CY25C01 Municipal water treatment

Sources of Water

Rivers and lakes are the most common sources of water used by municipalities.
Water should be free from:

1. Colloidal impurities – கூழ்ம மாசுக்கள் (நீரில் மிதந்து காணப்படும் மிக நுண்ணிய மாசுக்கள்).
2. Domestic sewage – வீடுகளில் இருந்து வெளியேறும் கழிவுநீர்.
3. Industrial effluents – தொழிற்சாலைகளிலிருந்து வெளியேறும் கழிவுநீர் மற்றும் கழிவுப் பொருட்கள்.
4. Disease-producing bacteria – நோய்களை உண்டாக்கும் பாக்டீரியாக்கள் (நோய்க்கிருமிகள்).

Screening

• Screening is the process of removing floating materials, such as leaves, wood pieces, etc., from water.
• Raw water is allowed to pass through a screen, which retains the floating materials.

Aeration

• Aeration is the process of mixing water with air.
• It helps to remove gases such as CO₂, H₂S, and other volatile impurities.
• It helps to remove ferrous and manganous salts by converting them into insoluble forms.

Sedimentation

• Sedimentation is the process of removing suspended impurities by allowing water to stand undisturbed in a large tank for 2–6 hours.
• Due to the force of gravity, most of the suspended impurities settle at the bottom of the tank.
• The settled impurities are then removed from the tank.

Coagulation

• in this method, certain chemicals called coagulants are added to water to remove colloidal impurities.
• Examples of coagulants include alum  Al₂(SO₄)₃ (Aluminum sulfate)
• When alum is added to water, it undergoes hydrolysis and forms a gelatinous precipitate of aluminum hydroxide  Al(OH)₃ 
• The gelatinous precipitate traps the colloidal impurities, causing them to form larger particles that settle at the bottom of the tank.

Filtration

Filtration is the process of removing bacteria, color, taste, odour, and suspended particles from water by passing it through filter beds.

Filter bed materials: Fine sand, coarse sand, and gravel.

Sterilisation or Disinfection

• Disinfection is the process of destroying harmful bacteria and other disease-causing microorganisms in water.
• The chemicals used in this process are called disinfectants.

Methods of Disinfection

1. Using ozone.
2. Using ultraviolet (UV) radiation.
3. Chlorination.