Following method is followed for the design of sewers: 1. Zone Formation 2. Sewer Network 3. Sewage Quantity 4. Flow Velocity 5. Sewer Section 6. Grade of Sewers.
1. Zone Formation:
The whole town or city in which sewerage system is to be provided is divided into different zones. The layout of sewers is plotted along the roads, and the each zone is marked separately. The sewer which will carry the sewage and the storm water of a particular zone is also marked.
2. Sewer Network:
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The proposed arrangement or network of sewers for different zones is then worked out and marked on the plan. The low lying areas are marked separately and are isolated from the main sewerage system, because pumping is to be done in that case and pumps are to be installed at suitable places. As the sewage flows under gravitational force only, the sewage starts from the high level zones to the low level zones and finally flows to the point of disposal.
3. Sewage Quantity:
The quantity of sewage (maximum as well as minimum) to be carried by each sewer is worked out. The variation factors are also determined and the actual quantity of sewage for which the sewer is to be designed is determined.
4. Flow Velocity:
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The suitable velocity of flow between the self-cleansing velocity and limiting velocity is chosen for the design purpose. The value can also be determined by the empirical formulae.
5. Sewer Section:
Now the sectional area of the sewer is determined by the relation
A = Q/V. From the area the section of sewer is determined.
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6. Grade of Sewers:
Now the slope of the sewer line is determined and the longitudinal section of each sewer line is drawn to a suitable scale. These sections show invert levels and all sewer appurtenances. Now keeping suitably the depth of the sewer from the ground level at the highest point or point of disposal, the laying of sewers can be started.
Example 1:
Determine the velocity of the flow in a sewer running one half full. The sewer is laid at 1 in 550 slope. The diameter of the sewer is 150 cm. Also determine the discharge flowing through the sewer. Assume N = 0.012 in Manning’s formula.
Example 2:
With the help of crimp and Burge’s formula, determine the velocity and discharge of a sewer of 40 cm diameter laid at a gradient of 1 in 350.
Solution:
Crimp and Burge’s formula states
Example 3:
Design the section of a combined circular sewer from the following data:
Area to be served = 150 hectares
Population of the locality = 50,000
Max. permissible velocity = 3.2 m/sec
Time of entry = 5 minutes
Time of flow = 20 minutes
Rate of water supply = 270 litres/day/capita
Impermeability factor = 0.45
Solution:
