Why Ammonia Stays High—Even with Strong DO

In municipal and industrial wastewater treatment, high ammonia levels are a common headache. Operators often respond by increasing aeration—hoping to boost nitrifying bacteria and convert ammonia (NH₄⁺) into nitrate (NO₃⁻).

But sometimes, even with strong aeration, ammonia remains high. That’s because the root problem may not be oxygen—but deeper system issues.

Here are 5 key reasons why ammonia stays high even after intensive aeration:

 

🌡️ Temperature Outside Ideal Range

Nitrifying bacteria are highly sensitive to temperature.

• 💡 Optimal range: 20–30°C
• <5°C or >35°C → activity drops sharply

• 40°C → nitrification may stop completely

If water temperature is outside this window, ammonia removal will be very slow—even with high DO.

 

🧪 pH or Alkalinity Not in the Right Range

When pH > 9, ammonium (NH₄⁺) turns into free ammonia (NH₃), which is toxic to nitrifiers.

• ✅ Keep pH at 6.5–8.5
• 🧂 Maintain stable alkalinity to support nitrification

If pH and alkalinity are unstable, the system can’t sustain consistent ammonia removal.

 

🧬 Inhibition from High Ammonia or Nitrite

Both ammonia and nitrite can become toxic at high concentrations.

• Too much ammonia = overload for bacteria
• Nitrite buildup = process bottleneck

🛠️ Solutions:

• Use air stripping to reduce ammonia before bio-treatment
• Apply denitrification first to remove nitrite and restart nitrification

 

⚖️ Excess Organic Load: Heterotrophs Take Over

If COD is too high, heterotrophic bacteria grow faster and outcompete nitrifiers for space and oxygen.

This causes:

• Nitrifier population to drop
• Ammonia removal rate to decline

Fix it by:

• Adjusting C/N ratio
• Increasing MLSS
• Seeding with nitrifying cultures

 

☣️ Sludge Toxicity or Heavy Metal Interference

Industrial wastewater may contain:

• Heavy metals
• Toxic organics

These can poison the sludge, killing sensitive nitrifying bacteria.

🧯 Solutions:

• Trace the contamination source
• Replace toxic sludge
• Use detox agents or specialized bacteria to recover performance

 

🧩 Final Thoughts

More air doesn’t always mean more nitrification. ✅ Solving high ammonia requires a full-system review, not just more aeration.

By understanding these 5 key factors, operators can:

• Fix the real cause
• Improve ammonia removal
• Lower overall operating cost

💬 Have similar issues or solutions? Share in the comments or reach out for technical support.