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Week -4

Environmental Engineering – II

Design of Sanitary Sewers

Dr.Amir Farooq

PhD. Environment Management

MSc. Environmental Engineering

BSc. Civil Engineering

Procedure for designing of Sanitary Sewer

i) Draw line to present proposed sewer in each street to be

served with an arrow head to indicate the direction of

flow of sewage

ii) Locate the manholes by giving Identification Numbers to

ease to detect fault in system

iii) Sketch limits of the service area for each lateral.

iv) Measure the service areas for each lateral

v) Prepare tabulation (Hydraulic Statement)

PARTIAL FLOW IN SEWERS

It is necessary to determine velocity and depth

of sewage in a pipe when it is flowing partially

full.

Various graphs for determining hydraulic

elements for partial flow are available; they are

used to determine depths of flow and velocities

A 915-mm (36 inches) circular sewer pipe is laid on a slope of

0.003. n = 0.013 when the sewer is full. What will the velocity of

the flow be when a sewer is carrying 8.5 m3/min?

Problem

Slope = 0.003

Dia = 36”

n = 0.013

INVERT LEVEL

INVERT LEVEL:

The lowest inside level at any cross-section of a sewer

is known as the INVERT LEVEL at that cross section.

SIGNIFICANCE:

Sewers must be laid at a particular slope to attain self

cleansing velocities. The required slope (while laying the

sewers) is achieved through calculations of invert levels.

INVERT LEVEL CALCULATIONS

IL = NGSL/RL – Depth of Sewer – Thickness of Sewer – Dia of Sewer

IL = Invert level

Minimum Cover = 3 feet or 1 meter

Know Upper & Lower Inver level of Sewer, gradient can be calculated as

given under

S = (IL Upper – IL Lower) / Span of Sewer

INVERT LEVEL CALCULATIONS

U/S IL = NGSL/RL – Depth of Sewer – Thickness of Sewer – Sewer Dia

D/S IL = U/S Invert Level – Drop (Length x slope)

SINGLE SEWER

TWO OR MORE SEWERS OF SAME SIZE

When equal dia sewers discharge in a manhole and the same

dia sewers receives the total discharge, LOWEST D/S I.L.

among the discharging sewers will be carried as U/S I.L. for the

receiving sewer.

SEWERS OF DIFFERENT SIZE

When receiving sewer dia is greater than the discharging sewer;

Keep the crowns at the same level

Drop the U/S I.L. of the receiving sewer by the difference in the

dia of the two sewers.

Cutting

It is the depth of the trench to be dug for the sewer line &

Cutting = IL – thickness of Pipe – Bedding provided

Variation in Sewage Discharge Results in Variation depth of

flow

Thus, leads to variation in hydraulic mean depth (r)

Changes in hydraulic mean depth, affects flow velocity (which

depends directly on r2/3)

Sewer must be checked and maintain a minimum velocity of

about 0.6 m/s at the time of minimum flow (assumed to be

1/3rd of average flow).

Design should also ensure development of a maximum

velocity of 2.6 m/s at least at the time of maximum flow and

preferably during the average flow periods also.

However, velocity generated must not exceed the scouring

value at time of maximum flow.

Effects of Flow Variation on Velocity in a Sewer

Assignment

Design of Sanitary Sewer

Q-1 A circular sewer is to carry 2.5 m3/min of sanitary

sewage when flowing full. Conditions are such that minimum

allowable grade must be adopted. Taking n=0.015 determine

the commercial pipe size

Q-2 A 300 mm sewer is laid at minimum slope. What is

maximum population that can be served by the sewer if the

average water consumption is 300 lpcd.

Q-3 Design a sanitary sewer to serve a population of

15,000 person supplied with a water at a rate of 300 lpcd.

Assume necessary data to arrive at design flow.

Q-4 Calculate the size and slope of a trunk sewer serving

a population of 0.5 million. Water consumption is estimated to

be 350 lpcd. A value of 0.013 may used for n. Assume other

necessary data.

Q-5 A sewer length of 100 m and 300 diameter has its

two ends at levels of 200m and 199.7m respectively.

Calculate the capacity of the sewer when running full.

Sewer Material

Materials used for transport of water can be used for carriage

of sewage

However, use of less expensive material is common

Types of Pipes based on material

PVC, AC, PCC, RCC, C.I., Steel, Clay

Cast Iron and Steel Pipes

Used when sewage line is under pressure

PCC Pipes

Normally used for small storm drains and sanitary

sewers.

Made in three classes with three wall thicknesses.

Sizes: 100 mm – 610 mm.

