12.2 Treatment of Domestic Water Supply

This topic explains how raw water is treated to make it safe for drinking and domestic use.

Definition

The treatment of domestic water supply involves several processes to ensure that water is safe for human use.

Simple explanation: Water treatment makes dirty water clean and safe to drink.

Main Idea

Raw water is treated in a municipal water purification plant to make it fit for drinking and domestic purposes.

Various stages in this treatment are:

• Sedimentation — suspended matter sinks to the bottom
• Coagulation — chemicals are added to remove fine particles
• Filtration — water is passed through sand and gravel
• Chlorination — chlorine is added to kill bacteria

Simple explanation: Water goes through four main steps: settle, clump, filter, and disinfect.

Core Concept

The Four Stages of Water Treatment

Detailed Explanation

Stage 1: Sedimentation

Sedimentation is the process in which water is allowed to stand in a reservoir. The suspended matter sinks to the bottom.

Simple explanation: Water is left to sit still so that heavy particles settle at the bottom.

• Removes large suspended particles (sand, silt, mud).
• Prepares water for the next stage.

Stage 2: Coagulation

Coagulation is the process in which water is treated with slaked lime and alum. These materials react to form a gelatinous mass of aluminium hydroxide.

Equation from the book:

3Ca(OH)_2(aq) + Al₂(SO₄)_3(aq) → 2Al(OH)_3(s) + 3CaSO_4(aq)

• Slaked lime = calcium hydroxide, Ca(OH)₂
• Alum = aluminium sulphate, Al₂(SO₄)₃
• They react to form aluminium hydroxide, Al(OH)₃ — a gelatinous (jelly-like) mass.
• This gelatinous mass traps fine particles that are too small to settle on their own.
• The mass then settles to the bottom, carrying the fine particles with it.

Simple explanation: Chemicals are added to form a jelly-like substance that grabs tiny particles and sinks.

Stage 3: Filtration

The water is then filtered through sand and gravel. Sometimes it is filtered through charcoal to remove coloured and odorous compounds.

Simple explanation: Water is passed through layers of sand, gravel, and sometimes charcoal to remove remaining particles, colour, and odour.

• Removes remaining suspended particles.
• Removes colour and odour (with charcoal).
• Makes water clear.

Stage 4: Chlorination

In the final step, chlorine is added to kill any remaining bacteria.

Equation from the book:

Cl_2(aq) + H₂O_(l) → HClO_(aq) + H⁺_(aq) + Cl^-_(aq)

• Chlorine (Cl₂) reacts with water to form hypochlorous acid (HClO).
• Hypochlorous acid kills bacteria.
• This is the disinfection step.

Simple explanation: Chlorine is added to kill any remaining bacteria and make the water safe to drink.

Flow Diagram of Water Treatment

The book gives a flow diagram:

Raw Water → Sedimentation → Coagulation → Filtration → Chlorination → Pure Water

Simple explanation: Water flows through these stages one by one, becoming cleaner at each step.

Intermediate Level

Why Each Stage Is Needed

• Removes large particles that would clog filters.
• Simple and cheap first step.

• Removes fine particles that don't settle on their own.
• Alum and lime form a gelatinous mass (Al(OH)₃) that traps particles.

• Removes remaining particles.
• Charcoal removes colour and odour.
• Makes water clear and pleasant.

• Kills bacteria and viruses.
• Makes water safe for drinking.
• Protects against waterborne diseases.

The Chemistry of Coagulation

3Ca(OH)₂ + Al₂(SO₄)₃ → 2Al(OH)₃ + 3CaSO₄

• Slaked lime (Ca(OH)₂) + alum (Al₂(SO₄)₃) → aluminium hydroxide (Al(OH)₃) + calcium sulphate (CaSO₄)
• Al(OH)₃ is the gelatinous mass that traps fine particles.
• This is why the process is called coagulation — the particles "coagulate" (clump together).

Simple explanation: Lime and alum react to form a jelly that grabs tiny particles.

The Chemistry of Chlorination

Cl₂ + H₂O → HClO + H⁺ + Cl^-

• Chlorine (Cl₂) reacts with water to form hypochlorous acid (HClO).
• HClO is a strong disinfectant that kills bacteria.
• This is why chlorine is used in swimming pools and drinking water.

