Wastewater Treatment MCQ with answer and solution

Concept

IS 2470 (Part 1) fixes a minimum width of 75 cm for a septic tank so it can be cleaned and works properly.

Step-by-step solution

1. IS 2470 (Part 1) gives the design rules for septic tanks. 2. For small tanks (5 users), the code table gives length about 1.5 m, width 0.75 m and liquid depth about 1.0 m. 3. The code also says the minimum width of any septic tank is 75 cm. 4. So the answer is 75 cm.

Explanation

IS 2470 says a septic tank must be at least 75 cm wide. Even for very few users (like 5), the width is not taken less than 75 cm. This gives enough space for settling of solids and for cleaning.

Why the other options are wrong

A (55 cm): less than the code minimum. C (60 cm): less than the code minimum of 75 cm. D (85 cm): more than the minimum, so it is not the minimum width.

Shortcut

Septic tank minimum: width 75 cm, liquid depth 1 m, capacity 1 m³.

Common mistake

Students mix up the minimum width (75 cm) with the free board (about 30 cm) or other small values.