6.4 Design and Construction of Sewers

6.4 Design and Construction of Sewers

Introduction to Sewerage Systems

The safe and efficient collection, conveyance, and disposal of wastewater (sewage) is as critical to public health as the supply of clean water. A well-designed sewerage system prevents the spread of waterborne diseases, mitigates environmental pollution, and contributes to overall urban sanitation. This unit covers the fundamental engineering principles behind estimating wastewater flows, selecting appropriate sewer systems, designing the hydraulic elements of sewers, choosing suitable materials, and understanding construction practices. The goal is to create a self-cleansing, leak-proof, and durable underground network that reliably transports wastewater to a treatment facility.

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1. Estimation of Quantity of Wastewater

1.1 Sources of Wastewater

1. Domestic/Sanitary Sewage: Wastewater from residential areas (toilets, bathrooms, kitchens, laundries).

2. Industrial Wastewater: Effluent from manufacturing and processing industries. Often requires pre-treatment before discharge into public sewers.

3. Infiltration/Inflow:

   • Infiltration (I): Groundwater that seeps into sewers through defective joints, cracks, or pipe walls.

   • Inflow (I): Stormwater that enters sewers from unintended direct connections (yard drains, roof leaders, manhole covers).

1.2 Key Terms and Relationships

1. Dry Weather Flow (DWF): The flow of wastewater in sewers during periods of dry weather (no storm runoff). It consists of domestic + industrial + infiltration.

2. Wet Weather Flow (WWF): The total flow in sewers during rainfall, comprising DWF + stormwater inflow.

3. Average Daily Flow: The total volume of wastewater generated per day, averaged over a year.

4. Peak Factor: The ratio of maximum flow to average flow. Used to size sewers for peak conditions.

1.3 Estimation Methods

The quantity of sanitary sewage is directly related to the water consumption.

1. Average Rate of Water Supply (Per Capita Demand):

   • Typically, 80-85% of the water supplied becomes wastewater. This accounts for losses in consumption (e.g., gardening, evaporation).

   • If water supply is 200 LPCD, wastewater generation is: $200 \times 0.85 = 170 \, \text{LPCD}$.

2. Accounting for Infiltration/Inflow:

   • Infiltration is estimated based on pipe material, length, and groundwater conditions. Common design allowance: 5-10% of average DWF or a fixed rate (e.g., 0.15 L/s/ha).

   • Inflow is highly variable and should be minimized through proper design and construction.

3. Peak Flow Calculation:

   • For design, sewers must carry the maximum instantaneous flow.

   • Peak Factor (PF) formulas are empirical. A common one (Babbitt's formula): $\text{PF} = \frac{5}{P^{0.2}}$ where $P$ is the contributing population in thousands.

   • Simplified Rule: For small communities (< 20,000), PF ≈ 3-4. For large cities (> 500,000), PF ≈ 2-2.5.

   • Design Flow (Q) for a separate sanitary sewer: $Q_{\text{design}} = \text{Peak Factor} \times \text{Average Domestic Flow} + \text{Industrial Flow} + \text{Infiltration}$

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2. Sewerage System and Types

2.1 Separate System

• Description: Uses two entirely independent networks of pipes.

  • Sanitary Sewers: Carry only domestic and industrial wastewater.

  • Storm Sewers: Carry only surface runoff (rainwater).

• Advantages:

  • Smaller sanitary sewer sizes (carry only sewage).

  • No dilution of sewage, making treatment more efficient and economical.

  • Stormwater can be discharged directly to natural water bodies with minimal treatment.

• Disadvantages:

  • Higher initial cost due to two separate pipe networks.

  • Risk of wrong connections (stormwater into sanitary sewer or vice-versa).

2.2 Combined System

• Description: Uses a single network of pipes to carry both sanitary sewage and stormwater runoff.

• Advantages:

  • Single set of pipes, lower construction cost for piping.

  • Efficient flushing action during rains due to high flow.

• Disadvantages:

  • Massive pipe sizes required to carry wet weather flow.

  • Sewage is highly diluted, increasing treatment volume and cost.

  • During heavy rains, combined sewer overflows (CSOs) discharge untreated sewage directly to rivers, causing pollution.

• Modern Preference: The Separate System is now almost universally preferred for new developments due to its environmental and treatment efficiency.

2.3 Partially Separate System

• Description: A compromise. A single sewer carries sewage and part of the stormwater (typically from roofs and yards). Another separate system carries the remaining stormwater (from streets).

• Rationale: Roof water is relatively clean and can help in flushing the sewers.

