Hotel Service Unit 13 Reverse Osmosis Q3 – Answer

7
. (a) Describe, with the aid of a sketch, the operation of the silver ion method of water sterilisation.(6)
(b) Describe how Ultra Violet Light can be used to kill bacteria in potable water.(2)
(c) Explain the disadvantages of using Ultra Violet Light as a method of sterilisation, stating how this may be overcome.(2)

(a) Silver Ion Method of Water Sterilisation (6 marks)

Operation:

  • The silver ion method sterilises potable water by releasing controlled quantities of silver ions (Ag⁺) into the water.
  • Electrodes made of silver are fitted in a sterilisation unit and connected to a low-voltage DC supply.
  • As current passes, silver ions are released into the water.
  • These ions attach to bacterial cell walls, disrupting enzyme activity and preventing reproduction, effectively killing or inactivating bacteria.
  • The silver ions remain in the water for some time, providing a residual bactericidal effect (unlike UV light, which only acts at the point of treatment).
  • Concentrations are carefully controlled (typically below 0.1 mg/L) to ensure safety for human consumption.

Sketch (exam-style):

  • Show a water pipeline with a steriliser chamber.
  • Inside: two silver electrodes connected to a DC power source.
  • Label: “Water Inlet → Chamber with Silver Electrodes → Treated Water Outlet”.
  • Add arrows for DC supply, and note “release of Ag⁺ ions into flow”.

(b) Ultraviolet (UV) Light Sterilisation (2 marks)

  • Water passes through a chamber containing a UV lamp enclosed in a quartz sleeve.
  • The UV light penetrates the water and destroys bacteria and viruses by damaging their DNA, preventing them from reproducing.
  • It is highly effective at the point of contact with the light.

(c) Disadvantages of UV Sterilisation & How to Overcome Them (2 marks)

Disadvantages:

  1. No residual effect – once the water leaves the UV chamber, there is no ongoing protection; bacteria may re-enter downstream.
  2. Effectiveness reduced by water turbidity or fouling of the lamp’s quartz sleeve.

Overcome by:

  • Using pre-filtration to reduce turbidity before UV treatment.
  • Combining UV sterilisation with a secondary disinfectant (e.g. chlorine or silver ion dosing) to maintain residual bactericidal action in the storage and distribution system.