of the sewage can
favorable by adopted under the following conditions:
• When the sewage is comparatively fresh i.e. it is
discharged within 3-4 hours of its collection.
• When the floating matter and settlable solids have been
removed by primary treatment.
• When the diluting water has high DO content, so that
not only the BOD is satisfied, but sufficient DO remains
available for the aquatic life.

Condition Favoring Disposal by
Dilution
• Where the dilution waters are not used for the purpose of
navigation or water supply for at-least some reasonable
distance on the downstream from the point of sewage
disposal.
• Where flow current of the diluting waters are favorable,
causing no deposition or destruction to aquatic life. Its
means that swift forward currents are helpful, as they
easily carry away the sewage to the point of unlimited
dilution. On the other hand, slow back currents tend to
cause sedimentation, resulting in large sludge deposits.
• Where the wastewater does not contain industrial
wastewater having toxic substances.
• When the outfall sewer of the city or the treatment plant is
situated near some natural waters.

Standards Of Dilution For Discharge
Of Wastewater Into Rivers
• The bureau of Indian standards has therefore laid
down its guiding standards for sewage effluents, vide
4764-1973 and for industrial effluents vide IS 2490 -
1974
• These standards are the national guide lines for each
state pollution control board, using which they
prescribed their legally enforceable standards
depending upon the water quality and dilution
available in their respective surface water sources.
• When the industrial waste water are disposed of in to
public sewers, their quality should be control by using
the standards IS 3306-1974

Environmental Engineering -II Unit I

Types Of Receiving Waters For
Dilution
• The following are the types of receiving waters into which wastewater or
effluent can be discharged for dilution:
• Perennial rivers and streams
• Lakes
• Oceans or Sea
• Estuaries
• Creeks
• Perennial rivers or streams are probably the best type of receiving waters,
since the water is in continuous motion. Also in the natural streams there is
balance between plant and animal life, with considerable interaction
among the various life forms. However the discharge flowing during
summer and during winter varies. During summer, there may be
minimum flow in the stream, so the dilution factor may be low, and also
high temperature of water may result in low solubility of oxygen,
necessitating proper treatment before dilution.

Dilution
• Sometimes, especially when the perennial streams are not
available lakes may be used for dilution. Various
characteristics of lakes, such as its size, shape, volume of
fresh water flowing into it etc.. should be critically
examined before deciding the self purifying capacity.
• Ocean has abundant water so that the dilution factor is
unlimited. However, sea water has about 20 % less DO
than river or stream. The water is turbid due to dissolved
impurities and penetration of sun rays is less. Due to this
proper care has to be taken in dilution by sea otherwise
anaerobic conditions would occur resulting in forming of
sludge banks and emission of foul odour.

Dilution
• A creek is in the form of an inlet on sea coast,
which may not have dry weather flow during
some part of the year. Due to this, great care
should be taken in disposal of effluent in to it.
• Estuary is wide lower tidal part of the river.
Hence dilution in an estuary is affected both by
ocean water as well as river water. However, the
process of dilution is generally satisfactory in
estuaries.

Dilution

Self Purification of Natural Streams
• When the wastewater or effluents is discharged into
natural body of water, the receiving waters gets
polluted due to the waste products, present in the
sewage effluent. But the conditions do not remain so
for ever, because the natural forces of purification,
such as dilution, sedimentation, oxidation, reductions
etc.. go on acting upon the pollutants and bring back
the water into original condition. This automatic
purification of polluted water, in due course of time is
called the self purification. The organic matter
present in the sewage is broken down by bacteria
(present in the stream water to ammonia, nitrate,
sulphates, carbon dioxide etc..

• In this process the Dissolved Oxygen (DO) content of the
natural water decreases and a deficiency of DO is created.
The DO is replenished by the diffusion form the atmosphere
(re-aeration). The stable by product of oxidation mentioned
above are utilized by the plants and algae forms the food for
protozoa, rotifers, and crustaceans. They in turn becomes
food for minnows and tiny fish which become food for man.
The man discharges his waste back to stream. Thus the
natural cycle of self purification continues and the rivers
water regains its original quality. The complete self
purification is only possible if the concentration of pollutants
is less than the assimilative capacity of the river, it becomes
polluted and aquatic life gets affected as some minimum DO
is required for survival of fish.

