- With reference to sewage treatment plants:
(a) describe the biological operating principle of an aerobic sewage treatment plant, explaining the dangers if a supply of oxygen is not present;(8)
(b) state how a sufficient supply of oxygen is ensured.(2)
Sewage Treatment Plants
(a) Biological operating principle of an aerobic sewage treatment plant (8 marks)
- Sewage enters the aeration tank, where it is mixed with a culture of aerobic microorganisms.
- These microorganisms use oxygen to break down organic matter into stable end-products such as carbon dioxide, water, and non-harmful sludge.
- Continuous aeration keeps solids in suspension and maintains the growth of healthy bacteria.
- The treated mixture then passes to a settlement tank, where solids (sludge) settle out.
- The clarified effluent is disinfected (usually chlorinated) before being discharged overboard in compliance with MARPOL Annex IV.
- Dangers if oxygen is absent:
- Conditions become anaerobic, encouraging growth of anaerobic bacteria.
- This produces toxic gases such as hydrogen sulphide and methane.
- Causes strong odours, corrosion of plant and pipework, and reduced treatment efficiency.
- Risk to crew health if gases accumulate in enclosed spaces.
(b) Ensuring a sufficient supply of oxygen (2 marks)
- Oxygen is supplied by blowers or compressors, which bubble air through the effluent in the aeration tank.
- Some systems use mechanical surface aerators or diffusers to maintain dissolved oxygen at the required level.