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

 Page 1


Module II
The typical functions of each unit operations are given in the following table: 
Functions of Water Treatment Units
Unit treatment Function (removal)
Aeration, chemicals 
use
Colour, Odour, Taste
Screening Floating matter
Chemical methods Iron, Manganese, etc.
Softening Hardness
Sedimentation Suspended matter
Coagulation Suspended matter, a part of colloidal matter and bacteria
Filtration Remaining colloidal dissolved matter, bacteria
Disinfection
Pathogenic bacteria, Organic matter and Reducing 
substances
The types of treatment required for different sources are given in the following table:
Source Treatment required
1. Ground water and spring water fairly free from 
contamination
No treatment or Chlorination
2. Ground water with chemicals, minerals and 
gases
Aeration, coagulation (if 
necessary), filtration and 
disinfection
3. Lakes, surface water reservoirs with less 
amount of pollution
Disinfection
4. Other surface waters such as rivers, canals 
and impounded reservoirs with a considerable 
amount of pollution
Complete treatment
Aeration
• Aeration removes odour and tastes due to volatile gases like hydrogen sulphide and due 
to algae and related organisms.
• Aeration also oxidise iron and manganese, increases dissolved oxygen content in water, 
removes CO2 and reduces corrosion and removes methane and other flammable gases.
Page 2


Module II
The typical functions of each unit operations are given in the following table: 
Functions of Water Treatment Units
Unit treatment Function (removal)
Aeration, chemicals 
use
Colour, Odour, Taste
Screening Floating matter
Chemical methods Iron, Manganese, etc.
Softening Hardness
Sedimentation Suspended matter
Coagulation Suspended matter, a part of colloidal matter and bacteria
Filtration Remaining colloidal dissolved matter, bacteria
Disinfection
Pathogenic bacteria, Organic matter and Reducing 
substances
The types of treatment required for different sources are given in the following table:
Source Treatment required
1. Ground water and spring water fairly free from 
contamination
No treatment or Chlorination
2. Ground water with chemicals, minerals and 
gases
Aeration, coagulation (if 
necessary), filtration and 
disinfection
3. Lakes, surface water reservoirs with less 
amount of pollution
Disinfection
4. Other surface waters such as rivers, canals 
and impounded reservoirs with a considerable 
amount of pollution
Complete treatment
Aeration
• Aeration removes odour and tastes due to volatile gases like hydrogen sulphide and due 
to algae and related organisms.
• Aeration also oxidise iron and manganese, increases dissolved oxygen content in water, 
removes CO2 and reduces corrosion and removes methane and other flammable gases.
• Principle of treatment underlines on the fact that volatile gases in water escape into 
atmosphere from the air-water interface and atmospheric oxygen takes their place in 
water, provided the water body can expose itself over a vast surface to the atmosphere. 
This process continues until an equilibrium is reached depending on the partial pressure 
of each specific gas in the atmosphere.
Types of Aerators
1. Gravity aerators
2. Fountain aerators
3. Diffused aerators
4. Mechanical aerators.
Gravity Aerators (Cascades): In gravity aerators, water is allowed to fall by gravity such that a 
large area of water is exposed to atmosphere, sometimes aided by turbulence.
Fountain Aerators: These are also known as spray aerators with special nozzles to produce a 
fine spray. Each nozzle is 2.5 to 4 cm diameter discharging about 18 to 36 l/h. Nozzle spacing 
should be such that each m3 of water has aerator area of 0.03 to 0.09 m2 for one hour.
Injection or Diffused Aerators: It consists of a tank with perforated pipes, tubes or diffuser 
plates, fixed at the bottom to release fine air bubbles from compressor unit. The tank depth is 
kept as 3 to 4 m and tank width is within 1.5 times its depth. If depth is more, the diffusers must 
be placed at 3 to 4 m depth below water surface. Time of aeration is 10 to 30 min and 0.2 to 0.4 
litres of air is required for 1 litre of water.
Mechanical Aerators: Mixing paddles as in flocculation are used. Paddles may be either 
submerged or at the surface.
Settling
Solid liquid separation process in which a suspension is separated into two phases -
• Clarified supernatant leaving the top of the sedimentation tank (overflow).
