Wastewater Treatment MCQ with answer and solution

Concept

Detention time in primary sedimentation tanks is kept short (1-2 hours) to allow heavy suspended solids to settle without causing anaerobic decomposition or sludge bulking.

Formula

Detention Time (t) = Volume (V) / Flow Rate (Q)

Step-by-step solution

1. Understand the purpose of a primary settling tank: removal of readily settleable organic and inorganic solids. 2. Recall standard design parameters for primary settling tanks in wastewater treatment plants. 3. The standard detention time ranges between 1 to 2 hours (or 1.5 to 2 hours for plain sedimentation).

Explanation

In wastewater treatment, primary settling tanks (sedimentation tanks) are designed to remove settleable solids through gravity. The standard detention time for a primary settling tank typically ranges from 1 to 2 hours.

Why the other options are wrong

B: 6-8 hours is too long and may lead to anaerobic conditions in primary sewage. C: 12 hours is typical for smaller septic tanks or specific chemical precipitation processes, not primary clarifiers. D: 24 hours is excessive for primary settling and would require massive tank volumes.

Shortcut

Primary treatment is faster than secondary biological treatment; remember primary = 1-2 hours.

Common mistake

Confusing primary settling tank detention time with secondary sedimentation (clarifiers) or water treatment sedimentation tanks, which can sometimes have different design periods.