In fully oxidized sewage (end of nitrification), the nitrogen is predominantly present in which form?

Introduction / Context:
Understanding nitrogen transformations in wastewater is essential for process control and environmental compliance. As wastewater undergoes aerobic biological treatment, reduced nitrogen forms are progressively oxidized, culminating in nitrate under fully aerobic, fully oxidized conditions.

Given Data / Assumptions:

• Aerobic conditions sufficient for complete nitrification.
• Microbial community includes nitrifying bacteria (e.g., Nitrosomonas, Nitrobacter).
• Organic matter has been largely oxidized.

Concept / Approach:

The nitrification pathway converts ammonia to nitrite and then to nitrate. In a fully oxidized (stable) effluent, nitrite is transient and typically low; nitrate is the dominant oxidized inorganic nitrogen species. Ammonia would be low due to oxidation, and “albuminoid nitrogen” refers to organic nitrogen that has largely been mineralized during treatment.

Step-by-Step Solution:

Verification / Alternative check:

Typical secondary effluents post-nitrification show NO3− in the tens of mg/L as N, with negligible NO2− and low NH3-N unless anoxic conditions or short-circuiting occur.

Why Other Options Are Wrong:

(a) Nitrite is intermediate and unstable under strong aeration; (c) ammonia indicates incomplete nitrification; (d) albuminoid nitrogen is largely removed by oxidation/biodegradation.

Common Pitfalls:

Confusing “fully oxidized” with denitrification (which reduces nitrate to nitrogen gas under anoxic conditions); misreading grab samples where short-term nitrite spikes can occur.

Final Answer:

Nitrates (NO3−)