Wastewater Treatment MCQ with answer and solution

Concept

Depletion of dissolved oxygen in wastewater leads to anaerobic conditions, causing the sewage to turn septic and produce foul odors.

Step-by-step solution

1. Understand that fresh sewage contains dissolved oxygen. 2. When wastewater is stagnant or takes a long time to travel through sewers, microorganisms consume the available oxygen. 3. Once oxygen is completely absent (absence of oxygen), anaerobic bacteria take over the decomposition process. 4. This anaerobic decomposition results in septic conditions.

Explanation

Sewage becomes septic when dissolved oxygen is completely depleted, leading to anaerobic decomposition of organic matter by anaerobic bacteria. This process produces foul-smelling gases like hydrogen sulfide, methane, and ammonia, characteristic of septic sewage.

Why the other options are wrong

A: Aerobic conditions keep sewage fresh with dissolved oxygen. B: pH rise does not directly cause sewage to become septic; in fact, septic sewage often becomes more acidic due to organic acids. D: Chlorination is used to disinfect and prevent sewage from becoming septic.

Shortcut

No oxygen = Anaerobic = Septic.

Common mistake

Confusing aerobic decomposition (which keeps sewage fresh and odorless) with anaerobic/septic conditions.