Lecture Notes: Desalination and Water Disinfection

Introduction

Definition: Process to remove minerals from saline water to produce potable (safe for human consumption) water.
Main Methods:
- Thermal Distillation
  - Common in the Middle East due to low rainfall and fossil fuel availability.
  - Process: Seawater heated, evaporates, and condenses into freshwater.
  - Produces brine, safely returned to the sea.
- Reverse Osmosis
  - Filtration technique forcing water under pressure through a semi-permeable membrane.
  - Pre-treatment includes intake screens, flotation units, and sand filtration.
  - Requires energy to move water from high to low solute concentration.
  - Also produces concentrated brine, disposed of safely.

Advantages:
- Produces potable water.
- Reduces demand on freshwater resources.
- Independent of climate factors (e.g., rainfall).
Disadvantages:
- Energy-intensive and expensive to build/maintain.
- Environmental concerns due to heat disposal and marine life impact.

Chemicals Used: Chlorine, chloramine, hypochlorous acid.
Process:
- Chlorine gas injected into water, forming equilibrium with hydrochloric acid and hypochlorous acid.
- Equilibrium affected by pH levels.
Effect of pH:
- Low pH increases hydronium ions, shifts equilibrium left, increases chlorine gas (undesirable).
- High pH increases hypochlorous acid (desirable).
Hypochlorous Acid: Key disinfecting agent due to ability to penetrate bacterial cell walls.

Function: Oxidizes proteins and enzymes in bacteria, causing denaturation.
Comparison: Hypochlorous acid more effective than hypochlorite ions.
- Neutral charge allows better penetration of bacterial cell walls.

Desalination and water disinfection are crucial processes for producing potable water.
Understanding the balance of pH in disinfection crucial for effectiveness.
Hypochlorous acid is a potent disinfectant, especially compared to hypochlorite ions.