A technician is called to investigate a recurring blockage in a building sewer serving a multi-unit residential building. The sewer is 150 mm in diameter and appears to be sagging in several locations, creating belly-like depressions. What is the most likely cause of the recurring blockages?

Question: A technician is called to investigate a recurring blockage in a building sewer serving a multi-unit residential building. The sewer is 150 mm in diameter and appears to be sagging in several locations, creating belly-like depressions. What is the most likely cause of the recurring blockages?

Answer options:

• Excessive slope causing rapid water flow and leaving solids behind. ✅ Accumulation of solids due to insufficient flow velocity in sags.
• Poorly installed cleanouts preventing proper maintenance.
• Corrosion of the pipe material causing internal roughness.

Correct answer: Accumulation of solids due to insufficient flow velocity in sags.

Explanation: Sags in a building sewer create areas where wastes can settle and accumulate, leading to blockages due to insufficient flow velocity to carry solids away. Excessive slope would generally increase flow velocity and reduce blockages, while proper cleanouts are for access, not prevention of a sag issue itself.