• Concentrated sludge leaving the bottom of the sedimentation tank (underflow).
Purpose of Settling
• To remove coarse dispersed phase.
• To remove coagulated and flocculated impurities.
• To remove precipitated impurities after chemical treatment.
• To settle the sludge (biomass) after activated sludge process / tricking filters.
Principle of Settling
• Suspended solids present in water having specific gravity greater than that of water tend 
to settle down by gravity as soon as the turbulence is retarded by offering storage.
• Basin in which the flow is retarded is called settling tank.
• Theoretical average time for which the water is detained in the settling tank is called the 
detention period.
Page 3


Module II
The typical functions of each unit operations are given in the following table: 
Functions of Water Treatment Units
Unit treatment Function (removal)
Aeration, chemicals 
use
Colour, Odour, Taste
Screening Floating matter
Chemical methods Iron, Manganese, etc.
Softening Hardness
Sedimentation Suspended matter
Coagulation Suspended matter, a part of colloidal matter and bacteria
Filtration Remaining colloidal dissolved matter, bacteria
Disinfection
Pathogenic bacteria, Organic matter and Reducing 
substances
The types of treatment required for different sources are given in the following table:
Source Treatment required
1. Ground water and spring water fairly free from 
contamination
No treatment or Chlorination
2. Ground water with chemicals, minerals and 
gases
Aeration, coagulation (if 
necessary), filtration and 
disinfection
3. Lakes, surface water reservoirs with less 
amount of pollution
Disinfection
4. Other surface waters such as rivers, canals 
and impounded reservoirs with a considerable 
amount of pollution
Complete treatment
Aeration
• Aeration removes odour and tastes due to volatile gases like hydrogen sulphide and due 
to algae and related organisms.
• Aeration also oxidise iron and manganese, increases dissolved oxygen content in water, 
removes CO2 and reduces corrosion and removes methane and other flammable gases.
• Principle of treatment underlines on the fact that volatile gases in water escape into 
atmosphere from the air-water interface and atmospheric oxygen takes their place in 
water, provided the water body can expose itself over a vast surface to the atmosphere. 
This process continues until an equilibrium is reached depending on the partial pressure 
of each specific gas in the atmosphere.
Types of Aerators
1. Gravity aerators
2. Fountain aerators
3. Diffused aerators
4. Mechanical aerators.
Gravity Aerators (Cascades): In gravity aerators, water is allowed to fall by gravity such that a 
large area of water is exposed to atmosphere, sometimes aided by turbulence.
Fountain Aerators: These are also known as spray aerators with special nozzles to produce a 
fine spray. Each nozzle is 2.5 to 4 cm diameter discharging about 18 to 36 l/h. Nozzle spacing 
should be such that each m3 of water has aerator area of 0.03 to 0.09 m2 for one hour.
Injection or Diffused Aerators: It consists of a tank with perforated pipes, tubes or diffuser 
plates, fixed at the bottom to release fine air bubbles from compressor unit. The tank depth is 
kept as 3 to 4 m and tank width is within 1.5 times its depth. If depth is more, the diffusers must 
be placed at 3 to 4 m depth below water surface. Time of aeration is 10 to 30 min and 0.2 to 0.4 
litres of air is required for 1 litre of water.
Mechanical Aerators: Mixing paddles as in flocculation are used. Paddles may be either 
submerged or at the surface.
Settling
Solid liquid separation process in which a suspension is separated into two phases -
• Clarified supernatant leaving the top of the sedimentation tank (overflow).
• Concentrated sludge leaving the bottom of the sedimentation tank (underflow).
Purpose of Settling
• To remove coarse dispersed phase.
• To remove coagulated and flocculated impurities.
• To remove precipitated impurities after chemical treatment.
• To settle the sludge (biomass) after activated sludge process / tricking filters.
Principle of Settling
• Suspended solids present in water having specific gravity greater than that of water tend 
to settle down by gravity as soon as the turbulence is retarded by offering storage.
• Basin in which the flow is retarded is called settling tank.
• Theoretical average time for which the water is detained in the settling tank is called the 
detention period.
Types of Settling
Type I: Discrete particle settling - Particles settle individually without interaction with 
neighboring particles.
