Wastewater Engineering Notes for SSC JE & RRB JE – Complete Junior Engineer Civil Preparation Guide

 

1️⃣ Introduction of the Chapter

🔹 Importance in JE/AE Exams

• Covers 8–15% weightage in SSC JE, RRB JE, State AE.

• 3–6 direct theory questions + 1–2 numericals expected.

• Frequently asked: BOD, COD, Manning’s formula, Activated Sludge Process.

🔹 Real-Life Engineering Application

• Urban drainage planning

• STP (Sewage Treatment Plant) design

• River pollution control

• Smart city sanitation projects

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🟦 SEWERAGE SYSTEMS

Types of Sewerage Systems

1. Separate System

• Two separate pipes:

  • Sanitary sewage

  • Storm water

✔ Economical treatment

❌ High initial cost

2. Combined System

• Single pipe carries both sewage + storm water
  ✔ One pipe system
  ❌ Very large size required

3. Partially Separate System

• Small storm water allowed in sanitary sewer.

🔑 Exam Trick:
Separate → Two Pipes
Combined → One Pipe
Partial → Mix Type

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Dry Weather Flow (DWF)

$$\text{DWF} = \text{Average Sewage Flow} + \text{Infiltration}$$

• Occurs in absence of rainfall

• Used for sewer design

Storm Water Flow (Rational Method)

$$Q = C I A$$

Where:
Q = discharge (m³/s)
C = runoff coefficient
I = rainfall intensity
A = area (hectare)

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🟦 DESIGN OF SEWERS

Manning’s Formula (Most Important)

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

Where:
V = velocity (m/s)
R = hydraulic radius
S = slope
n = Manning’s coefficient

✔ Valid for turbulent flow
✔ Most used in JE exams

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Chezy’s Formula

$$V = C \sqrt{RS}$$

C = Chezy’s constant

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Self-Cleansing Velocity

• Minimum velocity to prevent sedimentation

• 0.6 – 0.9 m/s (commonly asked)

🔑 Trick: “0.6 keeps sewer clean

Shape	Use
Circular	Most common
Egg-shaped	Combined sewer
Horseshoe	Large tunnels

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🟦 QUALITY & CHARACTERISTICS OF SEWAGE

BOD (Biochemical Oxygen Demand)

• Oxygen required by microorganisms

• Standard: 5 days at 20°C

$$BOD_5 = L_0 (1 - e^{-kt})$$

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COD (Chemical Oxygen Demand)

• Oxygen required for chemical oxidation

• Always COD > BOD

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ThOD (Theoretical Oxygen Demand)

• Based on chemical formula

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Relative Stability

$$\text{Relative Stability} = \frac{DO}{BOD}$$

🟦 SEWAGE TREATMENT

Primary Treatment

• Screening

• Grit chamber

• Skimming tank

• Sedimentation

Removal efficiency: 25–35% BOD

Secondary Treatment

1. Activated Sludge Process (ASP)

•     Aeration tank + Secondary clarifier
•     Uses microorganisms

2. Trickling Filter

Fixed media biological system

3. Oxidation Pond

Natural treatment

Large land required

Sludge Digestion

• Anaerobic digestion

• Produces methane gas

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🟦 SEWAGE DISPOSAL

1. Dilution

• Discharge into rivers

• Requires high DO

2. Land Disposal (Sewage Farming)

• Used as irrigation water

• Provides nutrients

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📌 Important Definitions (Exam-Oriented)

• BOD: Oxygen required by microorganisms to stabilize organic matter in 5 days at 20°C.

• Self-cleansing velocity: Minimum velocity preventing deposition.

• DWF: Sewage flow during dry weather.

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📌 Quick Formula Sheet

$$V = \frac{1}{n} R^{2/3} S^{1/2}$$ $$V = C \sqrt{RS}$$ $$Q = CIA$$ $$BOD_t = L_0 (1 - e^{-kt})$$

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📌 20 Important MCQ Concept Questions

1. COD is always greater than?

2. Self-cleansing velocity value?

3. Egg-shaped sewer used in?

4. BOD test temperature?

5. DWF includes?

6. Manning’s ‘n’ depends on?

7. Activated sludge requires?

8. Trickling filter is?

9. Relative stability depends on?

10. Sludge digestion is?

11. Oxidation pond type?

12. Most economical system?

13. Peak factor in DWF?

14. Chezy constant depends on?

15. Anaerobic process product?

16. Minimum sewer slope criteria?

17. Primary treatment removes?

18. Storm flow formula?

19. Hydraulic radius definition?

20. Combined system disadvantage?

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🟨 FINAL QUICK REVISION SHEET

✔ BOD test → 5 days @ 20°C
✔ COD > BOD
✔ Self-cleansing velocity = 0.6 m/s
✔ Manning’s most important
✔ Egg-shaped → Combined sewer
✔ Primary removes 30% BOD
✔ ASP → Most efficient secondary method