6.With reference to sewage treatment plants:
(a)state the purpose of the vent, and explain why it is fitted with a gauze;(3)
(b) explain the difference between aerobic and anaerobic micro organisms;(3)
(c) explain why air is bubbled through the effluent in the aeration tank;(2)
(d) explain why calcium hypochlorite is added to the treated sewage before discharge overboard.(2)
Sewage Treatment Plants
(a) Purpose of the vent and gauze (3 marks)
- The vent allows gases produced during sewage breakdown (e.g. methane, hydrogen sulphide) to escape safely.
- Prevents dangerous pressure build-up in the system.
- The gauze prevents insects, vermin, and foreign objects from entering, maintaining hygiene and preventing contamination.
(b) Difference between aerobic and anaerobic microorganisms (3 marks)
- Aerobic microorganisms: Require oxygen to survive; break down sewage efficiently into stable products like water and carbon dioxide with minimal odour.
- Anaerobic microorganisms: Live without oxygen; produce methane, hydrogen sulphide, and other harmful gases with unpleasant odours.
- Aerobic treatment is preferred in marine sewage plants due to efficiency and fewer harmful by-products.
(c) Why air is bubbled through the effluent in the aeration tank (2 marks)
- Supplies oxygen to sustain aerobic microorganisms.
- Ensures effective biological breakdown of sewage and prevents septic conditions.
(d) Why calcium hypochlorite is added before discharge (2 marks)
- Provides disinfection by killing any remaining bacteria or pathogens.
- Ensures compliance with MARPOL Annex IV by making effluent safe for discharge into the sea.