Sewerage Systems MCQ with answer and solution

Concept

Optimal sewer cross section for combined systems

Formula

NONE

Step-by-step solution

1. Combined sewerage systems carry both domestic sewage and stormwater runoff. 2. An egg-shaped sewer section provides a narrow bottom that maintains self-cleansing velocity during low dry-weather flow. 3. Hence, the egg-shaped section is the most suitable design for combined sewers. Answer: C

Explanation

Combined sewers carry both sanitary sewage and stormwater runoff, leading to large flow rate variations between dry and rainy periods. An egg-shaped sewer section maintains a narrow bottom channel that ensures adequate velocity and self-cleansing action during low dry-weather flows. The broader upper portion provides the necessary capacity for heavy stormwater flows.

Why the other options are wrong

A) Rectangular sewer sections are structurally inefficient and fail to maintain self-cleansing velocity at low flow rates. B) Circular sewer sections do not maintain adequate self-cleansing velocity during low dry weather flow in combined systems. D) Option C is correct as the egg-shaped section provides higher velocity during low dry weather flows.

Shortcut

Egg-shaped sewers are ideal for combined systems to maintain self-cleansing velocity during low flows.

Common mistake

Selecting a circular section suitable for separate sewerage systems instead of an egg-shaped section required for combined systems.