- Explain, with the aid of a sketch, how an Aerobic Sewage Treatment plant operates.(10)
Aerobic Sewage Treatment Plant – Operation (10 marks)
Process explanation:
- Inlet Stage
- Raw sewage enters the aeration tank.
- Air is continuously bubbled through by blowers or diffusers.
- Aeration Tank (Biological Stage)
- Oxygen supports the growth of aerobic microorganisms.
- These bacteria digest organic matter, converting it into stable products (CO₂, water, and sludge).
- Continuous mixing keeps solids in suspension.
- Settlement/Clarifier Tank
- Effluent flows into a clarifier where sludge settles to the bottom.
- Some sludge is recirculated back to the aeration tank to maintain bacterial population.
- Excess sludge is removed and sent to a sludge holding tank.
- Disinfection Stage
- The clarified effluent passes to a disinfection tank.
- Calcium hypochlorite or chlorine solution is added.
- This kills remaining pathogens and ensures effluent meets MARPOL Annex IV discharge standards.
- Final Discharge
- Treated, disinfected effluent is discharged overboard.
Sketch (exam style)
Your sketch should include:
- Inlet → Aeration Tank (with air bubbles shown) → Settlement Tank (sludge settling & recirculation arrow) → Disinfection Tank (chlorine dosing) → Overboard discharge.
- Label air supply, recirculated sludge, and chlorine addition.
- Use arrows to show flow direction.