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3. Design Criteria of Sewers

The hydraulic design of sewers is governed by the need for self-cleansing velocity to prevent sedimentation and blockage.

3.1 Flow Formula

Sewers are designed as open channels (not flowing full under normal conditions). The Manning's Formula is universally used:

$V = \frac{1}{n} R^{2/3} S^{1/2}$

Where,

• $V$ = Velocity of flow (m/s)

• $n$ = Manning's roughness coefficient

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

• $S$ = Slope of the sewer (energy gradient, m/m)

3.2 Key Design Parameters

1. Minimum (Self-Cleansing) Velocity:

   • To prevent deposition of solids, a minimum velocity must be maintained even at minimum flow (usually at the start of the design period).

   • For sanitary sewers: 0.6 m/s (or 0.75 m/s as per some codes).

   • For storm sewers: 0.75 - 1.0 m/s (to carry grit and sand).

2. Maximum Velocity:

   • To prevent erosion and abrasion of the pipe interior.

   • For concrete/stoneware pipes: 2.5 - 3.0 m/s.

   • For PVC/DI pipes: 4.0 - 6.0 m/s.

3. Minimum Pipe Diameter:

   • To prevent clogging. For public sanitary sewers, the minimum internal diameter is typically 150 mm (6 inches). For house connections, it can be 100 mm.

4. Depth of Cover (Burial Depth):

   • Minimum cover to protect the sewer from surface loads and frost penetration. Typically 1.0 to 1.5 m.

5. Slope (Gradient):

   • Determined by solving Manning's formula to achieve self-cleansing velocity at the expected full flow or half-full flow condition.

   • Slope must also follow the general ground gradient to minimize excavation.

6. Flow Depth (Partial Flow Conditions):

   • Sewers are designed to flow full or half-full at peak design flow.

   • They typically flow at 0.5d to 0.8d depth (d = diameter) under average daily flow conditions.

3.3 Hydraulic Elements of Circular Sewers

For a circular pipe flowing partially full, relationships for velocity (V) and discharge (Q) relative to full flow conditions (V_f, Q_f) are used from partial flow diagrams/tables.

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4. Shapes of Sewers and Sewer Materials

4.1 Shapes of Sewers

1. Circular (Most Common):

   • Advantages: Uniform stress distribution, maximum hydraulic radius for a given area, easy to manufacture and install.

   • Used for almost all sizes.

2. Egg-Shaped:

   • Advantage: Provides good self-cleansing velocity even at low flows (the smaller bottom section accelerates the flow).

   • Disadvantage: Complex construction and formwork.

   • Historically used in combined systems, now less common.

3. Horse-Shoe Shaped: Used for very large sewers and culverts (often in-situ concrete).

4. Rectangular/Basket-Handle: Used for large storm drains and culverts.

4.2 Sewer Materials

Selection depends on strength, durability, corrosion/abrasion resistance, and cost.

Material	Key Properties	Common Use
Vitrified Clay Pipe (VCP)	Excellent corrosion resistance (inert), good abrasion resistance, rigid.	Small to medium diameter sanitary sewers.
Reinforced Concrete Pipe (RCP)	High strength, can be cast in large diameters, economical for large projects.	Large diameter storm and trunk sewers.
PVC (uPVC)	Lightweight, smooth interior (high n value ~0.009), corrosion-proof, easy to install.	Mainly for house connections and lateral sewers.
Ductile Iron (DI)	Very high strength and impact resistance, can be used under heavy loads, expensive.	Where high internal pressure or external loads exist (force mains, under railways).
High-Density Polyethylene (HDPE)	Flexible, excellent joint integrity (welded), good chemical resistance, good for trenchless tech.	Long pipelines, river crossings, rehab.
Brick or Stoneware Sewer	Used for large non-circular sewers constructed in-situ (now largely replaced by RCP).	Old construction, large storm drains.

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5. Design of Sewers for Separate and Combined Systems

5.1 Design Steps (Separate Sanitary Sewer)

1. Prepare Plan and Profiles: Obtain a contour map of the area. Mark the outfall (treatment plant) location.

2. Delineate Drainage Areas: Divide the total area into smaller sub-catchments for each sewer branch.

3. Layout the Network: Draw the proposed alignment of sewers (usually along the center of roads). Follow the natural slope of the ground.

4. Number all Manholes: These are placed at every change in direction, slope, pipe size, or at regular intervals (max 100-150 m).

5. Calculate Cumulative Contributing Population and Area for each pipe segment.

6. Estimate Flows:

   • Average Domestic Flow = Population × Per Capita Wastewater Flow.

