Besides COD removal wastewater treatment plants must be able to remove minerals from the wastewater. Here the focus is on nitrogen and phosphorous removal to protect the environment from excessive availability of such compounds. Compounds that could otherwise contribute to unwanted eutrophication of surface waters.

In many countries 40 mg/l total-N and 2 mg/l total-P is the discharge limit on surface waters. Limits in the trend of becoming much lower (like only 10 mg/l total-N and 0.15 mg/l total-P) and more and more strict. With sometimes also ‘sub limits’ for TKN, NH₄, NO₃, NO₂.

In the classical biological solution nitrogen will be converted from NH₄ (ammonia) via NO₂ (nitrite) to NO₃ (nitrate), named nitrification. That process is realised under aerobic conditions (with oxygen). To also remove those nitrogen compounds from the water denitrification is needed. That process will execute under the absence of oxygen, but with the availability of carbon (COD, BOD, TOC). Denitrification is to convert NO₃ (nitrate) via NO₂ (nitrite) to N₂ (nitrogen gas), which will be emitted to the air. Depending on the chosen process steps, a compliant final effluent is then easily possible, having all forms of nitrogen at low levels: low organic-N + low NH₄ + low NO_x = low total nitrogen (TN). In many countries TN must be below 30 mg/l to allow discharge to the environment.

In parallel to nitrogen removal from the water to the air, some nitrogen will also be removed with the grown biomass or sludge.

Over the years several alternatives are developed. Specially to deal with scenarios of limited availability of carbon. The so named C/N ratio must be high enough to remove enough nitrogen from the water via nitrification/denitrification processes. Several available solutions:

• External addition of carbon source.
  • It has the idea to overcome the carbon deficit for the denitrification process.
• Nitrite (NO₂) shunt.
  • Here the idea is to eliminate the nitrification step from NO₂ (nitrite) to NO₃ (nitrate) and the denitrification step from NO₃ (nitrate) to NO₂ nitrite).
  • It can save 25% on oxygen demand + 40% on carbon consumption!
• Anammox (ANaerobic AMMonium OXidation).
  • It’s the name of both the process and the bacteria involved.
  • The benefit of these bacteria is it’s ability to anaerobically (without oxygen) convert NH₄ (ammonia) to N₂ (nitrogen gas), by consumption of NO₂ (nitrite). That NO₂ (nitrite) originates as result of the first step of nitrification by other bacteria.
  • It can save 62% on oxygen demand + 100% on carbon consumption!
• Automation or Advanced Control
  • Optimising specially (above) solutions in favor of nitrogen removal. Instead of carbon removal.
• Struvite pathway. Also known as MgNH₄PO₄∙6H₂O, Magnesium Ammonium Phosphate (MAP) and ammonium struvite.
  • Under specific conditions like the sufficient availability of each of the needed compounds (magnesium, ammonium and phosphate) and the correct pH it will start to grow struvite granules in a special designed reactor.
  • With or without removing those granules separately from the sludge, such process can aim for low enough final effluent values on NH₄-N (ammonia nitrogen).
  • Main benefit is the recovery of the ammonia. Specially struvite can be used as fertiliser containing N and P.
• Non-biological and non-struvite solutions like ammonia stripping, ammonia sorption, electrodialysis, ion exchange, and membrane technologies.

Different from carbon and nitrogen removal, phosphor don’t have compounds that can be emitted to the air. Therefor all must be removed via the sludge for example. Some will be incorporated in biomass and disposed with the sludge. Unfortunately many wastewaters contain more phosphate then needed by the bacteria.

Therefor several solutions are developed over the years:

• Addition of chemicals. For chemical precipitation for example.
• Bio-P or Enhanced Biological Phosphorus Removal (EBPR).
  • Luxury uptake of phosphorus by bacteria. Some types of bacteria have the ability to take more phosphorous with them then they strictly need. It’s a process depending on having the different environmental conditions of anaerobic (without O₂ and NO_x) and aerobic (with O₂).
  • It can remove up to about 5x more phosphorus then by ‘standard’ sludge growth only.
• Automation or Advanced Control
  • Optimising specially (above) solutions in favor of phosphorus removal.
• Struvite pathway. Also known as MgNH₄PO₄∙6H₂O, Magnesium Ammonium Phosphate (MAP) and ammonium struvite. Or technically a mixture of different struvite types can be formed. Having potassium in the wastewater also MgKPO₄∙6H₂O, Magnesium Potassium Phosphate (MPP) or potassium struvite can be a product.
  • Under specific conditions like the sufficient availability of each of the needed compounds (magnesium, ammonium/potassium and phosphate) and the correct pH it will start to grow struvite granules in a special designed reactor.
  • With or without removing those granules separately from the sludge, such process can aim for low enough final effluent values on PO₄-P (orthophosphate).
  • Main benefit is the recovery of the phosphate. Phosphate is a raw material that most probably will become scarce in future. Specially struvite can be used as fertiliser containing N, P and K.

Here it’s even more difficult to simply remove it with WWTP’s. It still has limited attention too by researchers and the industry. That despite potassium might become scarce 3 times earlier then phosphate!?

Therefor several solutions are developed over the years:

• Struvite pathway. Chemically written as MgKPO₄∙6H₂O and named Magnesium Potassium Phosphate (MPP) or potassium struvite.
  • Under specific conditions like the sufficient availability of each of the needed compounds (magnesium, potassium and phosphate) and the correct pH it will start to grow struvite granules in a special designed reactor.
  • With or without removing those granules separately from the sludge, such process can aim for low enough final effluent values on potassium.
  • Main benefit is the recovery of the potassium. Potassium is a raw material that most probably will become scarce in future. Specially struvite can be used as fertiliser containing P and K.