Nitrogen Removal in Wastewater Treatment
Engineering approaches to meet stringent total nitrogen discharge limits.
Total nitrogen removal requires a combination of nitrification (converting ammonia to nitrate under aerobic conditions) and denitrification (converting nitrate to nitrogen gas under anoxic conditions). The revised EU Directive mandates TN < 10 mg/L for plants > 10,000 PE, with 80% minimum removal efficiency. Most existing plants achieve partial nitrogen removal but need process optimization or additional treatment stages to meet future limits.
Key Numbers
Technical Overview
Nitrogen exists in wastewater primarily as ammonium (NH4+) and organic nitrogen. Effective removal requires converting these forms through biological processes:
Nitrification: Autotrophic bacteria (Nitrosomonas, Nitrospira) oxidize ammonium to nitrate under aerobic conditions. This requires sufficient dissolved oxygen (>2 mg/L), adequate SRT (>8 days at 10°C), and alkalinity for pH buffering.
Denitrification: Heterotrophic bacteria reduce nitrate to nitrogen gas under anoxic conditions, using organic carbon as the electron donor. Internal recirculation returns nitrate-rich mixed liquor from the aerobic to the anoxic zone.
Advanced processes: For very low TN targets (<6 mg/L), post-denitrification with external carbon dosing or anammox-based processes may be required.
Engineering Insight: Carbon Management
Technology Comparison
Pre-Denitrification (MLE)
highModified Ludzack-Ettinger process with anoxic zone upstream of aerobic zone and internal recirculation.
- Uses influent carbon
- Proven and reliable
- Low chemical cost
- Good for retrofit
- Limited by recirculation ratio
- TN typically > 8 mg/L
- Needs adequate C:N ratio
Post-Denitrification
highSeparate denitrification stage after nitrification with external carbon dosing.
- Very low TN achievable
- Independent of influent C:N
- Consistent performance
- External carbon cost
- Additional reactor volume
- Carbon dosing control complexity
SBR (Sequencing Batch Reactor)
mediumTime-based alternation between aerobic and anoxic phases in the same reactor.
- Flexible operation
- Good for small plants
- Single tank design
- Simple to retrofit
- Batch operation
- Decanting complexity
- Limited throughput
MBBR Add-On
highMoving bed biofilm reactor carriers added to existing tanks to increase nitrification capacity.
- Minimal footprint
- Uses existing tanks
- Increases SRT
- Quick installation
- Carrier cost
- Screen maintenance
- Aeration redesign needed
For Existing Plants
EU Nitrogen Limits
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Last reviewed: August 2026. Regulatory requirements may vary by national implementation. This content is for informational purposes and does not constitute legal or engineering advice.