De-Nitrification, Only after Nitrification

Understanding the Nitrogen Removal Process

De-nitrification is when facultative (common) treatment bacteria change nitrate (NO₃⁻) to nitrogen gas (N₂), carbon dioxide (CO₂), and water (H₂O). This process is essential for removing nitrogen from wastewater and meeting increasingly strict effluent limits.

The De-nitrification Process

De-nitrification is a biological process that occurs under anoxic conditions (low dissolved oxygen, typically <0.5 mg/L). The bacteria use nitrate as an electron acceptor instead of oxygen, converting it to nitrogen gas that escapes to the atmosphere.

Prerequisites for De-nitrification

1. Nitrification Must Occur First

This is the critical requirement: de-nitrification can only occur after nitrification has converted ammonia to nitrate. The process sequence is:

1. Nitrification: NH₄⁺ → NO₃⁻ (aerobic conditions)
2. De-nitrification: NO₃⁻ → N₂ (anoxic conditions)

2. Anoxic Conditions

De-nitrifying bacteria require low dissolved oxygen levels:

• Optimal DO: <0.5 mg/L
• Maximum DO: <1.0 mg/L
• Complete absence of oxygen is not required

3. Available Carbon Source

De-nitrifying bacteria need organic carbon as an energy source:

• Readily biodegradable organic matter
• Typical ratio: 3-5 mg BOD₅ per mg NO₃⁻-N
• Sources: influent wastewater, endogenous decay, external carbon

4. Adequate Sludge Age

Sufficient time is needed for de-nitrifying bacteria to grow:

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

Process Configurations

Pre-Anoxic (Modified Ludzack-Ettinger)

An anoxic zone is placed before the aerobic zone:

• Uses influent BOD as carbon source
• Recycles nitrate from aerobic zone
• Typical removal: 60-80%
• Cost-effective for moderate removal requirements

Post-Anoxic

An anoxic zone is placed after the aerobic zone:

• Requires external carbon addition
• Higher removal efficiency possible
• Typical removal: 80-95%
• More expensive due to carbon addition

Sequential Batch Reactor (SBR)

Uses time-based sequencing in a single tank:

• Aerobic phase for nitrification
• Anoxic phase for de-nitrification
• Flexible operation
• Good for small to medium facilities

Monitoring and Control

Key Parameters

• Dissolved oxygen in anoxic zones
• Nitrate concentration in effluent
• Carbon availability
• Recycle rates
• Sludge age

Performance Indicators

Common Problems and Solutions

Insufficient Nitrification

If nitrification is incomplete, de-nitrification cannot occur:

• Symptoms: High effluent ammonia, low nitrate
• Solutions: Improve nitrification first (DO, sludge age, pH)

High Dissolved Oxygen

Too much oxygen inhibits de-nitrification:

• Symptoms: High effluent nitrate, low removal
• Solutions: Reduce aeration, improve mixing, adjust recycle rates

Insufficient Carbon

Lack of carbon limits de-nitrification:

• Symptoms: High effluent nitrate, incomplete removal
• Solutions: Add external carbon, optimize carbon utilization

Optimization Strategies

Process Control

• Maintain proper DO levels in each zone
• Optimize recycle rates
• Monitor and adjust carbon addition
• Control sludge age appropriately

Chemical Addition

• External carbon sources (methanol, acetate)
• pH adjustment if needed
• Nutrient supplementation

Benefits of De-nitrification

• Reduces nitrogen loading to receiving waters
• Helps meet effluent permit limits
• Prevents eutrophication
• Protects aquatic life
• Supports regulatory compliance

Understanding the relationship between nitrification and de-nitrification is essential for successful nitrogen removal. Proper sequencing, monitoring, and control of both processes are key to achieving consistent and reliable performance.