The various forces of purification which help in effecting
self-purification process are summarized below:
Physical forces which includes
(i) Dilution
(ii) Dispersion
(iii) Sunlight (acts through bio-chemical reactions)
Chemical forces aided by biological forces (called
bio chemical forces) which includes
(i) Oxidation (Bio Oxidation)
(ii) Reduction

The various forces of purification which help in
effecting self-purification process are summarized
below:

These forces are described below:
• (i) Dilution:
• When the wastewater is discharged into large volume of
water contained in the river or stream, its gets diluted, due
to which the concentration of organic matter is reduced
and thus the potential nuisance of sewage is also reduced.
• If Cs and Cr are the concentrations of any impurities such
as organic content, BOD, suspended solids, dissolved
oxygen in the sewage and river having discharge Qs and
Qr respectively, the resulting concentration C of the
mixture is given by
• Cs Qs + Cr Qr = C (Qs + Qr)
C= Cs Qs + Cr Qr
(Qs + Qr)

Dispersion due to currents
• Self purification of stream largely depends upon
currents which will readily disperse the wastewater
in the stream, preventing local high concentration of
pollutants. High velocity also increases the re-
aeration which reduces the time of recovery,
through length of the stream affected by wastewater
is increased.

Sedimentation
• The settle able solids, if present in sewage
effluents, will settle down into bed of river,
near the outfall of sewage, thus helping in the
self purification process as the concentration
of sewage may get reduced.

Sunlight
• Sunlight acts as disinfectants and stimulates
the growth of algae (they produce their own
food by the process of photosynthesis) which
produce oxygen during daylight but utilizes
oxygen at night. Hence wherever there is
algal growth, the water may be saturated with
DO during day light hours, through
anaerobic conditions exist in night.

Oxidation
• The oxidation of organic matter present in the
sewage effluents will start as soon as the sewage
is discharged into the river. The anaerobic
bacteria will oxidize the organic matter utilizing
the dissolved oxygen of the stream. The
deficiency of oxygen so created, will be filled up
by the atmospheric oxygen. The process of
oxidation, will continue till the organic matter
has been completely oxidized. This is the most
important action responsible for effecting the self
purification of the rivers.

Reduction
• Reduction occurs due to hydrolysis of organic
matter settled at the bottom either chemically
or biologically. Anaerobic bacteria will help in
splitting the complex organic constituents of
sewage into liquids and gases, thus paving the
way for their ultimate stabilization by
oxidation.

Factors affecting self purification
capacity of the river or stream.
Self purification capacity of a river or a stream
depends on following factors
• Temperature
• Hydrographic factors such as the velocity and
surface expanse of the river or stream
• Rate of re-aeration
• Amount and type of organic matter
• Available initial DO
• Types of microorganisms present.

Factors affecting self purification
capacity of the river or stream

Temperature
• The rate of biological activity increases with increase in
temperature and decreases with decrease in temperature,
hence the rate of oxygen demand also increases with
increase in temperature and decrease in temperature.
• The dissolved oxygen content of water, which is very
essential for maintaining aquatic life and aerobic
conditions, is also influence by the temperature. The
solubility of oxygen decreases with increase in
temperature. The solubility of oxygen decreases with
increase in temperature. So at higher temperature the rate
of biological activities is high and the DO concentration is
low, causing rapid depletion of DO. This can results in a
development of anaerobic conditions and subsequent
nuisance.

Hydrographic factors like velocity and
surface expanse of the river:
• In a swiftly moving stream i.e. with high velocity the oxygen is
replenished quite rapidly. The water surface at the air liquid
interface is being continuously replaced by the turbulence of the
stream movement. The oxygen saturated layer is overturned and
replaced by an oxygen deficit layer. In this way the oxygen-
saturated liquid is rapidly mixed throughout the stream to supply
the microorganisms with sufficient oxygen. Too high velocity is
also not preferable because it will created lot of turbulence which
would scour the bottom sediments, increase turbidity and retard
algal growth.
• In a slow moving stream, the surface water is not being turned
over. The stream tends to flow in distinct layers. At the air liquid
interface the water is saturated with oxygen which can mix with
the oxygen deficit layers only by slow diffusion. The failure to
replace the oxygen being used in the lower regions of the stream
allows anaerobic conditions to developed.

surface expanse of the river:
• Rivers having large surface expense will also
have better reaeration as large surface area is
open to atmospheric diffusion of oxygen.
• Rate of re-aeration:
• The rate at which the DO deficiency is
replenished, will considerably govern the self
purification process. The greater is this rate, the
quickere is the self purification. As discuss earlier
re-aeration is related to temperature and
hydrographic characteristics of river.

surface expanse of the river

Amount and type of organic matter
• If the Waste contains large amount of
biodegradable matter its oxygen demand
would be very high. So the DO of the stream
depletes faster than the rate of re-aeration
and anaerobic conditions are developed. Thus
the self purification capacity of the stream is
affected.

