Question

The minimum grade for the drains of 10 cm diameter should be:

• A. 1.00%
• B. 0.50%
• C. 0.20%
  (D) 0.10%
• D.

Hint:

A simple rule of thumb for minimum sewer grades is that the slope is inversely proportional to the diameter. Smaller pipes need steeper slopes. For a 10 cm (4") pipe, a grade of 1 in 500 (0.2%) is a good value to remember.

Answer 1

The Correct Option is C

Step 1: Understand the concept of minimum grade in drains. Drains and sewers are laid at a slope (grade or gradient) to ensure that the wastewater flows under gravity. The slope must be sufficient to generate a "self-cleansing velocity" (typically around 0.6 to 0.9 m/s) which prevents solid particles from settling and clogging the pipe.
Step 2: State the general rule for minimum grade. The required minimum grade depends on the pipe's diameter and its internal roughness. Smaller diameter pipes require a steeper slope to achieve self-cleansing velocity compared to larger pipes. For a small diameter drain, such as 10 cm (or 4 inches), a commonly recommended minimum grade in drainage and plumbing codes is 0.20% to 0.25%. A grade of 0.20% means a fall of 0.20 meters over a length of 100 meters.