Design of Sewers for Self-Cleansing Velocity:

Self-cleansing velocity is defined as the minimum velocity required in a sewer to keep the solids in suspension and prevent deposition of solids. The design of sewer is done in such a way that the velocity of flow in the sewer attains the self-cleansing velocity at the minimum hourly rate of flow.

Factors Affecting Self-Cleansing Velocity:

The self-cleansing velocity of a sewer depends upon the following factors:

1. Size and Shape of Sewer: The self-cleansing velocity is directly proportional to the size and shape of the sewer. The larger the size of the sewer, the higher the velocity required to achieve self-cleansing.

2. Type of Flow: The self-cleansing velocity is different for different types of flow such as laminar and turbulent flow. Turbulent flow requires a higher velocity for self-cleansing.

3. Nature of Sewage: The self-cleansing velocity is affected by the nature of sewage. Sewage with a higher concentration of solids requires a higher velocity for self-cleansing.

Calculation of Self-Cleansing Velocity:

The self-cleansing velocity can be calculated using the following formula:

Vc = K (Ry S)^1/3

Where,

Vc = Self-cleansing velocity in m/s
K = Coefficient of roughness
Ry = Hydraulic radius of the sewer in m
S = Slope of the sewer
^1/3 = Cube root

Design of Sewers:

The design of sewers is done in such a way that the velocity of flow in the sewer attains the self-cleansing velocity at the minimum hourly rate of flow. The minimum hourly rate of flow is the flow rate that occurs during the dry weather period. The sewer is designed to achieve a velocity of flow that is 25% higher than the self-cleansing velocity. This is done to ensure that even during the low flow period, the velocity is sufficient to prevent the deposition of solids.

Conclusion:

The self-cleansing velocity is an important parameter in the design of sewers. The sewer is designed to achieve a velocity of flow that is higher than the self-cleansing velocity to prevent the deposition of solids. The design of sewers is done in such a way that the velocity of flow in the sewer attains the self-cleansing velocity at the minimum hourly rate of flow.

Self-cleansing velocity in sewers

Self-cleansing velocity is the minimum velocity required to prevent deposition of solids in sewers. If the velocity of wastewater is less than the self-cleansing velocity, then the solid particles will settle down and accumulate in the sewer. This can lead to blockages and reduced capacity of the sewer. Therefore, it is essential to design a sewer system to achieve self-cleansing velocity.

Factors affecting self-cleansing velocity

The self-cleansing velocity of sewers depends on various factors, such as:

- Size and shape of the sewer
- Slope of the sewer
- Nature and quantity of solids in wastewater
- Temperature and viscosity of wastewater
- Hydraulic conditions, such as turbulence and eddies

Among these factors, the size and slope of the sewer are the most critical parameters that affect the self-cleansing velocity. A sewer should be designed to achieve self-cleansing velocity under the minimum hourly rate of flow.

Design of sewer for self-cleansing velocity

The design of sewer for self-cleansing velocity involves the following steps:

1. Calculation of minimum velocity

The minimum velocity required to achieve self-cleansing velocity is calculated based on the following formula:

Vmin = Ks * (g * R * S)^0.5

Where,
Vmin = minimum velocity (m/s)
Ks = shape coefficient (varies from 0.6 to 1.0 depending on the shape of the sewer)
g = acceleration due to gravity (9.81 m/s^2)
R = hydraulic radius (m)
S = slope of the sewer (%)

2. Selection of sewer size

Once the minimum velocity is calculated, the sewer size is selected based on the following criteria:

- The velocity of wastewater should be greater than the self-cleansing velocity.
- The velocity of wastewater should not exceed the maximum velocity (usually 3 m/s) to avoid erosion and damage to the sewer.

3. Verification of design

The design of the sewer is verified by checking the velocity of wastewater at different flow rates. The velocity of wastewater should be greater than the self-cleansing velocity under the minimum hourly rate of flow.

Conclusion

In conclusion, a sewer is commonly designed to attain self-cleansing velocity at the minimum hourly rate of flow. This ensures that the sewer is able to transport wastewater without any blockages or sedimentation. The design of the sewer involves calculating the minimum velocity, selecting the sewer size, and verifying the design for different flow rates.