Reverse osmosis (RO) is a process that purifies water by forcing it through a semi-permeable membrane. In this process, high pressure is applied to a solution with a high concentration of dissolved solids, such as saltwater, on one side of the membrane. This pressure overcomes the natural osmotic pressure, causing the pure water molecules to pass through the membrane while leaving behind the larger salt ions and other impurities. The membrane acts as a selective barrier, allowing only the water to pass, while the concentrated brine solution is discarded. For large-scale production, a large membrane surface area and a strong pump capable of generating high pressures are necessary.

1. Pretreatment Stage

Pretreatment is essential to protect the R.O. membranes from fouling and scaling.

• Scaling: Caused by soluble salts such as calcium carbonate and calcium sulphate depositing on the membrane.
• Fouling: Caused by micro-organisms, metal oxides, and colloidal particles coating the membrane surface.

Pretreatment methods include:

• Mechanical filtration: Multiple filter stages in series, e.g.:
  • Sand filters
  • Multi-layer filters
  • Microfilters (<10 ppm particle size)
• Chemical treatment:
  • Coagulants for fine particle removal
  • Biocides to kill micro-organisms
  • Acid dosing to neutralize calcium salts and prevent scale formation

  A pump takes suction from the sea chest through a coarse filter, delivering water at about 6 bar through the pretreatment system.

  2. High-Pressure Stage

  • A high-pressure piston pump raises the feed water pressure to above 50 bar.
  • This pressurized water enters the semi-permeable membrane modules.

  3. Separation Process

  • Due to the pressure difference between the concentrated brine side and the permeate side, water molecules pass through the membrane.
  • Dissolved salts, organics, and microbes are rejected.

  Outputs:

  • Permeate (Fresh Water): Low-salt content water used for drinking and domestic purposes.
  • Brine (Concentrated Reject): Discharged overboard (OVBD).

  4. Post-Treatment

  The fresh water (permeate) is further treated to make it suitable for shipboard use:

  • Hardness adjustment (to prevent excessive softness)
  • pH correction (maintained around 8 for taste and corrosion control)
  • Chlorination (for disinfection)

  Note: If pH rises too high, chlorine’s effectiveness against micro-organisms is reduced.

  Flow Summary:

  Sea Water → Coarse Filter → Pretreatment Filters & Chemicals → High-Pressure Pump → R.O. Membranes →

  → Permeate (Fresh Water) → Post-treatment → Ship’s Fresh Water System

  → Brine (Reject Water) → Overboard