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- Describe the operation of a Biological Sewage Treatment Plant.(10)
Operation of a Biological Sewage Treatment Plant
- Inlet Stage
- Raw sewage enters the plant through an inlet line.
- Screening or coarse filters may remove large solids before biological treatment.
- Aeration Tank (Biological Treatment Stage)
- Sewage flows into the aeration tank.
- Air is bubbled through by blowers or diffusers, supplying oxygen.
- Aerobic microorganisms use oxygen to break down organic matter into stable by-products (mainly CO₂, water, and sludge).
- Continuous agitation keeps solids in suspension and bacteria in contact with waste.
- Settlement/Clarifier Tank
- The mixed liquor flows into a settlement (clarifier) tank.
- Sludge settles to the bottom, while clarified effluent rises.
- A portion of sludge is recirculated back to the aeration tank to maintain a healthy bacteria population.
- Excess sludge is removed to a sludge holding tank for later disposal ashore.
- Disinfection Stage
- Clarified effluent passes to a disinfection tank.
- Chlorination (e.g. with calcium hypochlorite) kills harmful bacteria and pathogens.
- Ensures effluent is safe to discharge under MARPOL Annex IV.
- Final Discharge
- The disinfected effluent is discharged overboard at sea.
- Discharge must not cause visible solids or discolouration of surrounding water.
Key Notes for Full Marks
- The process is biological (uses bacteria, not just mechanical/chemical treatment).
- Oxygen supply is vital: without it, conditions become anaerobic, producing harmful gases (H₂S, methane).
- Sludge management and disinfection are critical for safe operation and MARPOL compliance.