Amount and type of organic matter

Available Initial DO
• Greater the concentration of DO at the time
of sewage discharge, the better and earlier the
self purification will occur.

Types of microorganisms present:
• If algae are present along with the bacteria
re-aeration would increase as algae carbon
dioxide and gives out oxygen. So oxygen gets
added by algae as well from the atmosphere,
thus increasing the DO concentration and
thereby increasing the self purification
capacity of the river.

Zones of pollution in a river or Stream
• A polluted stream undergoing self-
purification can be divided into following
four zones
• Zone of degradation
• Zone of active decomposition
• Zone of recovery
• Zone of clearer water
• These zones are explained below:

Zones of pollution in a river or Stream

Zone of Degradation
• This zone is found for a certain length just ahead of the
point where sewage is discharged into the river. This Zone
is characterized by water becoming dark and turbid with
formation of sludge deposits at the bottom. Bacteria will
start oxidation of the organic matter the DO is reduced to
about 40 % of the saturation valve 9 at 30 0C it is 7.6 mg/l
and at 20 0C it is 9.17 mg/l). There is an increase in carbon
dioxide content. Oxygenation occurs but is slower than
deoxygenating.
• These conditions are unfavorable to the development of
aquatic life; and as such, algae dies out, but certain type of
fish may be present feeding on fresh organic matter.
Moreover, certain typical bottom worms such as
limondrilus and Tubifex appear with sewage fungi.

Zone of Active Decomposition
• This zone is marked by heavy pollution. It is characterized by water
becoming grayish and darker than in the previous zone. DO
concentration falls down to zero, an anaerobic conditions may set
in with the evolution of gases like methane, carbon dioxide,
hydrogen sulphide, etc.. bubbling to the surface, with masses of
sludge forming ugly scum layer at the surface. As the organic
decomposition slackens due to stabilization of organic matter, rate
of deoxygenating becomes lesser than the re-aeration rate and DO
again rises to the original level (i.e. above 40 %)
• In this region, the bacteria flora will flourish. At the upper end,
anaerobic bacteria will replace aerobic bacteria, while at the lower
end, the position will replace aerobic bacteria, while at the lower
end the position will be reversed. Protozoa and fungi first
disappear then reappear. Fish life will be absent. Algae and tubifex
will also be absent. Maggots and Psychoda (Sewage fly) larvae will,
however, be present in all but the most septic sewage.

Zone of Recovery
• In this zone, the river stream tries to recover from its
degraded condition to its former appearance. The
water becomes clearer, and so the algae reappears
while fungi decreases. BOD decreases and DO
content rises above 40 % of the saturation value;
Protozoa, Rotifers, Crustaceans and large plants like
sponges, Bryozoans, etc.. The organic material will
be mineralised to form nitrate, phosphate,
carbonates, etc..

Zone of Clearer Water
• In this zone, the river attains its original
condition with DO rising up to the saturation
value. Water becomes attractive in
appearance, fish ( which requires at least 4 to
5 mg /l of DO) and usual aquatic life prevails.
Some pathogenic organisms may still,
however survive and remain present, which
confirms the fact that “ once the river water
has been polluted, it will not be safe for
drinking, unless it is properly treated.

Indices of Self Purification
• The stage of self purification process can be
determined by the physical, chemical, and
biological analysis of water.
• Physical Indices: Colour and turbidity
• Chemical Indices: DO, BOD, and suspended
solids.
• Biological indices: Different types of micro and
macro organisms ( Growth and presence of
different types of organism gives the idea of the
self purification process and zone of pollution).

The Oxygen Sag or Oxygen Deficit of
a Polluted River.
• The oxygen deficit D at any time in a polluted
river is the difference between the actual D.O.
Content at the water temperature
• Oxygen Deficit (D)= Saturation DO- Actual
DO
• In order to maintain clean conditions in a
river, the oxygen deficit must be nil, and this
can be found out by knowing the rates of de-
oxygenation and re-oxygenation.

