Class 4 (4th Engineer) MEP 📅 Sep 2024

Exam Question

A biological sewage system develops a fault, which necessitates opening the unit for repair.

(a) The risk associated with opening the unit. (6)

(b) The precaution taken to reduce the risk (6)

Reference Answer

### (a) The Risks Associated with Opening a Biological Sewage Unit (6 Marks)
Opening a biological sewage treatment plant (STP) for repair is a high-risk activity due to a combination of atmospheric, biological, chemical, and physical hazards. A comprehensive understanding of these risks is essential for safe operation.
* **Atmospheric Hazards (Asphyxiation, Toxicity, Explosion):** This is the most acute and immediate danger.
* **Toxic Gases:** Aerobic and anaerobic decomposition of sewage produces hazardous gases. The most significant is **Hydrogen Sulphide (H₂S)**, which is highly toxic, heavier than air, and paralyses the sense of smell at high concentrations, giving a false sense of safety. Other gases include ammonia and carbon monoxide.

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