   • Add Industrial Flow and Infiltration.

   • Apply Peak Factor to domestic flow to get Peak Sanitary Flow.

   • Design Flow Q = Peak Sanitary + Industrial + Infiltration.

7. Assume a Trial Diameter (starting from min. 150 mm).

8. Determine Required Slope:

   • Using Manning's formula (with n for selected material), find the slope S required to achieve self-cleansing velocity (0.6 m/s) when the pipe is flowing full or half-full with the design flow Q.

   • Alternatively, for a given ground slope, calculate the diameter needed.

9. Check Velocities:

   • Velocity at Full Flow, V_f: Using the designed D and S in Manning's.

   • Velocity at Minimum Flow (Q_min): Use partial flow charts to find depth and velocity at Q_min. Ensure it is ≥ 0.6 m/s.

   • Ensure V_f is less than the maximum allowable.

10. Finalize Design: Adjust diameter and slope iteratively to meet all criteria while following the ground profile as closely as possible. Create a sewer profile drawing showing pipe invert levels, ground levels, and slopes.

5.2 Design of Combined Sewers

The process is similar, but the Design Flow (Q) is vastly different:

$Q_{\text{combined}} = \text{Peak Sanitary Flow} + \text{Runoff from Design Storm}$

1. Storm Runoff Calculation: Done using the Rational Formula: $Q_{\text{runoff}} = C \cdot I \cdot A$ where C is the runoff coefficient, I is the design rainfall intensity (mm/hr) for a storm duration equal to the time of concentration, and A is the tributary area.

2. Design Storm Frequency: Typically a 5 to 10-year return period storm is used.

3. The combined sewer is then sized using Manning's formula to carry this total Q_combined, resulting in much larger diameters than a separate sanitary sewer.

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6. Construction of Sewers and Sewer Appurtenances

6.1 Construction Process

1. Trench Excavation:

   • Trenches must be wide enough for pipe laying and jointing.

   • Sloping or shoring is required for deep trenches to prevent collapse.

2. Bedding and Foundation:

   • A stable, uniform foundation is prepared. For rigid pipes (RCP, VCP), a granular bedding (sand or gravel) is used to distribute loads.

3. Laying and Jointing:

   • Pipes are laid from the downstream end upwards.

   • Joints must be watertight to prevent exfiltration (sewage leaking out) and infiltration (groundwater leaking in).

   • Joint types: Rubber gasket (push-on), mortar joints, welded (HDPE), solvent-welded (PVC).

4. Testing:

   • Water Test: Section is plugged and filled with water to check for leaks.

   • Air Test: Low-pressure air is applied; pressure drop indicates leaks.

   • Mandrel Test: A dummy mandrel is pulled through to check for deformation.

5. Backfilling:

   • Done in layers with selected material, compacted properly to avoid future settlement and pipe damage.

6.2 Sewer Appurtenances

These are structures built along the sewer line to facilitate inspection, cleaning, ventilation, and flow regulation.

1. Manholes:

   • Purpose: Provide access for inspection, cleaning, and maintenance. Located at every change in direction, gradient, pipe size, and at junctions.

   • Components: Cover and frame (at ground level), chimney, bench, and channel (invert).

   • Spacing: Not more than 100-150 m for sewers up to 1 m diameter.

2. Drop Manholes:

   • Used when an incoming sewer is significantly higher (≥0.5 m) than the outgoing sewer. A vertical drop pipe is provided to avoid splashing and erosion.

3. Lamp Holes:

   • Small-diameter shafts with covers, used for lowering a lamp to inspect sewers between manholes (cheaper than manholes).

4. Cleanouts:

   • Fitted with a removable plug, installed at the head of a lateral sewer for rodding access.

5. Inverted Siphon (Depressed Sewer):

   • A sewer section that dips below the hydraulic grade line to cross an obstacle (river, valley, other utility). It runs full under pressure.

   • Requires multiple barrels and flushing arrangements to prevent sedimentation.

6. Grease and Sand Traps:

   • Intercept grease, oil, and grit from industrial or commercial establishments before they enter the public sewer.

7. Ventilating Shafts:

   • To release foul and potentially explosive gases ($\text{H}_2\text{S}, \text{CH}_4$) from the sewer to the atmosphere.

Conclusion: The design and construction of sewers is a meticulous engineering task that integrates hydraulics, materials science, and geotechnical principles. The objective is to create a gravity-driven, self-cleansing, and maintenance-friendly underground network. A well-executed sewerage system is a long-term asset that protects public health and the environment by ensuring the reliable conveyance of wastewater to treatment facilities.