Short Notes: Environmental Engineering - Module III | Short Notes for Civil Engineering - Civil Engineering (CE) PDF Download

 Page 1


Module III
Wastewater Quantity Estimation
The flow of sanitary sewage alone in the absence of storms in dry season is known as dry 
weather flow (DWF).
Quantity= Per capita sewage contributed per day x Population
Sanitary sewage is mostly the spent water of the community draining into the sewer system. It 
has been observed that a small portion of spent water is lost in evaporation, seepage in ground, 
leakage, etc. Usually 80% of the water supply may be expected to reach the sewers.
Fluctuations in Dry Weather Flow
Since dry weather flow depends on the quantity of water used, and as there are fluctuations in 
rate of water consumption, there will be fluctuations in dry weather flow also. In general, it can 
be assumed that (i) Maximum daily flow = 2 x average daily flow and (ii) Minimum daily flow = 
2/3 x (average daily flow).
Population Equivalent
Population equivalent is a parameter used in the conversion of contribution of wastes from 
industrial establishments for accepting into sanitary sewer systems. The strength of industrial 
sewage is, thus, written as
Std. BOD5 = (Std. BOD5 of domestic sewage per person per day) x (population equivalent) 
Design Periods & Population Forecast
This quantity should be worked out with due provision for the estimated requirements of the 
future. The future period for which a provision is made in the water supply scheme is known as 
the design period. It is suggested that the construction of sewage treatment plant may be 
carried out in phases with an initial design period ranging from 5 to 10 years excluding the 
construction period.
Design period is estimated based on the following:
Page 2


Module III
Wastewater Quantity Estimation
The flow of sanitary sewage alone in the absence of storms in dry season is known as dry 
weather flow (DWF).
Quantity= Per capita sewage contributed per day x Population
Sanitary sewage is mostly the spent water of the community draining into the sewer system. It 
has been observed that a small portion of spent water is lost in evaporation, seepage in ground, 
leakage, etc. Usually 80% of the water supply may be expected to reach the sewers.
Fluctuations in Dry Weather Flow
Since dry weather flow depends on the quantity of water used, and as there are fluctuations in 
rate of water consumption, there will be fluctuations in dry weather flow also. In general, it can 
be assumed that (i) Maximum daily flow = 2 x average daily flow and (ii) Minimum daily flow = 
2/3 x (average daily flow).
Population Equivalent
Population equivalent is a parameter used in the conversion of contribution of wastes from 
industrial establishments for accepting into sanitary sewer systems. The strength of industrial 
sewage is, thus, written as
Std. BOD5 = (Std. BOD5 of domestic sewage per person per day) x (population equivalent) 
Design Periods & Population Forecast
This quantity should be worked out with due provision for the estimated requirements of the 
future. The future period for which a provision is made in the water supply scheme is known as 
the design period. It is suggested that the construction of sewage treatment plant may be 
carried out in phases with an initial design period ranging from 5 to 10 years excluding the 
construction period.
Design period is estimated based on the following:
• Useful life of the component, considering obsolescence, wear, tears, etc.
• Expandability aspect.
• Anticipated rate of growth of population, including industrial, commercial developments & 
migration-immigration.
• Available resources.
• Performance of the system during initial period.
Population forecasting methods:
The various methods adopted for estimating future populations are given below. The particular 
method to be adopted for a particular case or for a particular city depends largely on the factors 
discussed in the methods, and the selection is left to the discretion and intelligence of the 
designer.
1 . Arithmetic Increase Method
2. Geometric Increase Method
3. Incremental Increase Method
4. Decreasing Rate of Growth Method
5. Simple Graphical Method
6. Comparative Graphical Method
7. Ratio Method
8. Logistic Curve Method
Wastewater Characterization
To design a treatment process properly, characterization of wastewater is perhaps the most 
critical step. Wastewater characteristics of importance in the design of the activated sludge 
process can be grouped into the following categories:
Temperature
pH
Colour and Odour 
Carbonaceous substrates 
Nitrogen 
Phosphorous 
Chlorides
Total and volatile suspended solids (TSS and VSS)
Toxic metals and compounds
Design of Sewers
The hydraulic design of sewers and drains, which means finding out their sections and 
gradients, is generally carried out on the same lines as that of the water supply pipes. However,
Page 3


Module III
Wastewater Quantity Estimation
The flow of sanitary sewage alone in the absence of storms in dry season is known as dry 
weather flow (DWF).