Type II: Flocculent Particles - Flocculation causes the particles to increase in mass and settle 
at a faster rate.
Type III: Hindered or Zone settling -The mass of particles tends to settle as a unit with 
individual particles remaining in fixed positions with respect to each other. 
Type IV: Compression - The concentration of particles is so high that sedimentation can only 
occur through compaction of the structure.
Type I Settling
• Size, shape and specific gravity of the particles do not change with time.
• Settling velocity remains constant.
If a particle is suspended in water, it initially has two forces acting upon it: 
force of gravity: Fg =pp gVp
Buoyant force quantified by Archimedes as: Fb =pgVp
If the density of the particle differs from that of the water, a net force is exerted and the particle 
is accelaratd in the direction of the force: Fn e t=(pp -p)gVp 
This net force becomes the driving force.
Once the motion has been initiated, a third force is created due to viscous friction. This force, 
called the drag force, is quantified by: Fd =CD Ap pv2 /2 
CD = drag coefficient.
Ap = projected area of the particle.
Because the drag force acts in the opposite direction to the driving force and increases as the 
square of the velocity, accelaration occurs at a decreasing rate until a steady velocity is reached 
at a point where the drag force equals the driving 
force:
(pp- p )gVp = Cd Ap pv2 /2
For spherical particles,
Vp =nd3 /6 and Ap = n d2 /4
Thus, v2 = 4g(Pp - p )d 
3CdP
Expressions for CD change with characteristics of different flow regimes. For laminar, transition, 
and turbulent flow, the values of C D are:
CD = 24 (laminar) 
R e
Cd= 24 +_3
Re Re1 /2
+0.34 (transition)
Page 4


Module II
The typical functions of each unit operations are given in the following table: 
Functions of Water Treatment Units
Unit treatment Function (removal)
Aeration, chemicals 
use
Colour, Odour, Taste
Screening Floating matter
Chemical methods Iron, Manganese, etc.
Softening Hardness
Sedimentation Suspended matter
Coagulation Suspended matter, a part of colloidal matter and bacteria
Filtration Remaining colloidal dissolved matter, bacteria
Disinfection
Pathogenic bacteria, Organic matter and Reducing 
substances
The types of treatment required for different sources are given in the following table:
Source Treatment required
1. Ground water and spring water fairly free from 
contamination
No treatment or Chlorination
2. Ground water with chemicals, minerals and 
gases
Aeration, coagulation (if 
necessary), filtration and 
disinfection
3. Lakes, surface water reservoirs with less 
amount of pollution
Disinfection
4. Other surface waters such as rivers, canals 
and impounded reservoirs with a considerable 
amount of pollution
Complete treatment
Aeration
• Aeration removes odour and tastes due to volatile gases like hydrogen sulphide and due 
to algae and related organisms.
• Aeration also oxidise iron and manganese, increases dissolved oxygen content in water, 
removes CO2 and reduces corrosion and removes methane and other flammable gases.
• Principle of treatment underlines on the fact that volatile gases in water escape into 
atmosphere from the air-water interface and atmospheric oxygen takes their place in 
water, provided the water body can expose itself over a vast surface to the atmosphere. 
This process continues until an equilibrium is reached depending on the partial pressure 
of each specific gas in the atmosphere.
Types of Aerators
1. Gravity aerators
2. Fountain aerators
3. Diffused aerators
4. Mechanical aerators.
Gravity Aerators (Cascades): In gravity aerators, water is allowed to fall by gravity such that a 
large area of water is exposed to atmosphere, sometimes aided by turbulence.
Fountain Aerators: These are also known as spray aerators with special nozzles to produce a 
fine spray. Each nozzle is 2.5 to 4 cm diameter discharging about 18 to 36 l/h. Nozzle spacing 
should be such that each m3 of water has aerator area of 0.03 to 0.09 m2 for one hour.
Injection or Diffused Aerators: It consists of a tank with perforated pipes, tubes or diffuser 
plates, fixed at the bottom to release fine air bubbles from compressor unit. The tank depth is 
kept as 3 to 4 m and tank width is within 1.5 times its depth. If depth is more, the diffusers must 
be placed at 3 to 4 m depth below water surface. Time of aeration is 10 to 30 min and 0.2 to 0.4 
litres of air is required for 1 litre of water.