Simple explanation: Chlorine forms hypochlorous acid, which kills bacteria.

Why Chlorination Is Important

• Kills bacteria and viruses — prevents waterborne diseases.
• Prevents diseases like cholera, typhoid, dysentery, and hepatitis.
• Chlorination is not effective against all viruses — the book notes that it is not effective against viruses such as those that cause hepatitis.

Simple explanation: Chlorine kills most bacteria, but not all viruses — so water treatment must be done carefully.

Additional Treatment Methods

• Aeration — to remove dissolved gases and improve taste.
• Reverse osmosis — to remove dissolved salts and impurities.
• UV treatment — to kill bacteria and viruses.
• Ozonation — using ozone to disinfect water.

Simple explanation: Different methods are used depending on the water quality and what needs to be removed.

Important Points

• - Raw water is treated in a municipal water purification plant.
• - Four main stages:
• 1. Sedimentation — suspended matter sinks to the bottom.
• 2. Coagulation — water is treated with slaked lime and alum to form a gelatinous mass of aluminium hydroxide.
• 3. Filtration — water is filtered through sand and gravel (sometimes charcoal).
• 4. Chlorination — chlorine is added to kill any remaining bacteria.
• - Coagulation equation:
• - 3Ca(OH)₂(aq) + Al₂(SO₄)₃(aq) → 2Al(OH)₃(s) + 3CaSO₄(aq)
• - Chlorination equation:
• - Cl₂(aq) + H₂O(l) → HClO(aq) + H⁺(aq) + Cl⁻(aq)
• - Flow: Raw Water → Sedimentation → Coagulation → Filtration → Chlorination → Pure Water.
• - Chlorination is not effective against all viruses (e.g., hepatitis).

Remember

• - Four stages: Sedimentation, Coagulation, Filtration, Chlorination.
• - Sedimentation → particles sink.
• - Coagulation → lime + alum → gelatinous Al(OH)₃.
• - Filtration → sand, gravel, charcoal.
• - Chlorination → chlorine kills bacteria.
• - Flow: Raw Water → Sedimentation → Coagulation → Filtration → Chlorination → Pure Water.
• - Chlorination → hypochlorous acid (HClO) kills bacteria.

Common Mistakes

• 1. Confusing sedimentation and coagulation. Sedimentation = particles settle naturally; coagulation = chemicals added to form a gelatinous mass.
• 2. Thinking filtration removes all impurities. Filtration removes particles; chlorination kills bacteria.
• 3. Forgetting the role of alum and lime. They react to form aluminium hydroxide, which traps fine particles.
• 4. Thinking chlorine kills all viruses. Chlorination is not effective against all viruses (e.g., hepatitis).
• 5. Confusing the order of stages. The correct order is: sedimentation → coagulation → filtration → chlorination.
• 6. Thinking water treatment removes all minerals. Good treatment keeps beneficial minerals while removing harmful substances.

Quick Revision

• Treatment of domestic water supply involves several processes to ensure that water is safe for human use.
• Raw water is treated in a municipal water purification plant.
• Various stages in this treatment:
• 3Ca(OH)₂(aq) + Al₂(SO₄)₃(aq) → 2Al(OH)₃(s) + 3CaSO₄(aq)
• Cl₂(aq) + H₂O(l) → HClO(aq) + H⁺(aq) + Cl⁻(aq)
• Flow diagram: Raw Water → Sedimentation → Coagulation → Filtration → Chlorination → Pure Water.
• Note: Chlorination is not effective against viruses such as those that cause hepatitis.

60-Second Revision

• Four stages: Sedimentation → Coagulation → Filtration → Chlorination.
• Sedimentation → particles sink.
• Coagulation → lime + alum → Al(OH)₃.
• Filtration → sand, gravel, charcoal.
• Chlorination → HClO kills bacteria.
• Flow: Raw Water → Sedimentation → Coagulation → Filtration → Chlorination → Pure Water.
• Warning: chlorination is not effective against all viruses.