Wastewater Treatment MCQ with answer and solution

Concept

Biological nitrogen removal mechanism (Nitrification-Denitrification)

Formula

Nitrification: NH4+ + 2O2 -> NO3- + 2H+ + H2O; Denitrification: NO3- + Organic Carbon -> N2 + CO2 + H2O

Step-by-step solution

1. Raw wastewater contains organic nitrogen and ammonia (NH4+). 2. Aerobic nitrification converts ammonia into nitrate (NO3-). 3. Subsequent anoxic denitrification reduces nitrate into nitrogen gas (N2) that escapes into the atmosphere. 4. Thus, the correct sequence is nitrification followed by denitrification. Answer: A

Explanation

Biological nitrogen removal in wastewater treatment occurs in two sequential biochemical stages: (1) Aerobic Nitrification, where autotrophic bacteria (Nitrosomonas and Nitrobacter) oxidize ammonia (NH4+) to nitrite (NO2-) and then nitrate (NO3-); followed by (2) Anoxic Denitrification, where heterotrophic bacteria reduce nitrate (NO3-) to harmless nitrogen gas (N2).

Why the other options are wrong

B) Denitrification cannot precede nitrification because nitrate must first be formed from ammonia. C) Phosphorus precipitation is a chemical process for phosphorus, not nitrogen removal. D) Nitrogen fixation adds nitrogen into biomass, the opposite of removal.

Shortcut

Biological Nitrogen Removal: Ammonia -> Nitrification (Aerobic) -> Denitrification (Anoxic) -> N2 gas.

Common mistake

Reversing the process sequence or confusing nitrogen removal with chemical phosphorus removal.