The Oxygen Sag or Oxygen Deficit of
a Polluted River

De- Oxygenation Curve:
• In a polluted steam, the DO content goes on reducing
due to decomposition of volatile organic matter. The
rate of de-oxygenation depends upon the amount of
the organic matter remaining to be oxidized at the
given time (Lt) as well as the temperature of reaction
(T). Hence, at a given temperature, the curve showing
depletion of DO with time which is called de-
oxygenation curve is similar to the first stage BOD
Curve described earlier. It can also be mathematically
expressed by the first stage BOD equation
𝐿𝑡
𝐿
=
10−𝑘𝐷.𝑡

Re-Oxygenation Curve
• Oxygen which is consumed by the bacteria in oxidation of organic
matter is replenished from the atmosphere, this process is called
re-oxygenation. The rate at which the oxygen is supplied by the
atmosphere to the polluted water depends upon:
• Velocity of the stream and the resulting turbulence created, more
the velocity more is the rate.
• The depth of the receiving water, lesser is depth more is the rate.
• Temperature of water, rate decreases with increase in
temperature.
• The saturation deficit or the oxygen deficit below the saturation
valve.
• Depending upon these factors, the rate of de-oxygenation can be
expressed mathematically and plotted in the form of a curve called
re-oxygenation curve.

Oxygen Sag Curve or Oxygen Deficit
Curve
• In running polluted stream exposed to atmosphere, the
deoxygenation and re-oxygenation process occurs simultaneously.
If the de-oxygenation is more rapid than the re-oxygenation, an
oxygen deficit occurs.
• The amount of the resultant oxygen deficit can be obtained by
algebraically adding the de-oxygenation and re-oxygenation
curve. The resultant curve so obtained is called the oxygen sag
curve or oxygen deficit curve. From the curve, the oxygen deficit
D and oxygenation balance (100-D)% in a stream after a certain
lapse of time can be found out.
• It can be seen that when de-oxygenation rate exceeds the
reoxygenation rate, the oxygen sag curve shows increasing deficit
of oxygen; but when both rates become equal, the critical point is
reached, and finally when the rate of de-oxygenation falls below
that of re-oxygenation, the oxygen deficit goes on decreasing till
becoming zero.

Oxygen Sag Curve or Oxygen Deficit
Curve

Disposal of Wastewaters in Lake
• Disposal of wastewater in confined in much more harmful
than its disposal in flowing streams and rivers. Water
quality management is entirely different from that in
rivers. River is a flowing water body while lake has
stagnant waters, so in lakes only top surface would become
saturated with DO, but the bottom layers would not have
enough oxygen. Overturning of layers would not occur
frequently, so that DO content would not be uniform
through out the depth of lake. Overturning takes place
only when there is change in the season due to which there
will be temperature difference between water in different
layers which cause change in the densities of different
layers and overturning occurs.

Disposal of Wastewaters in Lake

Lake Pollutants
• In Lake the phosphorous a nutrient largely
contained in industrial domestic wastewater
is seriously affecting the quality of lakes and
hence it is considered as the prime lake
pollutant. Oxygen demanding wastes may be
the other important lake pollutant. The toxic
chemicals from industrial wastewater can be
present.

Eutrophication
• Phosphorous acts as the nutrients for the algal
growth. Increase in the phosphorous content
would increase the algal growth. Excessive in
the phosphorous content would increase the
algal growth. Excessive algal growth (algal
Bloom) will create lot of the problems like
taste, odour, problems in oxygen diffusion in
lower layer. Ultimately the entire lake can get
covered with algae and it may become useless
for other organisms

Disposal of Wastewater in Sea or Ocean
• The saturation concentration of dissolved oxygen in water decreases
with increasing salt concentration of DO in sea water is
approximately 80 % of that in water. In addition to this deficiency, the
temperature of sea water is lower than the sewage temperature,
whereas the specific gravity is higher.. Due to this reasons, when the
sewage is discharged into the sea water, the lighter and warmer
sewage will rise up to the surface, resulting in lighter and warmer
sewage will rise up to the surface, resulting in spreading of the sewage
at the top surface of sea in a thin film or ‘sleek’ Moreover seawater
contains a large amount of dissolved matter which chemically reacts
with the sewage solids, resulting in the precipitation of some of the
sewage solids, giving a milky appearance to the sea water and
resulting in formation of sludge banks and thin milky layer formed at
the top of sea water produce offensive hydrogen sulphide gas by
reacting with sulphate rich sea water. As such the capacity of seawater
to absorb sewage solids is not as high as that of fresh water of a river.
Also the DO content is less. However, since the sea contains large
volume of water, most of these deficiencies can be overcome if the
sewage is discharged deep into the sea, much away from the coast line,
with extreme care.

