sewerage system notes

7/25/2019 Sewerage System notes

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Sewerage System

Dr. Sudipta Sarkar


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Preliminary Requirements

It is meant for the transport stormwater and wastewater from the generation

point to the treatment plant. So it should be laid as deep as possible so that allwastewater or storm water flow can be collected and transported.

Erosion and corrosion resistant. Should be structurally strong enough to resist

impact loads or overburden and live loads

Size and slope to be designed to carry the peak load as well as to carry average

flow in such a manner that the deposition shall be minimized.

Maintenance should be easy, economical and safe for the workers.

Aims of the design are: a) make the system operational and b) Economical to build

and c) make the system durable through out its entire design life


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Layout of Sewer Lines

Steps followed for making the layout:

Selection of an outlet or disposal points

Fixing limits to the drainage area or zone boundaries

Finalizing the location of Trunk and Main sewers

Finalizing the location of Pumping stations wherever necessary

Trunk sewer is the sewer in the network

with the largest diameter that extends

farthest from the sewage outfall

All other sewers are considered as

branches

Whenever two sewers meet at a point, the

incoming one with larger diameter is called the

main sewer.

Trunk SewerOutfall


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Nomenclature System Followed in Sewer Systems

Trunk Sewer

32

4

L.3.1

R.3.1

R.3.2

L1.R.3.1.1

L1.R.3.1.2

L2.R.3.1.1 L2.R.3.1.2

Outfall

Network

manhole


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Most common location of laying sanitary

sewer is along the center of the streets

House

House

The individual domestic connections

can be from either side of the streets

For very wide streets the sewers are

laid on each side of the streets in the

curb or under the sidewalk

House

House

Street

Street

Sewer

Sewer

Sewer

To avoid any contamination sewerlines are never laid near to the water

mains. If it is unavoidable, the sewers

are encased in concrete or in

Slope of the sewers generally follow the

natural slope of the ground or the street


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Design Approach

1. On a map of the area locate all the sewer lines and measure the contributory

area to each of the sewer lines or points.

2. Also, draw the longitudinal section or profiles of the sewer lines. Mark on the

profile view the critical points such as basements of the low lying houses, levels

of existing sewers, disposal points, etc.

3. Design all the branch sewers, main sewers and trunk sewers, starting fromthe farthest point in the network and based on the following considerations:

a) A self cleansing velocity is maintained at present peak flow

b) The sewer should run 0.8 full at the design ultimate peak flow

c) Minimum velocity of 0.6 m/s is obtained

d) Maximum velocity should not be beyond 3 m/s


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Small Bore Sewer System

They are designed to carry only the liquid part of the domestic sewage generated

for off-site treatment or disposal

Septic Tank or

interceptor tank

Sewer

Solids are separated at a septic tank or at the

aqua-privies before the sewage reaches the

sewers

The advantages:

a) The sewer can have less velocity and flowrateas it receives only settled wastewater

b) Economic as it requires less cost of

excavation, material and treatment

c) Upgradation from on-site treatment system to

conventional treatment system is easily done

d) Maintenance of strict sewer gradients is notrequired as there is no self-cleansing velocity

requirement

Minimum diameter of the sewer pipes is recommended to be 100 mm


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Small Bore Sewer System

The small bore sewer system outfall can be any of the following:

a)The conventional sewer systemb) Waste stabilization ponds

c) Any other low cost treatment systems followed by fish ponds or land-

based disposal with precautions

Limitations:

a) Interceptor tank requires periodical cleaning and disposal of solids

b) Any illegal connection without any interceptor tank shall ruin the

system. So, strict vigilance is required.


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Shallow Sewer System

These are modification of surface drain with covers and consist of a network of

pipework laid in the areas away from the places where heavy sewage loads are

expected.

Pipes are laid in flat gradients following the natural slope of the ground. The

minimum depth is 0.4 m

System contains:

a) House connectionsb) Inspection

chambers

c) Laterals

d) Street-collector

sewers

e) Pumping stations

The laterals are minimum diameter 100 mm

The street collectors have a minimum diameter of 150 mm


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Shallow Sewer System

Suitability of the system:

1. High density habitats such as slums or squatter settlements ( with populationdensity more than 170 per hectre)

2. Ground-condition is adverse and on-site disposal is not possible

3. Sewage has to be disposed of and minimum water consumption is 25 lpcd.

Limitations:a) It is suitable when suitable ground slope is available

b) Unless flushed out at peak flowrates, there is a possibility of solids

deposition if there is not enough ground slope available

c) May require frequent cleaning


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Example of a Profile of a Sewer Line


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A view inside a sewer in London


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Sewer Appurtenances

These are devices necessary (except pipes and conduits) for proper functioning

of the sanitary, storm and combined sewers

The appurtenances include:

1. Manhole

2. Drop Manhole

3. Lampholes

4. Gully-traps

5. Intercepting chambers

6. Flushing tanks

7. Street Inlets

8. Siphons

9. Grease traps

10. Side-flow weirs

11. Leaping weirs

12. Venturi flumes

13. Outfall structures


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MANHOLES

Manholes are RCC or masonry chambers, constructed at suitable

intervals along the sewer lines, for providing access to the inside of

the sewers.

