Question

In the context of biological wastewater treatment, which of the following pairs CORRECTLY match the electron donor/acceptor with the corresponding biological process?

• A. Nitrification: Ammonium as electron donor, oxygen as electron acceptor
• B. Denitrification: Organic carbon as electron donor, nitrate as electron acceptor
• C. Sulfate reduction: Oxygen as electron donor, sulfate as electron acceptor
• D. Methanogenesis: Hydrogen as electron donor, carbon dioxide as electron acceptor

Hint:

Recall each redox pair: nitrification oxidizes NH4+ using O2; denitrification oxidizes organic carbon using NO3-; sulfate reduction uses organic matter/H2 (not O2) to reduce sulfate; methanogenesis uses H2 to reduce CO2.

Answer 1

The Correct Option is A, B, D

Step 1: Examine nitrification (option A). Nitrification is carried out by autotrophic bacteria (Nitrosomonas, Nitrobacter) that oxidize ammonium (NH4+) as their electron donor and energy source, using oxygen as the terminal electron acceptor, producing nitrite then nitrate. This matches option (A).

Step 2: Examine denitrification (option B). Denitrification is carried out by facultative heterotrophic bacteria under anoxic conditions; they use organic carbon (BOD) as their electron donor/energy source and nitrate (NO3-) as the electron acceptor in place of oxygen, reducing it to nitrogen gas. This matches option (B).

Step 3: Examine sulfate reduction (option C). Sulfate-reducing bacteria are strict anaerobes that use organic matter or hydrogen as the electron donor and sulfate (SO4^2-) as the electron acceptor, producing hydrogen sulfide. Oxygen is toxic to these obligate anaerobes and is never used as the electron donor in this process, so option (C), which states oxygen as the electron donor, is factually incorrect.

Step 4: Examine methanogenesis (option D). Methanogenic archaea, under strictly anaerobic conditions, can use hydrogen (H2) as the electron donor and carbon dioxide (CO2) as the electron acceptor, producing methane (CO2 + 4H2 -> CH4 + 2H2O), which correctly matches option (D).

Step 5: Conclude. The correctly matched pairs are (A) nitrification, (B) denitrification, and (D) methanogenesis; option (C) is incorrect because sulfate reduction does not use oxygen as an electron donor.