Sanitary-chapter-3-Rashna-PanditTheory.pdf

2
5/8/2021 Rashna Pandit
❑ To Know the Condition Of Sewer In Nepal.
❑ To Know and understand the Design Procedure Of Sewer.
❑ To Gain the Knowledge about the Various Sewer Appurtenances.

Presentation Outline
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3.1. Shapes of sewer
3.2. Sewer Materials - salt glazed stoneware, cement concrete, cast iron
3.3. Design criteria of sewers - design period, minimum and maximum
velocities, self cleansing velocity, sewer size range, sewer gradient
3.4. Hydraulic formulae for design Manning's, and Hazen Williams formulae,
hydraulic elements of circular sewers for partial flow condition, partial flow
diagrams
3.5. Numerical
3.6 Construction of sewers

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There are various types of sewer among them most common is circular for
closed sewers. Anyway shape of the sewer section should be selected in
such a way that it should:
• Develop self cleansing velocity during DWF
• Be easy in cleaning and maintenance
• Withstand the internal and external pressure
• Be structurally safe and stable
• Br hydraulically efficient
• Have sufficient free board to prevent from overflow during monsoon

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1. Rectangular section
2. Circular section
3. Semi Elliptical section
4. Horse Shoe section
5. Basket Handle section
6. Egg section

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1. Rectangular section:
It is not preferred in a sewer line because it is not so
hydraulically efficient.

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2. Circular section:
It is the most common and used type of section in sewer
line.
Advantages:
• This section gives the least perimeter for a given area;
therefore it has the maximum hydraulic mean depth for
running full and half full depth of the section.
• This section is also very economical as the quantity of
materials required is less.
• This section can be easily manufactured in the factories,
transported and laid at the required places.
• This section is useful in separate system of sewerage,
where discharge is more or less uniform.

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2. Circular section:
Disadvantage:
• But it is not suitable for combined system, because
in dry weather it is very difficult to develop the self-
cleaning velocity.

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3. Semi Elliptical section:

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4.Horse Shoe section

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5. Basket Handle section

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3.2 Sewer Materials

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3.2 Sewer Materials
Requirements of sewer materials
➢It should be resistant to corrosion and abrasion de to impurities.
➢It should be durable and have sufficient strength.
➢It should be structurally safe.
➢It should have minimum weight so that it can be easily handled.
➢Should be impervious.
➢Should be cheap and easily available.

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3.3. Design criteria of sewers
1. Specific gravity of sewage: Sewage contains 99% water hence its sp.gr. Nearly 1
2. Design period: Consideration of design period (25-30 years).
3. Velocity of flow: Sewers are design to ensure the neither silting nor scouring
velocity i.e. self cleansing velocity and limiting velocity.
𝑉𝑠 =
8𝐾
𝑓
𝐺𝑠 − 1 𝑔 × 𝑑
Where, Vs is self cleansing velocity
K is constant (taken as between 0.04 to 0.8)
f Darcy Weisbach friction factor (taken as 0.03)
Gs is specific gravity (range from 1.2 for organic to 2.65 inorganic)
g is acceleration due to gravity = 9.8 m/sec
d diameter of the solid particles carried organic sediments (m)

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4. Minimum velocity: Minimum or self-cleansing velocity may be defined as that
velocity at which the solid particles will remain in suspension, without settling at
the bottom of the sewer. The criteria for determining minimum velocity which
depends upon:
• Sewer size
• Sewerage system
• Solid particles
S.N. Sewer diameter (m) Self cleaning velocity
1. 15-25 1
2. 30-90 0.75
3. >60 0.6
Self cleaning velocity for different diameter of sewer

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5. Maximum or limiting velocity: It is the necessary velocity of flow of sewage in a
sewer should not be excessive to cause scouring or erosion of its inner surface. For
general design, non-scouring or maximum velocity is considered as 3 m/sec but
limiting velocity depends on the material of sewers.
6. Minimum size of sewers: Sewer size should not less than 15 cm but
recommended is 20 cm. The commercially available size are of 15, 20, 25, 30, …….
,60 etc. maximum available size is 3 m.
7. Sewer grades: The sewer gradient are selected as per site condition. The
minimum gradient should be such that self cleansing velocity is achieved and
maximum gradient should be such that the velocity doesn’t exceed to non scouring
velocity. Minimum gradient should be 1: 100 and maximum is 1: 20.

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3.4 Hydraulic formulae

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1. Chezy’s Formula:
V=C√(RS)
Where, V = velocity of flow m/s
R = Hydraulic mean depth in mm
S = Slope or gradient of sewer
C= Chezy’s coefficient
2. Hazen-Williams formula:
V=0.85 CH R0.63 S0.54
Where, CH is Hazen-William’s coefficient
3. Manning’s Formula:
V=
1
𝑁
R2/3S1/2
Where, N is manning’s coefficient

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Hydraulic elements of circular section:
a) Circular sewers running full:
Let D be the internal diameter of the sewer
• Area of flow section: 𝐴 =
𝜋
4
𝐷2
• Wetted perimeter: 𝑃 = 𝜋D
• Hydraulic mean depth: 𝑅 =
𝐴
𝑃
=
𝐷
4
• Velocity of flow: V=
1
𝑁
R2/3S1/2
Where, N is manning’s coefficient and S is the slope or
gradient of sewer
• Discharge: 𝑄 = 𝐴 × 𝑉
b) Circular sewers running half:
Area of flow section: 𝐴 =
1
2
𝜋
4
𝐷2=
𝐷
4

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Hydraulic elements of circular section:
a) Circular sewers running partially full:
c)

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1. Chezy’s Formula:
V=C√(RS)
Where, V = velocity of flow m/s
R = Hydraulic mean depth in mm
S = Slope or gradient of sewer
C= Chezy’s coefficient
2. Hazen-Williams formula:
V=0.85 CH R0.63 S0.54
Where, CH is Hazen-William’s coefficient
3. Manning’s Formula:
V=
1
𝑁
R2/3S1/2
Where, N is manning’s coefficient

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3.5. Numerical
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