Wastewater denitrification — Which options are used to reduce nitrate (NO3−) in wastewater treatment applications?

Introduction / Context:
Denitrification removes nitrate by reducing it to gaseous nitrogen species, a key step in preventing eutrophication. Process engineers exploit heterotrophic denitrifiers and tailored reactor designs to achieve stable nitrate removal in industrial and municipal wastewaters.

Given Data / Assumptions:

• Heterotrophic denitrifiers require an electron donor (e.g., methanol) under anoxic conditions.
• Immobilized or encapsulated cells can improve stability and retention in reactors.
• Nitrifiers such as Nitrosomonas perform oxidation (nitrification), not nitrate reduction.

Concept / Approach:
Hypomicrobium species grow with methanol and can reduce nitrate in anoxic conditions. Micrococcus denitrificans has been used in immobilized systems (e.g., liquid membranes) to enhance nitrate removal and protect cells from washout or toxic shocks. Nitrosomonas europaea oxidizes ammonia to nitrite and is not a denitrifier.

Step-by-Step Solution:

Verification / Alternative check:
Standard design manuals specify methanol-fed denitrification and note benefits of immobilized biocatalysts for steady performance and low sludge yield.

Why Other Options Are Wrong:

• Nitrosomonas europaea: oxidizes NH3 to NO2−; does not reduce nitrate.
• Aeromonas strictly aerobic only: denitrification requires anoxic conditions and specific organisms capable of nitrate respiration.

Common Pitfalls:
Confusing the two-step nitrogen removal train: nitrification (aerobic) followed by denitrification (anoxic/anaerobic) with different microbial guilds.

Final Answer:
Both (a) and (b)