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P-Recycling is now a German law. We already have the answers to the legal requirements.

The P-recovery is no longer a trait of pioneer wastewater treatment plants, but recently it became a legal requirement (AbfKlärV) in Germany. And other EU countries will most probably follow soon.

We have developed three suitable process technologies that meet three particular requirements of the regulation. All patented or patent pending. AirPrex®-Micro and AirPrex®-Macro recover phosphorus as small and large struvite crystals. CalPrex® recovers phosphorus as brushite crystals. Both struvite and brushite are proven as excellent plant fertilizers. And thus both economically and ecologically equally attractive.

German sewage sludge ordinance (AbfKlärV 2017), makes phosphorus recovery from sewage sludge obligatory. Only sewage sludge from sewage treatment plants with a capacity of up to 50,000 PE may continue to be used for soil-related purposes in the future, provided that it meets the strict quality requirements.


In both processes, struvite (magnesium ammonium phosphate) is precipitated in the AirPrex® reactor. AirPrex® technology prevents uncontrollable forming of struvite leading to irreversible deposits in pipes and machines. It has been proven successfully many times with digested sludge before dewatering.

The AirPrex®-Micro process forms struvite as microcrystals that remain in the sludge. The positive effects of sludge optimization are fully exploited. It is suitable for P-recycling (sludge fertilizer) or for improved P-recycling from the ash after mono-incineration.

The AirPrex®-Macro process, in addition to the improved recovery of struvite, is based on an increased recovery of phosphate from the sludge in the second reaction tank. There is further crystal growth and the recovery of fine microcrystals.



The CalPrex® process recovers brushite (di-calcium-phosphate) crystals by adding lime and /or CaCl2. In contrast to AirPrex®, CalPrex® is placed upstream from the digestion process in combination with acidogenic hydrolysis.