Class 4 (4th Engineer) MEP 📅 Aug 2023

Exam Question

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

(i) The risk associated with opening the unit (5)

(ii) The precautions taken to reduce the risk (5)

(b) Explain the significance of biological oxygen demand (B.O.D.) (6)

Reference Answer

### (a) (i) The Risks Associated with Opening a Biological Sewage Unit (5 Marks)
Opening a biological sewage treatment plant (STP) for repair is a high-risk operation classified as a **Confined Space Entry**. The primary risks are associated with the hazardous atmosphere, direct contact with sewage, physical hazards, and environmental pollution.
1. **Hazardous Atmosphere:** The decomposition of organic waste by micro-organisms generates several dangerous gases and conditions:
* **Hydrogen Sulphide (H₂S):** A highly toxic, flammable gas heavier than air. It has a characteristic 'rotten egg' smell at low concentrations, but at higher, lethal concentrations, it paralyzes the olfactory nerves, rendering it undetectable by smell. It can cause immediate unconsciousness and death.

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