Quantity= Per capita sewage contributed per day x Population
Sanitary sewage is mostly the spent water of the community draining into the sewer system. It 
has been observed that a small portion of spent water is lost in evaporation, seepage in ground, 
leakage, etc. Usually 80% of the water supply may be expected to reach the sewers.
Fluctuations in Dry Weather Flow
Since dry weather flow depends on the quantity of water used, and as there are fluctuations in 
rate of water consumption, there will be fluctuations in dry weather flow also. In general, it can 
be assumed that (i) Maximum daily flow = 2 x average daily flow and (ii) Minimum daily flow = 
2/3 x (average daily flow).
Population Equivalent
Population equivalent is a parameter used in the conversion of contribution of wastes from 
industrial establishments for accepting into sanitary sewer systems. The strength of industrial 
sewage is, thus, written as
Std. BOD5 = (Std. BOD5 of domestic sewage per person per day) x (population equivalent) 
Design Periods & Population Forecast
This quantity should be worked out with due provision for the estimated requirements of the 
future. The future period for which a provision is made in the water supply scheme is known as 
the design period. It is suggested that the construction of sewage treatment plant may be 
carried out in phases with an initial design period ranging from 5 to 10 years excluding the 
construction period.
Design period is estimated based on the following:
• Useful life of the component, considering obsolescence, wear, tears, etc.
• Expandability aspect.
• Anticipated rate of growth of population, including industrial, commercial developments & 
migration-immigration.
• Available resources.
• Performance of the system during initial period.
Population forecasting methods:
The various methods adopted for estimating future populations are given below. The particular 
method to be adopted for a particular case or for a particular city depends largely on the factors 
discussed in the methods, and the selection is left to the discretion and intelligence of the 
designer.
1 . Arithmetic Increase Method
2. Geometric Increase Method
3. Incremental Increase Method
4. Decreasing Rate of Growth Method
5. Simple Graphical Method
6. Comparative Graphical Method
7. Ratio Method
8. Logistic Curve Method
Wastewater Characterization
To design a treatment process properly, characterization of wastewater is perhaps the most 
critical step. Wastewater characteristics of importance in the design of the activated sludge 
process can be grouped into the following categories:
Temperature
pH
Colour and Odour 
Carbonaceous substrates 
Nitrogen 
Phosphorous 
Chlorides
Total and volatile suspended solids (TSS and VSS)
Toxic metals and compounds
Design of Sewers
The hydraulic design of sewers and drains, which means finding out their sections and 
gradients, is generally carried out on the same lines as that of the water supply pipes. However,
there are two major differences between characteristics of flows in sewers and water supply 
pipes. They are:
• The sewage contain particles in suspension, the heavier of which may settle down at the 
bottom of the sewers, as and when the flow velocity reduces, resulting in the clogging of 
sewers. To avoid silting of sewers, it is necessary that the sewer pipes be laid at such a 
gradient, as to generate self cleansing velocities at different possible discharges.
• The sewer pipes carry sewage as gravity conduits, and are therefore laid at a continuous 
gradient in the downward direction upto the outfall point, from where it will be lifted up, 
treated and disposed of.
Hazen-William's formula; U=0.85 C rH 0 6 3S054
Manning's formula: U=1 /n rH 2/ 3S1/2
where, U= velocity, m/s; rH = hydraulic radius,m; S= slope, C= Hazen-William's coefficient, and n 
= Manning's coefficient.
Darcy-Weisbach formula: hL = (fLU2)/(2gd)
Minimum Velocity
The flow velocity in the sewers should be such that the suspended materials in sewage do not 
get silted up; i.e. the velocity should be such as to cause automatic self-cleansing effect. The 
generation of such a minimum self cleansing velocity in the sewer, atleast once a day, is 
important, because if certain deposition takes place and is not removed, it will obstruct free flow, 
causing further deposition and finally leading to the complete blocking of the sewer.
