AirPrex® Patented P-Recovery Process

The challenge

Increased phosphate levels lead to additional costs for sewage treatment plants

The biological phosphate elimination removes phosphates without the use of chemical precipitants.

This environmentally friendly and cost-saving process also has disadvantages:

  • In the anaerobic environment, the struvite deposits in piping, pumps and equipment, thereby reducing the performance of the treatment plant.
  • The high phosphate concentration in the anaerobic sludge leads to inefficient sludge dewatering.
  • The separated sludge liquor has a high phosphate back load.

CNP’s Solution

AirPrex®the future of sludge treatment

This is how AirPrex® works:

The AirPrex® process improves biological phosphate elimination. The digested sludge is fed into the AirPrex® reactor where it is subjected to CO2 stripping through aeration. The CO2 increases the pH of the sludge significantly. At the same time, the addition of magnesium salts leads to the formation and precipitation of magnesium-ammonium-phosphate (MAP). The crystals integrate homogeneously into the sludge matrix. The recovered MAP can then be used as fertilizer.


AirPrex® reduces costs and increases efficiency

AirPrex® can significantly increase the profitability of a sewage treatment plant. The unique solution for the struvite problem ensures smooth processes and best results in phosphate elimination:

  • Improved sludge dewatering
  • Lower polymer consumption
  • Lower Ortho-P chargeback to biological treatment
  • Reduced struvite deposits

Sludge treatment and P-Recovery

AirPrex® Micro Process

With AirPrex® Micro there is no struvite recovery, microcrystals remain in sludge for targeting  ≥ 80% P recovery from ash.


  • Increase of cake dryness up to 4%
  • Less polymer consumption (15-25%)
  • Reduction of ortho-P by 95 % in the filtrate           (internal recycling)
  • Prevention of crystallization in the sludge- and     centrate area

AirPrex®Macro Process

AirPrex® Macro, besides struvite harvesting, includes further P release into the second mixing tank where further struvite growth and sedimentation occurs, enabling micro crystals recovery.


The same benefits as AirPrex® Micro and additionally:

  • Increase of cake dryness up to 4%
  • Less polymer consumption (15-25%)
  • Reduction of ortho-P by 95 % in the filtrate (internal recycling)
  • Prevention of crystallization in the sludge- and centrate area