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) state how a sufficient supply of oxygen is ensured.(2)
(a) Biological Operating Principle of an Aerobic Sewage Treatment Plant (8 marks)
Sewage enters the aeration tank, where it is mixed with air.
Aerobic microorganisms use oxygen to break down organic matter in the sewage.
Organic material is converted into stable end-products: carbon dioxide, water, and sludge.
The effluent then passes to a settlement/clarifier tank, where solids (sludge) settle out.
Some sludge is recirculated back to maintain the bacterial culture; excess sludge is discharged to a holding tank.
The clarified effluent passes to a disinfection stage (e.g. chlorination) before being discharged overboard in compliance with MARPOL Annex IV.
If oxygen supply is absent:
The process turns anaerobic, favouring anaerobic bacteria.
This produces harmful gases such as hydrogen sulphide and methane.
Causes strong odours, toxic and flammable atmospheres, corrosion of equipment, and unsafe discharge quality.
(b) How a sufficient supply of oxygen is ensured (2 marks)
Oxygen is supplied using air blowers or compressors feeding diffusers in the aeration tank.
Some systems use mechanical aerators or agitators to maintain dissolved oxygen at the correct level.