Maximum Velocity
The smooth interior surface of a sewer pipe gets scoured due to continuous abrasion caused by 
the suspended solids present in sewage. It is, therefore, necessary to limit the maximum 
velocity in the sewer pipe. This limiting or non-scouring velocity will mainly depend upon the 
material of the sewer.
Effects of Flow Variation on Velocity in a Sewer
Due to variation in discharge, the depth of flow varies, and hence the hydraulic mean depth (r) 
varies. Due to the change in the hydraulic mean depth, the flow velocity (which depends directly 
on r2 /3 ) gets affected from time to time. It is necessary to check the sewer for maintaining a 
minimum velocity of about 0.45 m/s at the time of minimum flow (assumed to be 1/3r d of average 
flow). The designer should also ensure that a velocity of 0.9 m/s is developed atleast at the time 
of maximum flow and preferably during the average flow periods also. Moreover, care should be 
taken to see that at the time of maximum flow, the velocity generated does not exceed the 
scouring value.
Page 4


Module III
Wastewater Quantity Estimation
The flow of sanitary sewage alone in the absence of storms in dry season is known as dry 
weather flow (DWF).
Quantity= Per capita sewage contributed per day x Population
Sanitary sewage is mostly the spent water of the community draining into the sewer system. It 
has been observed that a small portion of spent water is lost in evaporation, seepage in ground, 
leakage, etc. Usually 80% of the water supply may be expected to reach the sewers.
Fluctuations in Dry Weather Flow
Since dry weather flow depends on the quantity of water used, and as there are fluctuations in 
rate of water consumption, there will be fluctuations in dry weather flow also. In general, it can 
be assumed that (i) Maximum daily flow = 2 x average daily flow and (ii) Minimum daily flow = 
2/3 x (average daily flow).
Population Equivalent
Population equivalent is a parameter used in the conversion of contribution of wastes from 
industrial establishments for accepting into sanitary sewer systems. The strength of industrial 
sewage is, thus, written as
Std. BOD5 = (Std. BOD5 of domestic sewage per person per day) x (population equivalent) 
Design Periods & Population Forecast
This quantity should be worked out with due provision for the estimated requirements of the 
future. The future period for which a provision is made in the water supply scheme is known as 
the design period. It is suggested that the construction of sewage treatment plant may be 
carried out in phases with an initial design period ranging from 5 to 10 years excluding the 
construction period.
Design period is estimated based on the following:
• Useful life of the component, considering obsolescence, wear, tears, etc.
• Expandability aspect.
• Anticipated rate of growth of population, including industrial, commercial developments & 
migration-immigration.
• Available resources.
• Performance of the system during initial period.
Population forecasting methods:
The various methods adopted for estimating future populations are given below. The particular 
method to be adopted for a particular case or for a particular city depends largely on the factors 
discussed in the methods, and the selection is left to the discretion and intelligence of the 
designer.
1 . Arithmetic Increase Method
2. Geometric Increase Method
3. Incremental Increase Method
4. Decreasing Rate of Growth Method
5. Simple Graphical Method
6. Comparative Graphical Method
7. Ratio Method
8. Logistic Curve Method
Wastewater Characterization
To design a treatment process properly, characterization of wastewater is perhaps the most 
critical step. Wastewater characteristics of importance in the design of the activated sludge 
process can be grouped into the following categories:
Temperature
pH
Colour and Odour 
Carbonaceous substrates 
Nitrogen 
Phosphorous 
Chlorides
Total and volatile suspended solids (TSS and VSS)
Toxic metals and compounds
Design of Sewers
The hydraulic design of sewers and drains, which means finding out their sections and 
gradients, is generally carried out on the same lines as that of the water supply pipes. However,
there are two major differences between characteristics of flows in sewers and water supply 
pipes. They are:
• The sewage contain particles in suspension, the heavier of which may settle down at the 
bottom of the sewers, as and when the flow velocity reduces, resulting in the clogging of 
sewers. To avoid silting of sewers, it is necessary that the sewer pipes be laid at such a 
gradient, as to generate self cleansing velocities at different possible discharges.
