Hotel Service Unit 13 Reverse Osmosis Q6 – Answer

  1. Explain, with the aid of sketches, the principles of reverse osmosis as a method of producing fresh water.(10)

Principles of Reverse Osmosis (RO) for Producing Fresh Water

1. Normal Osmosis (background principle):

  • If two solutions of different salinities are separated by a semi-permeable membrane, water naturally flows from the weaker (fresh) solution to the stronger (saline) solution.
  • This continues until equilibrium is reached, driven by osmotic pressure.

2. Reverse Osmosis:

  • In an RO plant, high pressure is applied to the saline seawater side.
  • This pressure exceeds the natural osmotic pressure (≈ 25–30 bar for brackish water, 55–70 bar for seawater).
  • As a result, the water flow is reversed:
    • Pure water molecules pass through the membrane.
    • Salts, minerals, and impurities are rejected and discharged as brine.

3. Semi-permeable Membrane:

  • Allows water molecules to pass but blocks salts, bacteria, and organic matter.
  • Salt rejection efficiency is typically 98–99%.

4. Flow Separation:

  • Feedwater enters the membrane module.
  • Two streams exit:
    • Permeate (fresh water) → collected for domestic or technical use.
    • Brine reject (concentrated seawater) → discharged overboard.

Sketches (Exam-style, black & white line diagrams)

  1. Osmosis vs Reverse Osmosis
    • Left: Fresh water side → Salt water side (arrow showing flow to saltier side).
    • Right: Reverse osmosis → Pump applying pressure to seawater side, arrows showing flow of fresh water through the membrane in the opposite direction.
  2. RO Plant Schematic
    • Seawater Inlet → High Pressure Pump → Membrane Module → Two outlets:
      • Permeate (fresh water) → To product tank.
      • Brine reject → Overboard.
    • Label “Semi-permeable Membrane” in the module.