Design of Sewer Systems Types and Components - Civil Engineering Optional for UPSC

What are the Different Types of Sewers?

• Sanitary Sewers: These sewers carry sanitary sewage, which includes wastewater from domestic and industrial sources.
• Storm Sewers: These sewers transport storm sewage, such as street wash and surface runoff.
• Combined Sewers: These sewers handle both sanitary sewage (from households and industries) and storm sewage.
• House Sewer: This type of sewer connects the plumbing system in a house to municipal sewers for sanitary sewage disposal.
• Lateral Sewers: These sewers carry sanitary sewage from multiple house sewers.
• Sub-Main Sewers: These sewers transport sewage from multiple lateral sewers.
• Main Sewer: The main sewer collects sewage from several sub-main sewers.
• Outfall Sewer: This sewer gathers all sewage discharge from different sewer systems for final disposal.

In simpler terms: - Sanitary Sewers handle waste from homes and industries. - Storm Sewers manage rainwater and runoff. - Combined Sewers deal with both waste and rainwater. - House Sewers link homes to main sewage systems. - Lateral Sewers carry waste from multiple homes. - Sub-Main Sewers transport waste from several lateral sewers. - Main Sewers collect waste from various sub-main sewers. - Outfall Sewers gather all sewage for final disposal.

What are the Components of a Sewer System?

• Sewers: Sewers are the pipes that carry wastewaters from various sources to water reclamation centers for treatment. These sewer pipes are typically made of concrete, PVC, and tilted pipes.
• Pumping Stations: They are responsible for pumping out sewage water that has deeply penetrated the ground. This water is then directed to water reclamation centers. Pumping stations also help prevent floods by discharging surface runoff that has overflowed the sewer into rivers and seas.

Water Reclamation Centers and Sewer Design

• Water Reclamation Centers: Water reclamation centers primarily treat sewage and the resulting sludge. They commonly utilize tanks for sewage treatment.
• Design and Construction of Sewers: When designing sewers, the initial step involves calculating the average sewage flow, considering total water consumption and population after the design period. The average flow is determined using specific formulae:
  • Sanitary Sewer Design Flow = Peak Sewage Flow (ratio of maximum sewage flow to average daily sewage flow) + Infiltration (water seeping into sewers through cracks and joints)
  • Partially Combined Sewer Design Flow = 2X Peak Sewage Flow + Infiltration

Design Equation

• The equation used for sewers under gravity flow is Manning's Equation.
• V = Velocity of flow in sewers (m/sec)
• R = Area / Wetted Perimeter
• S = Slope of the Sewer
• n = Roughness Coefficient for Pipes (0.013 - 0.015)

Minimum Velocity

• Minimum Velocity prevents solid particle settlement.
• Sanitary Sewer: 0.6 m/sec
• Partially Combined Sewer: 0.7 m/sec
• Storm Sewer: 1.0 m/sec

Maximum Velocity

• Maximum Velocity is the limit not to exceed.
• Exceeding the limit (2.4 m/sec) can cause sewer abrasion and scouring.

Sewer Size

• The recommended sewer size is 20 cm.
• It should not be less than 15 cm.
• In hilly areas, it can be 10 cm.
• The standard sizes of pipes are 20 cm, 25 cm, 30 cm, with an increase of 5 cm up to 2 m.

Sewer Grades

• It is understood as the slope at which the sewer rests.
• The general rule is to follow the natural slope of the ground.
• The slope should not be steeper than 1 in 20.
• A slope of 1 in 40 – 1 in 80 is preferred for house sewers.

Estimation of Design Flows

• Calculate the population (P) of the area.
• Calculate the sewage flow/day by the multiplication of population with flow/day per capita of sewage.
• Calculate the peak sewage flow (ratio between maximum sewage flow to average daily sewage flow).
• Calculate the industrial and commercial sewage allowance.
• Calculate the infiltration (the amount of water that seeps into the sewers through cracks and joints) as given by the Water and Sanitary Agency.
• Find the estimated design flow by adding peak sewage flow, industrial and commercial sewage allowance, and infiltration flow.

Hydraulics of Sewers

• Hydraulics of sewers involve the flow of wastewater through sewer pipes.
• The design of sewers for hydraulics depends on the location where the sewage system is built.
• For effective hydraulics, circular pipes with steeper gradients are typically used.
• Sewer pipes are laid at gradients that enable self-cleansing velocities to prevent clogging.
• Wastewater flows downward towards the discharge point.

Hydraulic Formulae

Manning's Equation:

• V = Velocity of flow in sewers (m/sec)
• R = Hydraulic Radius (Area / Wetted Perimeter)
• S = Slope of the sewer
• n = Roughness Coefficient for Pipes (0.013 - 0.015)

Bazin's Constant (K):

• 1. Very smooth inside sewer - K Value: 0.109
• 2. Smooth with bricks & concrete - K Value: 0.290
• 3. Smooth with rubble - K Value: 0.833
• 4. Good earthen material - K Value: 1.540
• 5. Rough with brick & concrete - K Value: 0.500
• 6. Rough earthen material - K Value: 3.170

Sewer Flow Formulas

• Chezy’s Formula

  Chezy’s FormulaChezy’s Formula

  Velocity (V) = C * √(R_i)

  Where,

  V = Velocity of flow in sewers in meters per second

  C = Chezy Coefficient = 157.6 / [1.81 * (K / √(R))]

  R = Hydraulic Radius = Area / Wetted Perimeter

• Crimp & Burge’s Formula

  Crimp & Burge’s Formula

  Velocity (V) = 83.47 * R^2/3 * S^1/2

  Where,

  V = Velocity of flow in sewers in meters per second

  R = Hydraulic Radius (m) = Area / Wetted Perimeter

  S = Slope of the water surface

Design of Sewer Lines

• Sewers ideally follow the natural slope of the ground.

• The recommended sewer size is 20 cm, not less than 15 cm. In hilly areas, it can be 10 cm. Standard sizes range from 20 cm to 2 m, increasing by 5 cm.
• To find the sewer diameter: Size of the sewer line = Area x Velocity

• It's best to align the sewer slope with the ground's natural slope, not steeper than 1 in 20. For house sewers, a slope of 1 in 40 - 1 in 80 is preferred.
• Manning's Formula helps calculate the sewer line slope: V = (1/n) * R^(2/3) * S^(1/2)

You can explore more about the Quality and Characteristics of Sewage.

Types and Constructions of Manholes

Manholes are built underground in sewers to clean, inspect, repair, and connect sewage systems to buildings, providing access to underground sewage.

Types of Manholes:

• Precast Concrete Manholes
• Plastic Manholes
• Fibreglass Manholes

Classification based on size:

• Shallow Manholes
• Normal Manholes
• Deep Manholes

Construction Steps:

• Calculate manhole size based on pipe diameter and depth from road to pipe crown.
• Decide on manhole material.
• Excavate the ground.
• Lay the foundation, build walls, and create benching.
• Construct the manhole roof and add a cover.

Laying, Joining and Construction of Sewers

The process of laying, joining, and constructing sewers involves several key steps:

• Sewers are placed horizontally on the ground with a starting and ending elevation.
• The pipe's gradient should align with the natural slope of the terrain.
• A trench is dug by excavating and leveling the ground.
• The area beneath the pipe must be firm and stable.
• The sewer is laid within the trench after a concrete base is prepared.
• Sewer pipes are connected using concrete joints.
• The joints are examined for any potential leaks.
• If the sewer passes the leak test, the trench is refilled with excavated soil.