• The sewer pipes carry sewage as gravity conduits, and are therefore laid at a continuous 
gradient in the downward direction upto the outfall point, from where it will be lifted up, 
treated and disposed of.
Hazen-William's formula; U=0.85 C rH 0 6 3S054
Manning's formula: U=1 /n rH 2/ 3S1/2
where, U= velocity, m/s; rH = hydraulic radius,m; S= slope, C= Hazen-William's coefficient, and n 
= Manning's coefficient.
Darcy-Weisbach formula: hL = (fLU2)/(2gd)
Minimum Velocity
The flow velocity in the sewers should be such that the suspended materials in sewage do not 
get silted up; i.e. the velocity should be such as to cause automatic self-cleansing effect. The 
generation of such a minimum self cleansing velocity in the sewer, atleast once a day, is 
important, because if certain deposition takes place and is not removed, it will obstruct free flow, 
causing further deposition and finally leading to the complete blocking of the sewer.
Maximum Velocity
The smooth interior surface of a sewer pipe gets scoured due to continuous abrasion caused by 
the suspended solids present in sewage. It is, therefore, necessary to limit the maximum 
velocity in the sewer pipe. This limiting or non-scouring velocity will mainly depend upon the 
material of the sewer.
Effects of Flow Variation on Velocity in a Sewer
Due to variation in discharge, the depth of flow varies, and hence the hydraulic mean depth (r) 
varies. Due to the change in the hydraulic mean depth, the flow velocity (which depends directly 
on r2 /3 ) gets affected from time to time. It is necessary to check the sewer for maintaining a 
minimum velocity of about 0.45 m/s at the time of minimum flow (assumed to be 1/3r d of average 
flow). The designer should also ensure that a velocity of 0.9 m/s is developed atleast at the time 
of maximum flow and preferably during the average flow periods also. Moreover, care should be 
taken to see that at the time of maximum flow, the velocity generated does not exceed the 
scouring value.
Sewer Appurtenances
Sewer appurtenances are the various accessories on the sewerage system and are necessary 
for the efficient operation of the system. They include man holes, lamp holes, street inlets, catch 
basins, inverted siphons, and so on.
Man-holes: Man holes are the openings of either circular or rectangular in shape constructed 
on the alignment of a sewer line to enable a person to enter the sewer for inspection, cleaning 
and flushing. They serve as ventilators for sewers, by the provisions of perforated man-hole 
covers. Also they facilitate the laying of sewer lines in convenient length.
Man-holes are provided at all junctions of two or more sewers, whenever diameter of sewer 
changes, whenever direction of sewer line changes and when sewers of different elevations join 
together.
Special Man-holes:
Junction chambers: Man-hole constructed at the intersection of two large sewers.
Drop man-hole: When the difference in elevation of the invert levels of the incoming and 
outgoing sewers of the man-hole is more than 60 cm, the interception is made by dropping the 
incoming sewer vertically outside and then it is jointed to the man-hole chamber.
Flushing man-holes: They are located at the head of a sewer to flush out the deposits in the 
sewer with water.
Lamp-holes: Lamp holes are the openings constructed on the straight sewer lines between two 
man-holes which are far apart and permit the insertion of a lamp into the sewer to find out 
obstructions if any inside the sewers from the next man-hole.
Street inlets: Street inlets are the openings through which storm water is admitted and 
conveyed to the storm sewer or combined sewer. The inlets are located by the sides of 
pavement with maximum spacing of 30 m.
Catch Basins: Catch basins are small settling chambers of diameter 60 - 90 cm and 60 - 75 cm 
deep, which are constructed below the street inlets. They interrupt the velocity of storm water 
entering through the inlets and allow grit, sand, debris and so on to settle in the basin, instead of 
allowing them to enter into the sewers.
Inverted siphons: These are depressed portions of sewers, which flow full under pressure 
more than the atmospheric pressure due to flow line being below the hydraulic grade line. They 
are constructed when a sewer crosses a stream or deep cut or road or railway line. To clean the 
siphon pipe sluice valve is opened, thus increasing the head causing flow. Due to increased 
velocity deposits of siphon pipe are washed into the sump, from where they are removed.
Page 5


Module III
Wastewater Quantity Estimation
The flow of sanitary sewage alone in the absence of storms in dry season is known as dry 
weather flow (DWF).
