Which of these sewers are preferred for combined system of sewage?
Egg shaped Sewer
Combined Sewer System: Preferred Sewer Shapes
A combined sewer system is designed to carry both domestic wastewater (sewage) and surface runoff (stormwater) in the same conduit. This dual function requires a sewer shape that is hydraulically efficient under varying flow conditions, from low dry-weather flows to high peak flows during storms.
Why Egg-Shaped Sewers Are Optimal
Egg-shaped sewers, also known as arched or composite-shaped sewers, are generally preferred for combined sewer systems due to their superior hydraulic characteristics compared to other shapes like circular or rectangular sewers.
Hydraulic Efficiency at Different Flows:
- Low Flows (Dry Weather): During periods with little or no rainfall, the flow consists mainly of sewage. The narrower, curved bottom profile of an egg-shaped sewer ensures that the flow is concentrated, maintaining a higher velocity. This higher velocity helps in achieving self-cleansing conditions, preventing the deposition of solids and sludge build-up, which is a common issue in circular sewers at low flows.
- High Flows (Storm Weather): When heavy rainfall occurs, the volume of stormwater entering the sewer system increases dramatically. The wider, upper portion of the egg shape provides a larger cross-sectional area and storage capacity, accommodating these high flows efficiently. This reduces the likelihood of the sewer surcharging (exceeding its capacity and backing up).
Comparison with Other Shapes:
Let's compare the egg shape with other common sewer profiles:
| Sewer Shape | Advantages | Disadvantages (for Combined Systems) |
|---|---|---|
| Circular Sewer | Structurally strong, easy to construct, efficient at high flows. | Less efficient at low flows; prone to sedimentation and self-cleansing issues. |
| Egg-Shaped Sewer | Excellent hydraulic efficiency at both low and high flows; promotes self-cleansing at low flows and handles high flows effectively. | Slightly more complex to construct than circular sewers. |
| Rectangular Sewer | Simple construction in some cases (e.g., box culverts). | Poor hydraulic efficiency, especially at corners; prone to settlement issues; difficult to clean. |
| Trapezoidal Sewer | Often used for open channels or canals. | Not typically suitable for enclosed combined sewer systems carrying wastewater; less efficient for sewage flow compared to egg shape. |
Conclusion
Given the need to efficiently manage both low-volume sewage flows and high-volume stormwater flows, the egg-shaped sewer provides the most suitable hydraulic performance for combined sewer systems. Its design optimizes velocity and capacity across a wide range of flow rates, minimizing sedimentation problems and maximizing transport efficiency.
Important Questions from Design of Sewer
Which of the following statements is NOT correct?
The average daily flow of wastewater generated from residential, commercial, and industrial sources, specifically excluding any contribution from rainfall or stormwater, is commonly referred to as:
As per NBO, the gradient required to generate self-cleansing velocity for a 100 mm Φ sewer is _____.
Which of the following problems does proper sewer ventilation help to mitigate?
Maximum velocity condition in a flow-through circular channel section is:
