CE 6001, Unit 2 Purification of water

 

2.1 Purification of Water

2.1.1 Objectives of Water Treatment:

Water from natural sources (rivers, lakes, wells) contains impurities like:

• Suspended solids (mud, sand),

• Microorganisms (bacteria, viruses),

• Dissolved salts and gases (like iron, fluoride, CO₂, etc.)

Main goals of treatment:

• Make water safe for human consumption.

• Remove taste and odor issues.

• Remove color, turbidity (cloudiness), and harmful chemicals.

• Meet drinking water quality standards (as per IS 10500).

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2.1.2 Aeration – Purpose and Methods

Why is aeration needed?

• To remove dissolved gases (like CO₂, H₂S) that cause bad taste or odor.

• To oxidize iron and manganese for easy removal.

• To increase dissolved oxygen, which helps in further purification.

Methods of Aeration:

1. Cascade Aerators: Water flows over steps or trays; oxygen mixes from air.

2. Fountain Aerators: Water is sprayed into the air, like a fountain.

3. Diffused Aeration: Air is pumped through perforated pipes at the tank bottom.

4. Mechanical Aerators: Blades or paddles stir water to mix air.

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2.2 Sedimentation

Sedimentation is the process of removing suspended particles by allowing them to settle under gravity.

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2.2.1 Plain Sedimentation

• Water is stored in a tank without chemicals.

• Heavy particles settle down slowly due to gravity.

• Efficiency depends on detention time and tank design.

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2.2.2 Sedimentation with Coagulation

• Fine particles and colloids (very small particles) do not settle easily.

• A coagulant is added to form flocs (sticky clumps) which settle faster.

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2.2.3 Types of Coagulants

1. Alum (Aluminum Sulphate) – most commonly used.

2. Ferric Chloride

3. Ferrous Sulphate

4. Polyelectrolytes – synthetic chemicals for better floc formation.

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2.2.4 Jar Test

• A lab test used to find optimum dose of coagulant.

• Steps:

  1. Take multiple jars of same water sample.

  2. Add increasing doses of coagulant.

  3. Stir and allow settling.

  4. Observe which jar gives best clarity.

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2.2.5 Process of Coagulation

1. Rapid Mixing: Coagulant is added and mixed quickly to disperse evenly.

2. Flocculation: Slow mixing allows particles to form bigger flocs.

3. Sedimentation: Flocs settle down in sedimentation tank.

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2.2.6 Types of Sedimentation Tanks

1. Rectangular Tank – water flows horizontally.

2. Circular Tank – water enters at center and flows out from the edge.

3. Upflow Tank – water moves from bottom to top.

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2.2.7 Clariflocculator

• Combined clarifier and flocculator in one unit.

• Saves space and cost.

• Used in large water treatment plants.

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2.3 Filtration

After sedimentation, some fine particles and microorganisms still remain. These are removed by filtration.

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2.3.1 Mechanism of Filtration

• Water passes through layers of sand, gravel, and stones.

• Dirt particles are trapped in the pores of sand.

• Some biological action also removes bacteria.

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2.3.2 Classification of Filters

2.3.2.1 Slow Sand Filter

• Rate: 0.1 – 0.4 m³/m²/hr (very slow).

• Media: Fine sand.

• Feature: Biological layer (schmutzdecke) forms on top, removes bacteria.

• Cleaning: Top sand layer is scraped off after clogging.

2.3.2.2 Rapid Sand Filter

• Rate: 5 – 15 m³/m²/hr (much faster).

• Media: Coarser sand and gravel.

• Backwashing: Cleaning by reversing flow of water.

• Used in: Urban and municipal plants.

2.3.2.3 Pressure Filter

• Closed steel tank; water is forced under pressure.

• Used in industries or where space is limited.

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2.3.3 Construction and Working

2.3.3.1 Slow Sand Filter

• Open tank with layers:

  • Supernatant water

  • Sand bed (60–90 cm)

  • Gravel bed (30–60 cm)

• Water enters from top, passes through layers, and gets collected at the bottom.

2.3.3.2 Rapid Sand Filter

• Includes:

  • Inlet and outlet pipes

  • Filter bed of coarse sand

  • Underdrain system

  • Backwashing system (for cleaning)

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2.3.4 Operational Problems in Filtration

• Clogging: Reduced flow due to trapped particles.

• Air binding: Air pockets prevent water from passing.

• Uneven flow: Leads to improper cleaning or leakage of impurities.

• Need for frequent backwashing in rapid sand filters.

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2.3.5 Disinfection

2.3.5.1 Objectives

• Kill harmful bacteria, viruses, and pathogens.

• Ensure safe water in the distribution system.

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2.3.5.2 Methods of Disinfection

1. Chlorination – most widely used.

2. Ozonation – effective but expensive.

3. Ultraviolet (UV) rays – kills bacteria quickly.

4. Boiling – for household use.

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2.3.5.3 Chlorination in Detail

2.3.5.3.1 Application of Chlorine

• Applied as:

  • Gas (Cl₂)

  • Bleaching powder (Calcium hypochlorite)

  • Liquid sodium hypochlorite

2.3.5.3.2 Forms of Chlorination

1. Pre-chlorination: Before filtration, controls algae and odor.

2. Post-chlorination: After treatment, for final disinfection.

3. Break-point chlorination: Complete oxidation of impurities.

4. Super chlorination: High dose in emergency (flood, outbreak).

2.3.5.3.3 Types of Chlorination Practices

• Continuous

• Intermittent

• Emergency

• Shock chlorination

2.3.5.3.4 Residual Chlorine and Its Importance

• Small amount (0.2 to 0.5 mg/L) left in water after chlorination.

• Prevents future contamination during storage and transport.

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2.3.5.3.5 Flow Diagram of Water Treatment Plant

Source of Raw Water
       ↓
Intake Structure
       ↓
Screening (removes large debris)
       ↓
Aeration (removes gases, oxidizes metals)
       ↓
Coagulation & Flocculation (forms flocs)
       ↓
Sedimentation (flocs settle down)
       ↓
Filtration (removes fine particles & microbes)
       ↓
Disinfection (kills bacteria & viruses)
       ↓
Storage Reservoir
       ↓
Distribution to Consumers