🌍 Overview

This lecture focused on the nitrogen cycle and its relation to resource recovery and pollution control — particularly in wastewater treatment.

Two main parts:

1. The nitrogen cycle in nature and technology
2. Nitrogen removal and recovery processes in wastewater treatment plants (WWTPs)

⚗️ The Nitrogen Cycle

🧩 Main Processes

1. Nitrogen fixation
   • Conversion of atmospheric N₂ → ammonia (NH₃).
   • Occurs naturally (microbes, lightning) and artificially via the Haber-Bosch process.
2. Nitrification
   • Oxidation of ammonia → nitrite (NO₂⁻) → nitrate (NO₃⁻).
   • Two groups of bacteria:
     • AOB (Ammonia-oxidizing bacteria): e.g., Nitrosomonas
     • NOB (Nitrite-oxidizing bacteria): e.g., Nitrospira, Nitrobacter
3. Denitrification
   • Reduction of NO₃⁻ → NO₂⁻ → NO → N₂O → N₂ gas
   • Removes nitrogen from ecosystems; produces greenhouse gas N₂O.
4. DNRA (Dissimilatory Nitrate Reduction to Ammonium)
   • Converts nitrate → ammonia instead of nitrogen gas.
5. Anammox (Anaerobic Ammonium Oxidation)
   • NH₄⁺ + NO₂⁻ → N₂ + H₂O
   • Carried out by Planctomycetes (e.g., Brocadia, Kuenenia).
   • Energy-efficient, no organic carbon needed.

🧪 Importance of Haber–Bosch

The Haber–Bosch process revolutionized agriculture by producing massive amounts of ammonia for fertilizers. ➡️ Enabled human population growth (roughly half of global nitrogen now synthetic). But also:

• 💀 Causes eutrophication in lakes & seas
• 🏭 Adds nitrogen pollution from agriculture and fossil fuels

🚰 Nitrogen in Wastewater

• Typical municipal wastewater contains 30–50 mg N/L.
• EU limit: <8 mg/L discharge.
• Each person produces ~4.4 kg nitrogen/year in wastewater.

🧴 Nitrogen Recovery (rarely done)

• Possible by:
  • Air stripping: raise pH to volatilize ammonia (inefficient).
  • Struvite precipitation: MgNH₄PO₄·6H₂O crystallization.
  • Single-cell protein production: bacteria grown on methane + ammonia.
• Usually not done because the Haber–Bosch route is cheaper.

🧫 Nitrification in Depth

Key Bacteria

• AOB: Nitrosomonas, Nitrosospira
• NOB: Nitrospira, Nitrotoga, Nitrobacter

Nitrospira are slow-growing, oxygen-sensitive, and require long sludge ages (10–20 days in cold climates).

Temperature effects:

• Warm (e.g., tropics): 5 days sludge age
• Cold (e.g., Finland): up to 20 days

🧠 COMAMMOX (Complete Ammonia Oxidizers)

• Discovery: Nitrospira inopinata (“surprising spiral”)
• Can oxidize ammonia → nitrate in one organism (no separate AOB/NOB).
• Discovered via genome sequencing of full nitrification reactors that lacked AOBs.
• Advantages:
  • High ammonia affinity
  • No N₂O production
  • Efficient at low concentrations
• Rare in Danish WWTPs (found in Ribe, Haderslev, etc.)

🧬 Denitrification

Steps:

1. NO₃⁻ → NO₂⁻ (nitrate reductase)
2. NO₂⁻ → NO (nitrite reductase)
3. NO → N₂O (nitric oxide reductase)
4. N₂O → N₂ (nitrous oxide reductase)

Each step handled by different enzymes and often different microbes. → Most species only perform part of the pathway.

Conditions:

• No oxygen (anoxic)
• Organic carbon as electron donor
• pH rises (opposite to nitrification)

Environmental issue: N₂O (nitrous oxide) is a potent greenhouse gas. Some Danish WWTPs face fines if emissions exceed limits.

🧫 Typical WWTP Designs

1. Two-stage system

• Tank 1: Nitrification (with O₂)
• Tank 2: Denitrification (without O₂ + organics added, e.g., methanol)

2. Modern single-sludge system (recirculating)

• Denitrification happens first using organics from wastewater
• Nitrification follows
• Nitrate-rich water is recirculated back
• ⚙️ Internal circulation keeps nitrates available for denitrification

Key parameters:

• Recirculation rate ≈ 6× inflow
• Requires balance between oxygen levels, organics, and sludge retention

🔴 Anammox Process in Practice

Features:

• Found in oxygen-free zones of sediments and reactors.
• Red cells due to anammoxosome organelle (contains rocket-fuel-like hydrazine!).
• Grow very slowly (doubling time ≈ 10 days).

Implementation:

1. Two-step system: partial nitrification → anammox
2. Single-reactor (SHARON-Anammox):
   • Limit oxygen so only ammonia → nitrite
   • Anammox bacteria convert NH₄⁺ + NO₂⁻ → N₂
   • No organic carbon needed

Advantages:

• ~50% less oxygen demand
• No carbon addition
• Lower sludge production

Challenges:

• Complex control (oxygen, temperature)
• Sensitive to disturbances
• Works best in warm, stable conditions

🇩🇰 Denmark’s Success Story

• Denmark introduced large-scale nitrogen removal ~30 years ago.
• One of the most effective countries worldwide.
• Now exploring Anammox-based systems to further cut costs and emissions.

🧭 Summary of Key Takeaways