Mechanical Aerators: Mixing paddles as in flocculation are used. Paddles may be either 
submerged or at the surface.
Settling
Solid liquid separation process in which a suspension is separated into two phases -
• Clarified supernatant leaving the top of the sedimentation tank (overflow).
• Concentrated sludge leaving the bottom of the sedimentation tank (underflow).
Purpose of Settling
• To remove coarse dispersed phase.
• To remove coagulated and flocculated impurities.
• To remove precipitated impurities after chemical treatment.
• To settle the sludge (biomass) after activated sludge process / tricking filters.
Principle of Settling
• Suspended solids present in water having specific gravity greater than that of water tend 
to settle down by gravity as soon as the turbulence is retarded by offering storage.
• Basin in which the flow is retarded is called settling tank.
• Theoretical average time for which the water is detained in the settling tank is called the 
detention period.
Types of Settling
Type I: Discrete particle settling - Particles settle individually without interaction with 
neighboring particles.
Type II: Flocculent Particles - Flocculation causes the particles to increase in mass and settle 
at a faster rate.
Type III: Hindered or Zone settling -The mass of particles tends to settle as a unit with 
individual particles remaining in fixed positions with respect to each other. 
Type IV: Compression - The concentration of particles is so high that sedimentation can only 
occur through compaction of the structure.
Type I Settling
• Size, shape and specific gravity of the particles do not change with time.
• Settling velocity remains constant.
If a particle is suspended in water, it initially has two forces acting upon it: 
force of gravity: Fg =pp gVp
Buoyant force quantified by Archimedes as: Fb =pgVp
If the density of the particle differs from that of the water, a net force is exerted and the particle 
is accelaratd in the direction of the force: Fn e t=(pp -p)gVp 
This net force becomes the driving force.
Once the motion has been initiated, a third force is created due to viscous friction. This force, 
called the drag force, is quantified by: Fd =CD Ap pv2 /2 
CD = drag coefficient.
Ap = projected area of the particle.
Because the drag force acts in the opposite direction to the driving force and increases as the 
square of the velocity, accelaration occurs at a decreasing rate until a steady velocity is reached 
at a point where the drag force equals the driving 
force:
(pp- p )gVp = Cd Ap pv2 /2
For spherical particles,
Vp =nd3 /6 and Ap = n d2 /4
Thus, v2 = 4g(Pp - p )d 
3CdP
Expressions for CD change with characteristics of different flow regimes. For laminar, transition, 
and turbulent flow, the values of C D are:
CD = 24 (laminar) 
R e
Cd= 24 +_3
Re Re1 /2
+0.34 (transition)
CD = 0.4 (turbulent)
where Re is the Reynolds number:
Re = p vd 
M
Reynolds number less than 1.0 indicate laminar flow, while values greater than 10 indicate 
turbulent flow. Intermediate values indicate transitional flow.
Stokes Flow
For laminar flow, terminal settling velocity equation becomes:
v= (p_p- pJad2 
18m
which is known as the stokes equation.
Transition Flow
Need to solve non-linear equations:
v2 = 4g(pp - p _ )d _ 
3Cd p
Cd= 24 +_3_ +0.34
R e R e
1 /2
Re= p _vd
M
• Calculate velocity using Stokes law or turbulent expression.
• Calculate and check Reynolds number.
• Calculate CD .
• Use general formula.
• Repeat from step 2 until convergence.
Types of Settling Tanks
• Sedimentation tanks may function either intermittently or continuously. The intermittent 
tanks also called quiescent type tanks are those which store water for a certain period 
and keep it in complete rest. In a continuous flow type tank, the flow velocity is only 
reduced and the water is not brought to complete rest as is done in an intermittent type.
• Settling basins may be either long rectangular or circular in plan. Long narrow 
rectangular tanks with horizontal flow are generally preferred to the circular tanks with 
radial or spiral flow.
Long Rectangular Settling Basin
• Long rectangular basins are hydraulically more stable, and flow control for large volumes 
is easier with this configuration.