Concrete pipes > 610 mm in size are reinforced.

RCC Pipes

RCC pipes are most commonly sued pipes for

conveyance of sewage.

These are normally used for combined, large storm

drains and sanitary sewers in urban areas.

These pipes are made in five classes with two wall

thicknesses in class-I and three wall thicknesses in

other four classes.

Dimensions available for RCC pipes are shown in

Table.

Strength of RCC Pipes

Three edge-bearing test is used to determine the strength of

RCC pipes.

Load is applied on the pipe to produce 0.25 mm crack.

It defines the load that can be safely supported by the sewer.

Sewer Shapes

Sewer Pipes can be Circular, rectangular and Square

Circular Shapes are Preferred following reasons

It gives maximum Cross-Sectional area for amount of

material in the walls

Pre-cast circular pipes can be casted more conveniently

Circular pipes are more stable in trenches

Posses good hydraulic properties

Strongest shape to withstand the external pressure,

hence circular shape is more important in case of deep

sewers

LOADS ON BURIED PIPES

Sewer design requires prior knowledge of soil

and site conditions to determine overburden

loads that will be placed on buries pipes.

Total load on buried pipes is the sum of live load

and backfill load.

Live loads on the surfaces rarely influences the

design of sanitary sewers because of their

greater depths.

Backfill load is of more concern.

Backfill Load on Sewers:

Backfill load on buries pipes can be calculated using

Marston’s Equation.

W = CωB2

Where;

W = Load on the pipe per unit length, Kg/m

ω = Weight of the backfill material per unit volume, Kg/m3

B = Width of the trench, m

C = Coefficient that depend on depth of trench, character of

construction and fill material.

Backfill Load on Sewers:

Backfill load on buries pipes can be calculated using

Marston’s Equation.

W = CωB2

Where;

W = Load on the pipe per unit length, Kg/m

ω = Weight of the backfill material per unit volume, Kg/m3

B = Width of the trench, m

C = Coefficient that depend on depth of trench, character of

construction and fill material.

For ordinary trench construction, C may be calculated from

Where;

H = depth of fill above pipe

B = Width of trench just below top of pipe

K = ratio of active lateral pressure to vertical pressure

μ’ = coefficient of sliding friction between fill material and sides

of trench

The product Kμ’ ranges from 0.1 to 0.16 for most soils as

shown in Table.

in developing the strength of the pipe,

assuring it is laid to the proper grade, and

preventing subsequent settlement.

In unfavorable soil conditions, bedding is particularly

important.

SEWER BEDDINGS

Provision of proper bedding is very important;

Load Factor:

Load Factor expresses the increase in strength of sewer

by provision of proper bedding

Depending upon the load factor, following four types of beddings are

provided for concrete pipes

Fig: Method of bedding concrete pipes and load factors applicable to strength

SEWER BEDDINGS IN LAHORE (WASA)

Brick Ballast Crushed Stone

Load Factor = 1.7

Used under poor subsoil

conditions, above the water table

Load Factor = 1.9

Used under poor subsoil

conditions, below water table

Concrete Cradle

Load Factor = 3.0

Used under increased strength

requirements

Sewer Construction

Construction of Sewer comprises of following steps

1) Excavation

2) Bracing

3) Dewatering

4) Pipe Installation/Laying

5) Backfilling

6) Construction of Appurtenances

1) Excavation

Locate the centre line of the trench.

Excavate according to size of the trench and required gradient.

Start Excavation and construction preferably from end point.

Excavation is relatively easy from the starting

Depth increases as work proceeds (Deep excavation &

Dewatering)

Increases probabilities of run off of contractors.

Required, If GWT is above the bottom of the trench.

Sheeting, bracing and pumping for de-watering) .

2) Bracing

Sheeting & Bracing for trenches in unstable material

prevent caving or collapse of the walls.

3) Dewatering

Pipes are inspected to ensure that they

have no crack or defects.

Chain and pulley arrangement (large &

deep trenches) or cranes

Placing of pipes on line and grade in

trenches (excavated and dewatered)

Joined & pressed together with a winch.

4) Pipe laying Laying of pipes Old method

a) Offset line is located (Avoid disturbance and covering)

b) Measure Lay out of the trench from offset line and excavate.

c) Batten boards are placed across the trench at 10-15m intervals.

