Hotel Service Unit 13 Reverse Osmosis Q5 – Answer

  1. Describe, with the aid of a sketch, the construction of a spirally wound membrane suitable for use in a Reverse Osmosis water making plant, identifying ALL component parts.(10)

Spirally Wound Reverse Osmosis (RO) Membrane – Construction

A spirally wound membrane is the most common design used in marine RO water makers. It is compact, efficient, and easy to replace.

Main Components:

  1. Permeate Collection Tube (central tube):
    • Runs through the centre of the element.
    • Collects the desalinated (permeate) water from the membrane layers.
    • Contains holes drilled along its length for water entry.
  2. Membrane Sheets:
    • Semi-permeable polyamide thin-film composite.
    • Rejects salts and dissolved solids while allowing water molecules to pass.
    • Two sheets are placed back-to-back to form a leaf.
  3. Feed Spacer (turbulence spacer):
    • A plastic mesh inserted between the feed channels.
    • Creates turbulence, minimises fouling, and maintains flow passages for feedwater.
  4. Permeate Spacer (carrier fabric):
    • Porous material between membrane layers.
    • Allows fresh water (permeate) to flow freely into the central collection tube.
  5. Adhesive Seal Strips:
    • Applied along three edges of each membrane leaf.
    • Directs permeate flow only towards the collection tube, preventing mixing with feedwater.
  6. Outer Wrap:
    • Fibreglass-reinforced shell holds the spiral assembly tightly wound.
    • Provides strength and allows insertion into the pressure vessel.

Principle of Construction/Operation:

  • Multiple membrane leaves are layered around the central tube in a spiral fashion.
  • Feedwater flows axially across the membrane surface (from one end to the other).
  • Permeate passes through the membrane, into the permeate spacer, and spirals inward to the central tube.
  • Concentrated brine continues along the length of the element and exits at the far end.

Sketch (Exam-Style)

A neat cross-sectional sketch should include:

  • Central permeate collection tube (labelled).
  • Spiral-wound membrane leaves.
  • Adhesive sealing edges.
  • Feed spacer between membranes.
  • Permeate spacer channel.
  • Outer fibreglass wrap.
  • Arrows showing:
    • Feed flow → across membrane surface.
    • Permeate flow → inward to central tube.
    • Brine flow → out to reject line.