Hotel Service Unit 13 Reverse Osmosis Q1 – Answer

  1. With reference to reverse osmosis plants:
    (a) describe the sea water pre-treatment process before the water enters the membrane modules;(6)
    (b) describe how the purity of the permeate is measured;(3)
    (c) state the limits of impurity in the permeate when produced to World Health Organisation Standards.

(a) Seawater pre-treatment process before entering the membrane modules (6 marks)

Before seawater passes into the reverse osmosis (RO) membranes, it must be treated to protect the membranes and ensure efficient operation:

  1. Screening – Coarse filters or strainers remove large debris such as seaweed, shells, or sand.
  2. Coagulation/flocculation – Chemicals (e.g., ferric salts or polyelectrolytes) may be added to cause fine suspended solids to clump together for easier removal.
  3. Filtration – Multi-media filters (sand, anthracite, or cartridge filters) remove fine suspended solids.
  4. Dechlorination – If chlorination was used for biofouling control, sodium bisulphite is added to neutralise chlorine, as chlorine would damage the membranes.
  5. pH adjustment / antiscalant dosing – Acids or antiscalant chemicals are added to reduce scaling from salts (e.g., calcium carbonate, sulphates).
  6. Final cartridge filtration – A fine micron filter (typically 5 µm) removes any last traces of suspended matter before the RO membranes.

(b) Measuring the purity of the permeate (3 marks)

  • Conductivity or Total Dissolved Solids (TDS) meters measure the electrical conductivity of the permeate, which is directly related to the salt concentration.
  • Salt rejection rate is calculated by comparing feedwater and permeate conductivity.
  • Continuous monitoring systems ensure the water meets potable standards before storage.

(c) Limits of impurities in the permeate under WHO Standards (1 mark each = 3 marks)

According to World Health Organisation (WHO) guidelines for potable water:

  1. Total Dissolved Solids (TDS): Should be less than 500 mg/L (acceptable up to 1000 mg/L, but <500 mg/L preferred for taste and health).
  2. Chloride content: Typically should not exceed 250 mg/L.
  3. Turbidity: Should be less than 1 NTU (Nephelometric Turbidity Unit).