🧪 1. The Phosphorus (P) Cycle

🔹 Nature of the Cycle

Unlike carbon or nitrogen, the phosphorus cycle is not a redox cycle. Phosphorus mainly exists as phosphate (PO₄³⁻) and cycles between rocks, soil, water, and living organisms — without changing oxidation state.

🔹 Geological Origins

• Most phosphorus is locked in rocks formed in ancient times.
• It is released through:
  • Natural weathering 🌦️
  • Mining for fertilizers 🧱

🔹 Human Impact

• Agriculture and industry release excess P into soils and water bodies, causing eutrophication (overgrowth of algae and oxygen depletion in water).
• 🌍 Phosphorus is a critical planetary boundary: too much leads to environmental damage, too little threatens food production.

🔹 Global Resources

• Large phosphate rock reserves are mainly in North Africa and China. → Europe has almost none, creating import dependency.
• The EU classifies phosphorus as a critical raw material due to supply risks.

🌿 2. Phosphorus Use and Recovery

🔹 Modern Use

Phosphorus is imported into countries via:

• Fertilizers
• Animal feed
• Food

💡 In Denmark, about 20% of fertilizer needs can be covered by phosphorus recovered from wastewater.

🔹 Recovery Methods

1. Biosolids reuse: Wastewater sludge applied directly on farmland (cheap but can contain contaminants).
2. Precipitation as struvite (magnesium ammonium phosphate):
   • Produces valuable crystals like 💎 Crystal Green.
   • Reduces unwanted “pipe scaling” (struvite buildup).
3. Incineration + Ash extraction:
   • Sludge is burned, and P is extracted from ash.
   • Clean but very expensive.

🌍 Some countries (e.g. Germany) ban direct sludge use → they incinerate and recover P from ash instead.

⚗️ 3. Biological Phosphorus Removal (Bio-P)

🔹 Basic Idea

Special bacteria known as PAOs (Polyphosphate-Accumulating Organisms) store phosphorus inside their cells as polyphosphate granules.

💡 They remove P biologically instead of via chemical precipitation (with iron/aluminum).

🧫 4. How PAOs Work

PAOs experience two alternating phases:

🔸 Anaerobic phase (no O₂, no nitrate):

• PAOs take up acetate (carbon source) from wastewater.
• They store it as PHA (polyhydroxyalkanoates) — an energy-rich polymer.
• To do this, they break down their stored polyphosphate → releasing phosphate into the water.
• Reaction powered by ATP from polyphosphate breakdown and reducing power from glycogen.

🧩 Key: Only PAOs can take up organics under anaerobic conditions — this gives them a competitive edge.

🔸 Aerobic phase (O₂ present):

• PAOs use the stored PHA for growth.
• They take phosphate back up from the water and rebuild their internal polyphosphate reserves.
• Result: Net phosphorus removal from wastewater.

♻️ Repeating these oxygen/no-oxygen cycles enriches PAOs in treatment plants.

🔬 5. Molecular Machinery

• PAOs have special transporters:
  • Pit transporter (energized by proton motive force)
  • Pst transporter (high-affinity phosphate uptake)
• Pit system is unique: it can generate energy while importing phosphate ⚡

Inside the cells, you find:

• Polyphosphate granules (energy storage)
• PHA globules (carbon storage)
• Glycogen (reducing power)

Up to half of a PAO cell’s biomass can be these storage polymers!

🌏 6. Who Are the PAOs?

Modern research (including global surveys) identifies key genera:

• Tetrasphaera
• Accumulibacter
• Dechloromonas

These are found in wastewater plants worldwide. Different species have different capabilities:

• Some can perform denitrification (reduce nitrate under anoxic conditions).
• Some specialize in fermentation or acetate vs. glucose uptake.

⚔️ 7. PAO Competitors: GAOs (Glycogen-Accumulating Organisms)

• GAOs behave similarly to PAOs but lack polyphosphate storage.
• They can take up acetate and store PHA but don’t remove P.
• Common GAOs: Competibacter, Defluviicoccus, Propionivibrio, Meganema.

💡 In practice: GAOs are rarely a major issue in full-scale plants — mainly a lab artifact.

🏭 8. Wastewater Plant Design

Two main configurations:

🅰️ Conventional EBPR (Enhanced Biological P Removal):

• Alternating anaerobic → aerobic → anoxic zones.
• Uses return sludge to recycle PAOs.
• Residence times: ~2–3 hours per tank.

🅱️ Side-stream hydrolysis system:

• Adds a small side tank with long retention (≥24h) for hydrolysis of particulates → produces acetate and fatty acids for PAOs.
• Increases process stability and P removal efficiency.

💡 9. Testing and Monitoring

Release test:

1. Take sludge sample → make anaerobic → add acetate.
2. Measure P released to water.
   • Good system: high P release.
   • Bad system: low P release.

Denmark’s EBPR plants show strong P-release activity (≈15 mg P released per test).

⚠️ 10. Operational Problems

• Too much nitrate → denitrification instead of P uptake.
• Too much iron → P chemically precipitates (unrecoverable).
• Lack of readily degradable organics → PAOs starve.
• Overabundance of GAOs → competition.

🧭 11. Summary of Key Points