CE 6001 Unit 5 Characteristics and treatment of Sewage

 

🔷 5.1 Analysis of Sewage

✅ 5.1.1 Characteristics of Sewage

Sewage contains both physical, chemical, and biological characteristics:

• Physical Characteristics:

  • Color: Fresh sewage is grey; old is black.

  • Odor: Smells bad due to decomposition.

  • Temperature: Slightly higher than atmospheric.

  • Solids: Suspended, dissolved, and floating solids.

• Chemical Characteristics:

  • pH: Normally between 6 to 8.

  • Organic Matter: Includes fats, proteins, carbohydrates.

  • Inorganic Matter: Salts, minerals, metals.

• Biological Characteristics:

  • Contains bacteria, viruses, protozoa, and parasites.

  • Microorganisms decompose organic waste.

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✅ 5.1.2 B.O.D. (Biochemical Oxygen Demand) and its Significance

• B.O.D. is the amount of oxygen required by microorganisms to decompose organic matter in sewage.

• Measured in mg/L over 5 days at 20°C.

• High BOD = High pollution.

• Used to measure strength of sewage and to design treatment plants.

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✅ 5.1.3 C.O.D. (Chemical Oxygen Demand) and its Significance

• C.O.D. is the amount of oxygen required to chemically oxidize all organic and inorganic matter.

• It gives faster results than BOD.

• Higher COD indicates more chemical pollution.

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✅ 5.1.4 CPCB Norms (Central Pollution Control Board)

• CPCB sets limits for the discharge of treated sewage into rivers or land.

• Common norms:

  • BOD ≤ 30 mg/L

  • Suspended Solids ≤ 50 mg/L

  • pH between 6.5 to 9.0

• Ensures the treated water does not harm the environment.

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✅ 5.1.5 Objectives of Sewage Treatment

1. Remove suspended solids.

2. Reduce BOD and COD.

3. Eliminate harmful pathogens.

4. Make sewage safe for disposal or reuse.

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✅ 5.1.6 Flow Diagram of Conventional Sewage Treatment Plant

1. Screening →

2. Grit Removal →

3. Primary Sedimentation →

4. Biological Treatment (Aerobic/Anaerobic) →

5. Secondary Sedimentation →

6. Sludge Treatment & Disposal →

7. Treated Effluent Discharge

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🔷 5.2 Treatment of Sewage (Introduction to Terms)

✅ 5.2.1 Screening

• First stage in treatment.

• Removes large objects like plastics, rags, sticks using metal bars or screens.

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✅ 5.2.2 Types of Screens

• Coarse Screen: Large openings for big objects.

• Fine Screen: Small openings to remove finer particles.

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✅ 5.2.3 Grit Removal

• Removes heavy inorganic particles like sand, stones.

• Grit chambers slow the flow to allow these particles to settle.

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✅ 5.2.4 Skimming

• Removes floating materials such as oil, grease, and soap foam.

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✅ 5.2.5 Sedimentation of Sewage

• Sewage is held in large tanks to allow solids to settle by gravity.

• These settled solids are called sludge.

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✅ 5.2.6 Aerobic and Anaerobic Process

• Aerobic Process: Uses oxygen to decompose organic matter (e.g., activated sludge process).

• Anaerobic Process: Occurs in the absence of oxygen (e.g., sludge digestion).

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✅ 5.2.7 Sludge Digestion

• Anaerobic bacteria break down organic matter in sludge.

• Reduces sludge volume and produces biogas.

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✅ 5.2.8 Trickling Filters

• Circular beds with stones or plastic media.

• Sewage trickles over the media; aerobic bacteria grow and consume organic matter.

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✅ 5.2.9 Activated Sludge Process

• Air is pumped into aeration tanks.

• Bacteria form flocs that eat waste.

• Mixture is then settled; sludge is recycled.

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✅ 5.2.10 Disposal of Sewage

• Treated sewage can be:

  • Discharged into rivers.

  • Used for irrigation.

  • Reused for industrial purposes.

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✅ 5.2.11 Oxidation Pond

• Shallow ponds where sewage is treated by sunlight, algae, and bacteria.

• Used in rural and semi-urban areas.

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✅ 5.2.12 Oxidation Ditch

• A type of aerated pond shaped like a ring.

• Provides continuous biological treatment with slow stirring and aeration.

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✅ 5.2.13 Septic Tank

• Underground tank where domestic sewage is partially treated.

• Solids settle, and anaerobic digestion occurs.

• Common in rural homes and small buildings.

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✅ 5.2.14 Recycling and Reuse of Domestic Waste

• Treated water can be used for:

  • Flushing toilets.

  • Watering plants.

  • Cooling in industries.

• Helps conserve water and reduce pollution.