Concept Notes (Deep Explanation + Examples)
🌍 Introduction
Sewage treatment is one of the most important topics in Environmental Engineering for ECET 2026 Civil. It deals with the removal of contaminants from domestic and industrial wastewater before it is released into natural water bodies or reused.
Untreated sewage contains:
- Suspended solids (sand, silt, organic matter)
- Dissolved organic matter (BOD/COD)
- Pathogens (bacteria, viruses)
- Nutrients (nitrogen, phosphorus)
If untreated sewage is discharged into rivers or lakes, it causes pollution, oxygen depletion, and health hazards.
🏗️ Stages of Sewage Treatment
There are three main stages in sewage treatment:
1️⃣ Primary Treatment (Physical Treatment)
This stage removes floating, settleable solids using physical processes.
- Screening: Removes large floating materials like plastics, rags, etc.
- Grit Chamber: Removes sand and gravel.
- Primary Sedimentation Tank: Removes settleable solids (about 60–70% suspended solids).
🧱 Example: In municipal sewage, almost 30–40% of BOD is removed after primary treatment.
2️⃣ Secondary Treatment (Biological Treatment)
This stage removes dissolved and colloidal organic matter using microorganisms.
👉 Common biological processes:
- Activated Sludge Process (ASP):
- Aeration tank → Secondary clarifier → Return activated sludge
- Removes 85–90% BOD.
- Trickling Filter:
- Fixed media (stones/plastic) → biofilm grows → organic matter oxidized.
- Oxidation Pond:
- Large shallow ponds → natural purification using algae and bacteria.
🧩 Real-life Example: In Vijayawada, secondary treatment plants use ASP with mechanical aerators for municipal wastewater.
3️⃣ Tertiary Treatment (Advanced Treatment)
Used when very high-quality effluent is required (e.g., reuse, discharge to sensitive water bodies).
Includes:
- Filtration (sand/carbon)
- Disinfection (chlorination, UV, ozonation)
- Nutrient removal (phosphorus, nitrogen)
💧 Example: After tertiary treatment, water can be reused for gardening, cooling towers, or construction.
⚗️ Sludge Treatment & Disposal
- Thickening → Digestion → Dewatering → Disposal
- Anaerobic digestion produces biogas, which can be used for energy.
📏 BOD & COD
- BOD (Biochemical Oxygen Demand): Oxygen required by microorganisms to decompose organic matter.
- COD (Chemical Oxygen Demand): Oxygen required to oxidize organic matter chemically.
- Typical ratio:
📈 Typical Efficiency
| Stage | BOD Removal (%) |
|---|---|
| Primary | 30–40% |
| Secondary | 85–90% |
| Tertiary | 95–99% |
📚 Exam-Important Points
- Screening → removes floating matter
- Grit chamber → removes heavy inorganic particles
- BOD test duration: 5 days at 20°C
- Sludge digestion → anaerobic process
- Disinfection → last step in sewage treatment
- Effluent standard: BOD < 30 mg/L for discharge
⚙️ Formulas (Plain LaTeX Only — No Boxes)
🔟 10 MCQs (GATE + ECET Mix)
- Screening in sewage treatment is used to remove:
A) Dissolved solids
B) Floating materials
C) Colloidal particles
D) Organic matter - The BOD test is conducted for how many days at 20°C?
A) 2 days
B) 3 days
C) 5 days
D) 7 days - The main purpose of a grit chamber is to:
A) Remove floating matter
B) Remove heavy inorganic solids
C) Remove organic solids
D) Aerate sewage - Which process removes dissolved organic matter biologically?
A) Screening
B) Sedimentation
C) Activated sludge process
D) Filtration - The typical BOD removal efficiency of the primary treatment is:
A) 10–20%
B) 30–40%
C) 50–60%
D) 70–80% - Trickling filters are used for:
A) Physical treatment
B) Chemical treatment
C) Biological treatment
D) Disinfection - Anaerobic sludge digestion produces:
A) Carbon dioxide only
B) Methane gas (biogas)
C) Oxygen
D) Nitrogen - The final stage of sewage treatment is:
A) Screening
B) Disinfection
C) Aeration
D) Sludge digestion - The ratio of BOD/COD for biodegradable waste is approximately:
A) 0.1
B) 0.3
C) 0.6
D) 0.9 - The efficiency of BOD removal in activated sludge process is about:
A) 40–50%
B) 60–70%
C) 80–90%
D) 95–100%
✅ Answer Key
Q.No Answer
1 B
2 C
3 B
4 C
5 B
6 C
7 B
8 B
9 C
10 C
🧠 Explanations (Step-by-Step)
1️⃣ B → Screening removes large floating materials like rags, paper, and plastics. Others are dissolved or colloidal solids.
2️⃣ C → BOD is measured over 5 days at 20°C, the standard ECET and IS method.
3️⃣ B → Grit chambers remove heavy inorganic solids like sand and gravel to protect pumps.
4️⃣ C → ASP uses microorganisms to oxidize dissolved organic matter biologically.
5️⃣ B → Primary treatment removes 30–40% BOD by physical sedimentation.
6️⃣ C → Trickling filters are biological treatment units using biofilm oxidation.
7️⃣ B → Anaerobic digestion produces methane-rich biogas used as energy.
8️⃣ B → Disinfection (chlorination/UV) is the final step to kill pathogens.
9️⃣ C → BOD/COD ≈ 0.6 indicates good biodegradability.
10️⃣ C → Activated sludge process achieves 80–90% BOD removal.
🎯 Motivation / Why Practice Matters (ECET 2026 Civil)
Sewage treatment is a repeated ECET topic every year because it combines concepts + numericals + practical understanding.
Mastering this helps in Environmental, Public Health, and Site Engineering.
Remember — understanding “how sewage is cleaned” connects classroom theory to real municipal projects.
Keep practicing and revising — concept clarity + consistency = Top Rank in ECET 2026!
📲 CTA
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