In ECET 2026 Chemistry, Wastewater Treatment is an important topic under Environmental Chemistry. It deals with removing contaminants from used water before releasing it back to nature or reusing it.
📘 Concept Notes
💧 What is Wastewater?
- Wastewater is water that has been used in homes, industries, or agriculture and contains impurities (organic, inorganic, and microbial).
- Examples: Domestic sewage, industrial effluents, agricultural runoff.
🧪 Composition of Wastewater
- Organic matter: Proteins, fats, carbohydrates, urea, etc.
- Inorganic salts: Nitrates, phosphates, chlorides, sulfates.
- Pathogens: Bacteria, viruses, protozoa.
- Suspended solids: Soil, sand, clay particles.
⚙️ Stages of Wastewater Treatment
Wastewater treatment is carried out in three main stages:
1️⃣ Primary Treatment (Physical Process)
- Purpose: Remove large solids and floating materials.
- Steps:
- Screening: Removes plastics, rags, and large debris.
- Sedimentation: Heavy solids settle at the bottom forming sludge.
- Floatation: Oils and grease float and are skimmed off.
No chemical reaction occurs here – purely mechanical.
2️⃣ Secondary Treatment (Biological Process)
- Purpose: Remove dissolved and colloidal organic matter by microbial action.
- Methods:
- Activated Sludge Process:
- Air is pumped into wastewater to promote growth of aerobic bacteria.
- Bacteria convert organic matter into
and water.
- Activated Sludge Process:
Trickling Filter Method:
- Wastewater trickled over stones covered with microbial film.
- Microbes degrade organic matter.
3️⃣ Tertiary Treatment (Chemical & Advanced Stage)
- Purpose: Remove remaining fine particles, dissolved salts, nutrients, and microbes.
- Methods include:
- Chemical coagulation: Alum or lime added to remove fine particles.
- Chlorination:
kills remaining microorganisms. - Ozonation:
is used as a strong disinfectant. - Reverse Osmosis (RO): Removes dissolved ions and salts.
🧮 Important Formulas
- Biochemical Oxygen Demand (BOD):
Indicates amount of oxygen required by microorganisms to decompose organic matter in water.
where
= Dissolved oxygen initially,
= Dissolved oxygen after 5 days at 20°C.
Chemical Oxygen Demand (COD):
Measures oxygen required to chemically oxidize organic matter.
where
= Volume of titrant for blank,
= Volume of titrant for sample,
= Normality of titrant,
= Volume of sample in mL.
Percentage of Removal Efficiency:
where
= Initial concentration,
= Final concentration.
🌱 Example Calculation
If initial BOD = 
and final BOD = 
, then:
This means 80% of organic matter has been biologically removed.
🔟 10 Expected MCQs – ECET 2026
Q1. Wastewater mainly contains:
A) Pure water only
B) Organic and inorganic impurities
C) Only gases
D) Only dissolved salts
Q2. The first stage of wastewater treatment is:
A) Secondary treatment
B) Primary treatment
C) Tertiary treatment
D) Final polishing
Q3. The process of adding alum for removing suspended solids is called:
A) Coagulation
B) Filtration
C) Chlorination
D) Sedimentation
Q4. BOD represents:
A) Chemical oxidation capacity
B) Oxygen required by microbes
C) Chlorine concentration
D) COD value
Q5. Typical BOD value for domestic sewage is:
A) 1–2 mg/L
B) 20–30 mg/L
C) 200–300 mg/L
D) 1000 mg/L
Q6. In activated sludge process, air is supplied to:
A) Kill bacteria
B) Support aerobic bacteria
C) Precipitate salts
D) Remove metals
Q7. Chemical used in chlorination:
A)
B)
C) 
D)
Q8. The process that removes fine suspended and colloidal particles is:
A) Primary
B) Secondary
C) Tertiary
D) None
Q9. The device that kills microorganisms by oxidation is:
A) Sand filter
B) Ozonator
C) Sedimentation tank
D) Aeration chamber
Q10. Efficiency of treatment is determined by:
A) Color of water
B) BOD and COD values
C) Temperature
D) Hardness
✅ Answer Key
| Q.No | Answer |
|---|---|
| Q1 | B |
| Q2 | B |
| Q3 | A |
| Q4 | B |
| Q5 | C |
| Q6 | B |
| Q7 | B |
| Q8 | C |
| Q9 | B |
| Q10 | B |
🧠 Explanations
- Q1 → B: Wastewater contains both organic & inorganic impurities.
- Q2 → B: Primary stage removes solids physically.
- Q3 → A: Alum promotes coagulation.
- Q4 → B: BOD = oxygen required by microbes to decompose waste.
- Q5 → C: Domestic sewage has ~250 mg/L BOD.
- Q6 → B: Aeration supports aerobic bacteria.
- Q7 → B: Chlorine gas disinfects water.
- Q8 → C: Tertiary treatment removes fine solids.
- Q9 → B: Ozonator kills pathogens via oxidation.
- Q10 → B: BOD and COD show treatment efficiency.
🎯 Why Practice Matters
- Wastewater treatment questions are conceptual + formula-based.
- You can easily score 2–3 marks by remembering stages, formulas (BOD, COD), and processes.
- Also useful for Environmental Engineering & Industrial Chemistry.
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