Hotel Service Unit 10 Sewage Q7 – Answer

  1. (a) State the THREE main stages of a Biological Sewage Treatment Plant.(3)
    (b) Describe the operation of a Biological Sewage Treatment Plant, explaining the process carried out in EACH of the stages stated in part (a).(7)

(a) Three main stages of a Biological Sewage Treatment Plant (3 marks)

  1. Aeration (biological treatment) stage
  2. Settlement/clarification stage
  3. Disinfection (chlorination) stage

(b) Operation and processes in each stage (7 marks)

  1. Aeration stage
    • Raw sewage enters the aeration tank.
    • Air is bubbled through by blowers or diffusers, supplying oxygen to aerobic microorganisms.
    • The bacteria digest organic matter, breaking it down into stable end-products (CO₂, water, sludge).
  2. Settlement/clarification stage
    • The aerated mixture passes to a settlement tank.
    • Sludge solids sink to the bottom and clarified effluent rises to the top.
    • Some sludge may be recirculated to maintain bacterial population, while excess sludge is removed for disposal.
  3. Disinfection (chlorination) stage
    • The clarified effluent is treated with calcium hypochlorite or chlorine solution.
    • This kills any remaining pathogens and ensures the discharge meets MARPOL Annex IV standards.
    • The disinfected effluent is then discharged overboard.