Disposal of Wastewater in Sea or
Ocean

Following points should be kept in mind while
discharging sewage into the sea.
• The sewage should be discharged in deep sea water.
• In order to mix sewage properly with the seawater, the
sewage should be released at a minimum depth of 3 to 5
m below the water level and taking it sufficiently inside
the shore, and thus preventing nuisance to baths and
recreation centres on the shore.
• To prevent the backing up and spreading of sewage on
the sea shore, the sewage should be disposed of only
during low tides, large sized tanks may, therefore be
constructed to hold the sewage during high tides.
Provision of a large sized sewer, grated with non return
valve at the end, is also an alternative to hold the sewage
during high tides.

Following points should be kept in mind while
discharging sewage into the sea.
• Before deciding the position of the outfall point, the sea current,
wind directions, velocity, etc., should be properly studied. The sea
currents, wind direction, velocity, etc.. should be properly studied.
The point of disposal should be such that the sewage is taken away
from the shore by the winds, and not brought back near the shore
by the winds, and not brought back near the shore.
• The outfall sewer should be placed on a firm rocky foundation, and
encased in thick stone masonry, so as to properly protect it from
wave action, floating debris, etc..
• The discharge of Industrial waste waters into sea should however,
be controlled in respect of the quality of the effluents, by adhering
to the following Indian standards., prescribed by IS 1968-1976.

Disposal of Sewage on land
• In this method, the sewage effluent either treated or raw is
disposed of by applying it on land. The most common forms of land
application are irrigation (Sewage farming) and rapid infiltration.
• When raw or partly treated sewage is applied on the land, a part of
it evaporates and remaining portion percolates in the soil. If proper
voids are maintained in the soil , the organic sewage solids are
oxidized by the bacteria present in the soil under aerobic
condition.
• However, if the soil is made of heavy, sticky, and fine grained
materials, the void space will soon get clogged resulting in non
aeration of these voids which would lead to anaerobic condition
and subsequent evolution of foul gases.
• Application of too strong or too hard of sewage will also result in
the quick formation of anaerobic conditions. The loads of sewage
can be reduced by dilution or pretreatment.

Sewage Farming
• In irrigation methods sewage is used for
irrigation the crop. This method in addition to
disposing the sewage, may help in increasing
crop yields as the sewage generally contains
lot of nutrients however the sewage effluent
must be made safe before using it for
irrigation.

Sewage Sickness
• When the sewage is applied continuously on a piece of
land, the soil pores or voids may get filled up and
clogged with sewage matter retained in them. The
time taken for such clogging will depend upon the
type of the soil and the load present in sewage. But
once these voids gets clogged, free circulation of air
will be prevented an anaerobic conditions will develop
within the pores. Decomposition of organic matter
would take place under anaerobic conditions with
evolution of foul gases like H2S, CO2 and methane.
This phenomenon of soil getting clogged, is known as
sewage sickness.

Methods of Preventing Sewage
Sickness
Primary Treatment of Sewage
• The sewage should be disposed of only after primary treatment,
such as screening, grit removal and sedimentation. This will help in
removing settle able solids and reducing the BOD load by 30 % or
so and soil will not get clogged frequently
Choice of Land
• The piece of land used for sewage disposal should normally be
sandy or loamy. Clayey lands should be avoided.
Under drainage of soil
The land, on which the sewage is being disposed of can be drained,
if a system of under drains is laid below, to collect the effluents
This will also minimize the possibility of sewage sickness.

Methods of Preventing Sewage
Sickness
Giving rest to the land
• The land being used for disposal should be given rest,
periodically by keeping some extra land as reserve and
standby for diverting the sewage during the rest
period. Moreover, during the rest period, the land
should be thoroughly ploughed, so that it gets broken
up and aerated.
Rotation of crops
• Sewage sickness can be reduced by planting different
crops This will help in utilizing the fertilizing elements
of sewage and help in aeration of soil.

References
Water Supply Engineering : By Prof S.K. Garg
Khanna Publishers
Environmental Engineering : By Prof B.R.Shah
Prof A M Malek
Mahajan Publishing House
Internet Websites

Environmental Engineering -II Unit I

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