Helps in:

a) Joining the sewer pipes

b) Inspection and cleaning of pipes

c) Maintenance

d) Ventilation if manholes are perforated

Water

main

Electric

cable

Gutter Curbmanhole

Sewer

Manholes are provided at every

transition points such as bends, junction,

change of gradient, or change in diameter

Between two adjacent manholes, the

sewer line runs straight with constant

slope or gradient


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Types of Manholes

Manholes with Depth less than 0.9 m

A. Shallow manholes

Suitable for branch sewers or places at noheavy traffic

It is also called an inspection chamber

Manholes with Depth 1.5 m

B. Normal or Medium manholes

Heavy cover is provided at the top

May be either square or rectangular (1m X

1m and 1.2m X 1m

900X 800 mm


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Manholes with Depth> 1.5 m

C. Deep Manholes

Heavy cover is provided at the top

May be either square or rectangular orcircular

Size in the upper portion is reduced by offset


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Access shaft: Minimum size is

0.75 X 0.6 m

Working chamber: Provides working

space for inspection and cleaning

operations, Minimum size 1,2 m X

0.9 m or 1.2 m dia; minimum height

is 1.8 m

Benching: concreted portion sloping

towards semicircular or U -shaped

bottom part of the main sewer, the

slope facilitates the entry of sewage

into the main sewer

Steps or ladders: for accessing

DROP MANHOLE


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DROP MANHOLE

It is used when a branch sewer joins a main sewer at a height more than 600 mm above the

main sewer or the drop is more than 600 mm.

Advantages: 1) Steep gradients in the branch sewer can be avoided ; 2) The sewage from

the branch sewers may fall on the person working; This is avoided.

PlugInspection Arm


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FLUSHING MANHOLE

Provided where it is not possible

to gain enough flow so as to

maintain a self-cleansing velocity.

Often such condition is prevalent

at the beginning of the branch

sewers.

Generally provided at the head

of the sewers where enough

storage is provided to

generate a high velocity to

flush out the obstructions


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Automatic Flushing Tanks


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Curb Inlet

Gratings


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Different Types of Street Inlets

GUTTER TYPE

CURB TYPE INLETS

COMBINATION MULTIPLE TYPE INLETS


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CATCH BASINS

SEWER

A Type of Street Inlet

The basin helps in settling the grit,

sand, debris, etc. before the

storm water enters the sewer line

Hood prevents the escape of the

foul gases into the sewer line andnetwork


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Oil and Grease Trap

Generally located near the sources which can generate oil and grease-contaminated wastewater. Restaurants, garages, automobile repair workshops

Oil and grease in the sewer system can : a) sticks to the inner surface of sewers

and reduces the sewer capacity; b)entraps suspended matter, further reducing

the capacity; c) adversely affect the performance of wastewater treatment

plants


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REGULATOR OR OVERFLOW DEVICES OR STORM-RELIEF WORKS

The regulators are provided to avoid overloading of sewers, pumping stations,

treatment plant or disposal arrangements by diverting excess flow to relief

sewers or overflow stream.

The overloading is caused by excess flow coming in a pipeline due to heavy

rainfall or excess stormwater. As they are not expected to carry huge pollutant

load, the excess stormwater can be safely disposed of to natural streams

without any treatment.

Three types of Regulator devices:

a) Leaping Weir

b) Side-flow or Overflow weir

c) Siphon spillway


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Leaping Weir

Arrangement consists of an opening at the invert of a storm drain through whichthe normal storm flow is taken into an intercepting sewer and excess flow leaps

over the combined sewer to flow to a neighboring stream

INCOMING FLOW

Intercepting Sewer

O fl Sid fl W i


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Overflow or Side-flow Weir

Excess water is allowed to overflow the

combined sewer in the manhole, from

where it is taken to another channel that

leads to stormwater drain or manhole.

The weir length has to be sufficiently

long for effective regulation

h ll


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Siphon Spillway

Air Line

Receiving StreamSewer

Spillway


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d/D a/A v/V q/Q

1.00 1.00 1.00 1.00

0.9 0.949 1.124 1.066

0.8 0.858 1.140 0.988

0.7 0.748 1.120 0.838

0.5 0.5 1.000 0.500

0.4 0.373 0.902 0.337

0.3 0.252 0.776 0.196

0.2 0.143 0.615 0.088

Advantages of a circular sewer diminishes when the sewer is not running at least half-full

Lesser the discharge, poorer is the performance

OVOID OR EGG-SHAPED SEWER


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OVOID OR EGG-SHAPED SEWER

At low discharges 2- 15%

higher velocities are available

for these type of sections

compared to HydraulicallyEquivalent Circular Sections

Standard Oval Shaped Sewers

New Type Oval Shaped Sewers

Hydraulically Equivalent Section: Two sewers of

different shape (i.e. different sections) are said to be of

hydraulically equivalent when they carry the same

discharge when running full at the same slope.

d/D v/V

Ovoid circular

0.25 0.7 0.698

0.20 0.61 0.62

0.10 0.4 0.44

0.05 0.25 0.29


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Design of Ovoid-Shaped Sewers

1. Calculate the approximate diameter of a hydraulically equivalent circular sewer that

would carry the same discharge at the same slope as the ovoid-shaped sewer.

2. Top horizontal diameter of the Ovoid-sewer = 0.84 X Diam. of the circular sewer

3. Find out the other dimensions from the following figures, according to the type of

sewer to be designed


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Horse-Shoe Type of Sections


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Open-Drain Sections

P

AR

2/13/21 sRn

V

VAQ *

sewerage system notes

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