Quantity= Per capita sewage contributed per day x Population
Sanitary sewage is mostly the spent water of the community draining into the sewer system. It 
has been observed that a small portion of spent water is lost in evaporation, seepage in ground, 
leakage, etc. Usually 80% of the water supply may be expected to reach the sewers.
Fluctuations in Dry Weather Flow
Since dry weather flow depends on the quantity of water used, and as there are fluctuations in 
rate of water consumption, there will be fluctuations in dry weather flow also. In general, it can 
be assumed that (i) Maximum daily flow = 2 x average daily flow and (ii) Minimum daily flow = 
2/3 x (average daily flow).
Population Equivalent
Population equivalent is a parameter used in the conversion of contribution of wastes from 
industrial establishments for accepting into sanitary sewer systems. The strength of industrial 
sewage is, thus, written as
Std. BOD5 = (Std. BOD5 of domestic sewage per person per day) x (population equivalent) 
Design Periods & Population Forecast
This quantity should be worked out with due provision for the estimated requirements of the 
future. The future period for which a provision is made in the water supply scheme is known as 
the design period. It is suggested that the construction of sewage treatment plant may be 
carried out in phases with an initial design period ranging from 5 to 10 years excluding the 
construction period.
Design period is estimated based on the following:
• Useful life of the component, considering obsolescence, wear, tears, etc.
• Expandability aspect.
• Anticipated rate of growth of population, including industrial, commercial developments & 
migration-immigration.
• Available resources.
• Performance of the system during initial period.
Population forecasting methods:
The various methods adopted for estimating future populations are given below. The particular 
method to be adopted for a particular case or for a particular city depends largely on the factors 
discussed in the methods, and the selection is left to the discretion and intelligence of the 
designer.
1 . Arithmetic Increase Method
2. Geometric Increase Method
3. Incremental Increase Method
4. Decreasing Rate of Growth Method
5. Simple Graphical Method
6. Comparative Graphical Method
7. Ratio Method
8. Logistic Curve Method
Wastewater Characterization
To design a treatment process properly, characterization of wastewater is perhaps the most 
critical step. Wastewater characteristics of importance in the design of the activated sludge 
process can be grouped into the following categories:
Temperature
pH
Colour and Odour 
Carbonaceous substrates 
Nitrogen 
Phosphorous 
Chlorides
Total and volatile suspended solids (TSS and VSS)
Toxic metals and compounds
Design of Sewers
The hydraulic design of sewers and drains, which means finding out their sections and 
gradients, is generally carried out on the same lines as that of the water supply pipes. However,
there are two major differences between characteristics of flows in sewers and water supply 
pipes. They are:
• The sewage contain particles in suspension, the heavier of which may settle down at the 
bottom of the sewers, as and when the flow velocity reduces, resulting in the clogging of 
sewers. To avoid silting of sewers, it is necessary that the sewer pipes be laid at such a 
gradient, as to generate self cleansing velocities at different possible discharges.
• The sewer pipes carry sewage as gravity conduits, and are therefore laid at a continuous 
gradient in the downward direction upto the outfall point, from where it will be lifted up, 
treated and disposed of.
Hazen-William's formula; U=0.85 C rH 0 6 3S054
Manning's formula: U=1 /n rH 2/ 3S1/2
where, U= velocity, m/s; rH = hydraulic radius,m; S= slope, C= Hazen-William's coefficient, and n 
= Manning's coefficient.
Darcy-Weisbach formula: hL = (fLU2)/(2gd)
Minimum Velocity
The flow velocity in the sewers should be such that the suspended materials in sewage do not 
get silted up; i.e. the velocity should be such as to cause automatic self-cleansing effect. The 
generation of such a minimum self cleansing velocity in the sewer, atleast once a day, is 
important, because if certain deposition takes place and is not removed, it will obstruct free flow, 
causing further deposition and finally leading to the complete blocking of the sewer.
Maximum Velocity
The smooth interior surface of a sewer pipe gets scoured due to continuous abrasion caused by 
the suspended solids present in sewage. It is, therefore, necessary to limit the maximum 
velocity in the sewer pipe. This limiting or non-scouring velocity will mainly depend upon the 
material of the sewer.