They are supported and fixed with ground as shown in figure.

d) The Centre line of the sewer is shown on the batten boards by a

nail, by the edge of upright cleat.

e) At the cleats, nails are fixed at the given gradient.

f) A cord is stretched along with these nails. This cord will be

according to required grade.

g) The centre line from the batten boards is transferred on the bottom

of the trench by means of a plumb rod. The grade is transferred

from the cord to the bed of trench by means of a stick marked in

even increments and having a short piece fastened at right angle to

its lower end.

h) Grade is checked by placing the short piece on the invert of each

length of sewer pipe and noting whether the proper mark touches

the cord.

Sewer Construction (Laying of Pipes)

JOINTS IN RCC SEWERS

“Bell and Spigot” and Tongue and Groove joints

Portland cement mortar or

bituminous material.

Entrance is reduced by

wrapping spigot with an seal

or pieces of old ropes cord of

appropriate thickness.

The gasket is driven into the

bell (calking Tool)

Joint is filled with mortar or

bitumen.

The inside of the pipe of

smoothen with a swab or

drag.

i) Bell and Spigot” joint

For 310-760 mm either joint can be used

For > 760 mm only tongue and groove is

used.

ii) Tongue and Groove Joint

v) Backfilling

Immediate Backfilling after laying and jointing of pipes

Delayed backfilling in case of class A bedding to

permit setting up of Concrete to support the backfill.

No water should be permitted to rise in backfilled

trenches.

Fill material (free of brush, debris, frozen material,

large rock and junks.

Tamping in layers ( 6” thickness to a depth of 2’)

Careful dropping upto 2’

Thereafter rapid backfilling .

Steps for Partially Combined Sewer for Urban areas

Find the present population of project area.

Find design population from given design period.

Find out average sewage flow for the design population. Using

this average sewage flow select Peak factor for the project

area (From Table No.1)

Draw layout of the sewer system keeping in view the layout of

the roads and streets. Represent sewer with line and

Manhole with dot.

Number the manholes and identify each sewer line e.g. M1M2,

M2M3 etc.

Allocate plots or area to each sewer line (Col 5,6,7,8)

Measure length of each sewer line according to the scale of

map. Indicate the direction of flow of sewer with help of arrow

head.

Find of sewage flow as 80 – 85 % of water consumption.

Calculate average sewage flow (Col. 10) and infiltration (Col.

11) for each sewer line.(For this particular design problem take

infiltration as 5% of average sewage flow).

Calculate Peak Sewage Flow (Col.12) and finally the design

flow (Col.14 & Col.15) for the sewer lines.

Using the method of back calculations, find appropriate

diameter (Col.17) and slope (Col.18) for your sewer assuming

that sewer is running full. For back calculation choose a

suitable design table with a suitable slef-cleansing velocity (0.6

m/sec). Refer Table-2.

Use graph from book by “Steel and “ to find the depth of flow

(Col.22) and actual velocity (Col.24 at “Design Flow”.

If actual velocity and depth of flow are satisfactory, then

diameter and slope of pipe are considered final.

If velocity is less than Self-Cleansing velocity, then increase the

slope of the sewer. For the new slope find Qf and then Qa/Qf

and d/D and finally Va/Vf and Va.

Find the Invert Levels (Col.28 & 29) for all the sewers and

complete the table of calculations called “ Hydraulic Statement”

(Note: a lot of care and vigilance should be exercised in

calculating the Invert Levels otherwise the whole scheme

may fail due to incorrect levels)

Draw profile or L-Section of all the sewer lines

Assignment

For ordinary trench construction, C may be calculated from

Where;

H = depth of fill above pipe

B = Width of trench just below top of pipe

K = ratio of active lateral pressure to vertical pressure

μ’ = coefficient of sliding friction between fill material and sides

of trench

The product Kμ’ ranges from 0.1 to 0.16 for most soils as

shown in Table.

in developing the strength of the pipe,

assuring it is laid to the proper grade, and

preventing subsequent settlement.

In unfavorable soil conditions, bedding is particularly

important.

SEWER BEDDINGS

Provision of proper bedding is very important;

Load Factor:

Load Factor expresses the increase in strength of sewer

by provision of proper bedding

Depending upon the load factor, following four types of beddings are

provided for concrete pipes

Fig: Method of bedding concrete pipes and load factors applicable to strength

SEWER BEDDINGS IN LAHORE (WASA)

Brick Ballast Crushed Stone

Load Factor = 1.7

Used under poor subsoil

conditions, above the water table

Load Factor = 1.9

Used under poor subsoil

conditions, below water table

Concrete Cradle

Load Factor = 3.0

Used under increased strength

requirements

Sewer Construction

Construction of Sewer comprises of following steps

1) Excavation

2) Bracing

3) Dewatering

4) Pipe Installation/Laying

5) Backfilling

6) Construction of Appurtenances

1) Excavation

Locate the centre line of the trench.