• A typical long rectangular tank have length ranging from 2 to 4 times their width. The 
bottom is slightly sloped to facilitate sludge scraping. A slow moving mechanical sludge
Page 5


Module II
The typical functions of each unit operations are given in the following table: 
Functions of Water Treatment Units
Unit treatment Function (removal)
Aeration, chemicals 
use
Colour, Odour, Taste
Screening Floating matter
Chemical methods Iron, Manganese, etc.
Softening Hardness
Sedimentation Suspended matter
Coagulation Suspended matter, a part of colloidal matter and bacteria
Filtration Remaining colloidal dissolved matter, bacteria
Disinfection
Pathogenic bacteria, Organic matter and Reducing 
substances
The types of treatment required for different sources are given in the following table:
Source Treatment required
1. Ground water and spring water fairly free from 
contamination
No treatment or Chlorination
2. Ground water with chemicals, minerals and 
gases
Aeration, coagulation (if 
necessary), filtration and 
disinfection
3. Lakes, surface water reservoirs with less 
amount of pollution
Disinfection
4. Other surface waters such as rivers, canals 
and impounded reservoirs with a considerable 
amount of pollution
Complete treatment
Aeration
• Aeration removes odour and tastes due to volatile gases like hydrogen sulphide and due 
to algae and related organisms.
• Aeration also oxidise iron and manganese, increases dissolved oxygen content in water, 
removes CO2 and reduces corrosion and removes methane and other flammable gases.
• Principle of treatment underlines on the fact that volatile gases in water escape into 
atmosphere from the air-water interface and atmospheric oxygen takes their place in 
water, provided the water body can expose itself over a vast surface to the atmosphere. 
This process continues until an equilibrium is reached depending on the partial pressure 
of each specific gas in the atmosphere.
Types of Aerators
1. Gravity aerators
2. Fountain aerators
3. Diffused aerators
4. Mechanical aerators.
Gravity Aerators (Cascades): In gravity aerators, water is allowed to fall by gravity such that a 
large area of water is exposed to atmosphere, sometimes aided by turbulence.
Fountain Aerators: These are also known as spray aerators with special nozzles to produce a 
fine spray. Each nozzle is 2.5 to 4 cm diameter discharging about 18 to 36 l/h. Nozzle spacing 
should be such that each m3 of water has aerator area of 0.03 to 0.09 m2 for one hour.
Injection or Diffused Aerators: It consists of a tank with perforated pipes, tubes or diffuser 
plates, fixed at the bottom to release fine air bubbles from compressor unit. The tank depth is 
kept as 3 to 4 m and tank width is within 1.5 times its depth. If depth is more, the diffusers must 
be placed at 3 to 4 m depth below water surface. Time of aeration is 10 to 30 min and 0.2 to 0.4 
litres of air is required for 1 litre of water.
Mechanical Aerators: Mixing paddles as in flocculation are used. Paddles may be either 
submerged or at the surface.
Settling
Solid liquid separation process in which a suspension is separated into two phases -
• Clarified supernatant leaving the top of the sedimentation tank (overflow).
• Concentrated sludge leaving the bottom of the sedimentation tank (underflow).
Purpose of Settling
• To remove coarse dispersed phase.
• To remove coagulated and flocculated impurities.
• To remove precipitated impurities after chemical treatment.
• To settle the sludge (biomass) after activated sludge process / tricking filters.
Principle of Settling
• Suspended solids present in water having specific gravity greater than that of water tend 
to settle down by gravity as soon as the turbulence is retarded by offering storage.
• Basin in which the flow is retarded is called settling tank.
• Theoretical average time for which the water is detained in the settling tank is called the 
detention period.
Types of Settling
Type I: Discrete particle settling - Particles settle individually without interaction with 
neighboring particles.
Type II: Flocculent Particles - Flocculation causes the particles to increase in mass and settle 
at a faster rate.
Type III: Hindered or Zone settling -The mass of particles tends to settle as a unit with 
individual particles remaining in fixed positions with respect to each other. 
Type IV: Compression - The concentration of particles is so high that sedimentation can only 
occur through compaction of the structure.
Type I Settling
• Size, shape and specific gravity of the particles do not change with time.
• Settling velocity remains constant.
If a particle is suspended in water, it initially has two forces acting upon it: 
force of gravity: Fg =pp gVp
Buoyant force quantified by Archimedes as: Fb =pgVp
If the density of the particle differs from that of the water, a net force is exerted and the particle 
is accelaratd in the direction of the force: Fn e t=(pp -p)gVp 
This net force becomes the driving force.