Effects of Flow Variation on Velocity in a Sewer
Due to variation in discharge, the depth of flow varies, and hence the hydraulic mean depth (r) 
varies. Due to the change in the hydraulic mean depth, the flow velocity (which depends directly 
on r2 /3 ) gets affected from time to time. It is necessary to check the sewer for maintaining a 
minimum velocity of about 0.45 m/s at the time of minimum flow (assumed to be 1/3r d of average 
flow). The designer should also ensure that a velocity of 0.9 m/s is developed atleast at the time 
of maximum flow and preferably during the average flow periods also. Moreover, care should be 
taken to see that at the time of maximum flow, the velocity generated does not exceed the 
scouring value.
Sewer Appurtenances
Sewer appurtenances are the various accessories on the sewerage system and are necessary 
for the efficient operation of the system. They include man holes, lamp holes, street inlets, catch 
basins, inverted siphons, and so on.
Man-holes: Man holes are the openings of either circular or rectangular in shape constructed 
on the alignment of a sewer line to enable a person to enter the sewer for inspection, cleaning 
and flushing. They serve as ventilators for sewers, by the provisions of perforated man-hole 
covers. Also they facilitate the laying of sewer lines in convenient length.
Man-holes are provided at all junctions of two or more sewers, whenever diameter of sewer 
changes, whenever direction of sewer line changes and when sewers of different elevations join 
together.
Special Man-holes:
Junction chambers: Man-hole constructed at the intersection of two large sewers.
Drop man-hole: When the difference in elevation of the invert levels of the incoming and 
outgoing sewers of the man-hole is more than 60 cm, the interception is made by dropping the 
incoming sewer vertically outside and then it is jointed to the man-hole chamber.
Flushing man-holes: They are located at the head of a sewer to flush out the deposits in the 
sewer with water.
Lamp-holes: Lamp holes are the openings constructed on the straight sewer lines between two 
man-holes which are far apart and permit the insertion of a lamp into the sewer to find out 
obstructions if any inside the sewers from the next man-hole.
Street inlets: Street inlets are the openings through which storm water is admitted and 
conveyed to the storm sewer or combined sewer. The inlets are located by the sides of 
pavement with maximum spacing of 30 m.
Catch Basins: Catch basins are small settling chambers of diameter 60 - 90 cm and 60 - 75 cm 
deep, which are constructed below the street inlets. They interrupt the velocity of storm water 
entering through the inlets and allow grit, sand, debris and so on to settle in the basin, instead of 
allowing them to enter into the sewers.
Inverted siphons: These are depressed portions of sewers, which flow full under pressure 
more than the atmospheric pressure due to flow line being below the hydraulic grade line. They 
are constructed when a sewer crosses a stream or deep cut or road or railway line. To clean the 
siphon pipe sluice valve is opened, thus increasing the head causing flow. Due to increased 
velocity deposits of siphon pipe are washed into the sump, from where they are removed.
Pumping of Sewage
Pumping of sewage is required when it is not possible to have a gravitational flow for the entire 
sewerage project.
Sufficient pumping capacity has to be provided to meet the peak flow, atleast 50% as stand by. 
Types of pumps :
1. Centrifugal pumps either axial, mixed and radial flow.
2. Pneumatic ejector pumps.
The raw sewage must be treated before it is discharged into the river stream. The extent of 
treatment required to be given depends not only upon the characteristics and quality of the 
sewage but also upon the source of disposal, its quality and capacity to tolerate the impurities 
present in the sewage effluents without itself getting potentially polluted. The layout of 
conventional wastewater treatment plant is as follows:
Indian Standards for discharge of sewage in surface waters are given in the table below.
Indian Standards for Discharge of Sewage in Surface Waters
Characteristic of the Effluent
Tolerance limit for Discharge of Sewage in Suface 
Water Sources
BOD5 20 mg/L
TSS 30 mg/L
The unit operations and processes commonly employed in domestic wastewater treatment, their 
functions and units used to achieve these functions are given in the following table:
Unit Operations/Processes, Their Functions and Units Used for Domestic Wastewater 
Treatment