Excavate according to size of the trench and required gradient.

Start Excavation and construction preferably from end point.

Excavation is relatively easy from the starting

Depth increases as work proceeds (Deep excavation &

Dewatering)

Increases probabilities of run off of contractors.

Required, If GWT is above the bottom of the trench.

Sheeting, bracing and pumping for de-watering) .

2) Bracing

Sheeting & Bracing for trenches in unstable material

prevent caving or collapse of the walls.

3) Dewatering

Pipes are inspected to ensure that they

have no crack or defects.

Chain and pulley arrangement (large &

deep trenches) or cranes

Placing of pipes on line and grade in

trenches (excavated and dewatered)

Joined & pressed together with a winch.

4) Pipe laying Laying of pipes Old method

a) Offset line is located (Avoid disturbance and covering)

b) Measure Lay out of the trench from offset line and excavate.

c) Batten boards are placed across the trench at 10-15m intervals.

They are supported and fixed with ground as shown in figure.

d) The Centre line of the sewer is shown on the batten boards by a

nail, by the edge of upright cleat.

e) At the cleats, nails are fixed at the given gradient.

f) A cord is stretched along with these nails. This cord will be

according to required grade.

g) The centre line from the batten boards is transferred on the bottom

of the trench by means of a plumb rod. The grade is transferred

from the cord to the bed of trench by means of a stick marked in

even increments and having a short piece fastened at right angle to

its lower end.

h) Grade is checked by placing the short piece on the invert of each

length of sewer pipe and noting whether the proper mark touches

the cord.

Sewer Construction (Laying of Pipes)

JOINTS IN RCC SEWERS

“Bell and Spigot” and Tongue and Groove joints

Portland cement mortar or

bituminous material.

Entrance is reduced by

wrapping spigot with an seal

or pieces of old ropes cord of

appropriate thickness.

The gasket is driven into the

bell (calking Tool)

Joint is filled with mortar or

bitumen.

The inside of the pipe of

smoothen with a swab or

drag.

i) Bell and Spigot” joint

For 310-760 mm either joint can be used

For > 760 mm only tongue and groove is

used.

ii) Tongue and Groove Joint

v) Backfilling

Immediate Backfilling after laying and jointing of pipes

Delayed backfilling in case of class A bedding to

permit setting up of Concrete to support the backfill.

No water should be permitted to rise in backfilled

trenches.

Fill material (free of brush, debris, frozen material,

large rock and junks.

Tamping in layers ( 6” thickness to a depth of 2’)

Careful dropping upto 2’

Thereafter rapid backfilling .

Steps for Partially Combined Sewer for Urban areas

Find the present population of project area.

Find design population from given design period.

Find out average sewage flow for the design population. Using

this average sewage flow select Peak factor for the project

area (From Table No.1)

Draw layout of the sewer system keeping in view the layout of

the roads and streets. Represent sewer with line and

Manhole with dot.

Number the manholes and identify each sewer line e.g. M1M2,

M2M3 etc.

Allocate plots or area to each sewer line (Col 5,6,7,8)

Measure length of each sewer line according to the scale of

map. Indicate the direction of flow of sewer with help of arrow

head.

Find of sewage flow as 80 – 85 % of water consumption.

Calculate average sewage flow (Col. 10) and infiltration (Col.

11) for each sewer line.(For this particular design problem take

infiltration as 5% of average sewage flow).

Calculate Peak Sewage Flow (Col.12) and finally the design

flow (Col.14 & Col.15) for the sewer lines.

Using the method of back calculations, find appropriate

diameter (Col.17) and slope (Col.18) for your sewer assuming

that sewer is running full. For back calculation choose a

suitable design table with a suitable slef-cleansing velocity (0.6

m/sec). Refer Table-2.

Use graph from book by “Steel and “ to find the depth of flow

(Col.22) and actual velocity (Col.24 at “Design Flow”.

If actual velocity and depth of flow are satisfactory, then

diameter and slope of pipe are considered final.

If velocity is less than Self-Cleansing velocity, then increase the

slope of the sewer. For the new slope find Qf and then Qa/Qf

and d/D and finally Va/Vf and Va.

Find the Invert Levels (Col.28 & 29) for all the sewers and

complete the table of calculations called “ Hydraulic Statement”

(Note: a lot of care and vigilance should be exercised in

calculating the Invert Levels otherwise the whole scheme

may fail due to incorrect levels)

Draw profile or L-Section of all the sewer lines

Assignment