Once the motion has been initiated, a third force is created due to viscous friction. This force, 
called the drag force, is quantified by: Fd =CD Ap pv2 /2 
CD = drag coefficient.
Ap = projected area of the particle.
Because the drag force acts in the opposite direction to the driving force and increases as the 
square of the velocity, accelaration occurs at a decreasing rate until a steady velocity is reached 
at a point where the drag force equals the driving 
force:
(pp- p )gVp = Cd Ap pv2 /2
For spherical particles,
Vp =nd3 /6 and Ap = n d2 /4
Thus, v2 = 4g(Pp - p )d 
3CdP
Expressions for CD change with characteristics of different flow regimes. For laminar, transition, 
and turbulent flow, the values of C D are:
CD = 24 (laminar) 
R e
Cd= 24 +_3
Re Re1 /2
+0.34 (transition)
CD = 0.4 (turbulent)
where Re is the Reynolds number:
Re = p vd 
M
Reynolds number less than 1.0 indicate laminar flow, while values greater than 10 indicate 
turbulent flow. Intermediate values indicate transitional flow.
Stokes Flow
For laminar flow, terminal settling velocity equation becomes:
v= (p_p- pJad2 
18m
which is known as the stokes equation.
Transition Flow
Need to solve non-linear equations:
v2 = 4g(pp - p _ )d _ 
3Cd p
Cd= 24 +_3_ +0.34
R e R e
1 /2
Re= p _vd
M
• Calculate velocity using Stokes law or turbulent expression.
• Calculate and check Reynolds number.
• Calculate CD .
• Use general formula.
• Repeat from step 2 until convergence.
Types of Settling Tanks
• Sedimentation tanks may function either intermittently or continuously. The intermittent 
tanks also called quiescent type tanks are those which store water for a certain period 
and keep it in complete rest. In a continuous flow type tank, the flow velocity is only 
reduced and the water is not brought to complete rest as is done in an intermittent type.
• Settling basins may be either long rectangular or circular in plan. Long narrow 
rectangular tanks with horizontal flow are generally preferred to the circular tanks with 
radial or spiral flow.
Long Rectangular Settling Basin
• Long rectangular basins are hydraulically more stable, and flow control for large volumes 
is easier with this configuration.
• A typical long rectangular tank have length ranging from 2 to 4 times their width. The 
bottom is slightly sloped to facilitate sludge scraping. A slow moving mechanical sludge
scraper continuously pulls the settled material into a sludge hopper from where it is 
pumped out periodically.
Drag of sedimentation tank
A long rectangular settling tank can be divided into four different functional zones:
Inlet zone: Region in which the flow is uniformly distributed over the cross section such that the 
flow through settling zone follows horizontal path.
Settling zone: Settling occurs under quiescent conditions.
Outlet zone: Clarified effluent is collected and discharge through outlet weir.
Sludge zone: For collection of sludge below settling zone.
Inlet and Outlet Arrangement
Inlet devices: Inlets shall be designed to distribute the water equally and at uniform velocities. 
A baffle should be constructed across the basin close to the inlet and should project several feet 
below the water surface to dissipate inlet velocities and provide uniform flow;
Outlet Devices: Outlet weirs or submerged orifices shall be designed to maintain velocities 
suitable for settling in the basin and to minimize short-circuiting. Weirs shall be adjustable, and 
at least equivalent in length to the perimeter of the tank. However, peripheral weirs are not 
acceptable as they tend to cause excessive short-circuiting.
Weir Overflow Rates
Large weir overflow rates result in excessive velocities at the outlet. These velocities extend 
backward into the settling zone, causing particles and flocs to be drawn into the outlet. Weir 
loadings are generally used upto 300 m3 /d/m. It may be necessary to provide special inboard 
weir designs as shown to lower the weir overflow rates.
Inboard Weir Arrangement to Increase Weir Length
Circular Basins
• Circular settling basins have the same functional zones as the long rectangular basin, 
but the flow regime is different. When the flow enters at the center and is baffled to flow 
radially towards the perimeter, the horizontal velocity of the water is continuously