Nitrification – What's Needed?

Understanding Nitrification

Nitrification is the biological oxidation of ammonia nitrogen to nitrate and accounts for most ammonia removal in wastewater treatment systems. This process is essential for meeting effluent ammonia limits and protecting receiving waters.

The Nitrification Process

Nitrification occurs in two steps:

1. Step 1: Ammonia (NH₄⁺) is converted to nitrite (NO₂⁻) by Nitrosomonas bacteria
2. Step 2: Nitrite (NO₂⁻) is converted to nitrate (NO₃⁻) by Nitrobacter bacteria

Requirements for Successful Nitrification

1. Adequate Dissolved Oxygen

Nitrifying bacteria are strict aerobes and require high dissolved oxygen levels:

• Minimum DO: 2.0 mg/L
• Optimal DO: 3.0-4.0 mg/L
• Higher DO levels may be needed during peak loading

2. Sufficient Alkalinity

Nitrification consumes alkalinity and can cause pH to drop:

• 7.14 mg/L of alkalinity consumed per mg/L of ammonia oxidized
• Maintain pH between 6.5-8.0
• Supplement with lime, caustic, or sodium bicarbonate if needed

3. Adequate Sludge Age

Nitrifying bacteria grow slowly and require longer sludge ages:

• Minimum sludge age: 10-15 days
• Optimal sludge age: 15-25 days
• Higher sludge ages provide more stability

4. Proper Temperature

Temperature affects nitrification rates:

• Optimal temperature: 20-30°C (68-86°F)
• Below 10°C (50°F): nitrification slows significantly
• Above 35°C (95°F): nitrification may be inhibited

5. Adequate Nutrients

Nitrifying bacteria need phosphorus and trace elements:

• Phosphorus: typically sufficient in domestic wastewater
• Trace elements: iron, manganese, copper, zinc
• Industrial wastewaters may be deficient

Common Nitrification Problems

Low Dissolved Oxygen

Symptoms and solutions:

• Symptoms: High effluent ammonia, low nitrate
• Solutions: Increase aeration, reduce organic loading, improve mixing

Low pH/Alkalinity

Symptoms and solutions:

• Symptoms: pH below 6.5, incomplete nitrification
• Solutions: Add lime, caustic, or sodium bicarbonate

Insufficient Sludge Age

Symptoms and solutions:

• Symptoms: High effluent ammonia, young sludge
• Solutions: Reduce wasting, increase MLSS concentration

Monitoring Nitrification

Key Parameters to Monitor

• Effluent ammonia (NH₄⁺-N)
• Effluent nitrate (NO₃⁻-N)
• Dissolved oxygen in aeration tanks
• pH and alkalinity
• Temperature
• Sludge age and MLSS concentration

Performance Indicators

Optimization Strategies

Immediate Actions

• Increase aeration to maintain DO >2.0 mg/L
• Add alkalinity if pH drops below 6.5
• Reduce wasting to increase sludge age
• Monitor and adjust for temperature changes

Long-term Improvements

• Upgrade aeration equipment for better oxygen transfer
• Implement process control systems
• Consider separate nitrification tanks
• Optimize sludge age management

Understanding the requirements for nitrification and maintaining proper operating conditions will ensure consistent ammonia removal and compliance with effluent limits.