Technology Spotlight

MAP recovered with AirPrex® 

What is your technology? 
AirPrex® is a sludge optimization process that removes and recovers orthophosphate through controlled formation of struvite (NH4MgPO4·6H2O). AirPrex is provided by CNP-Technology Water and Biosolids Corp. 

In a continuously mixing reactor, the boundary condition for struvite precipitation is achieved by stripping CO2 to adjust pH of sludge and elevating magnesium concentration by adding magnesium salt, namely MgCl2. Struvite crystals are harvested from the bottom cone of the reactor. The AirPrex reactor is designed to remove 90% of orthophosphate from sludge. The AirPrex is typically installed between anaerobic digestion and dewatering process, where uncontrolled precipitation of struvite is most prevalent causing scaling and clogging of pipes and process equipment. The AirPrex process is complimentary to a biological P-Removal process. The Bio-P process produces digestate high in orthophosphate which exacerbates uncontrolled precipitation of struvite. AirPrex reduces the amount of orthophosphate being recycled back to the wastewater treatment process and therefore improves the overall efficiency of phosphorus removal of the plant. In addition, removal of orthophosphate improves the dewaterability of sludge and reduces polymer consumption for sludge dewatering. Overall, the AirPrex system brings multiple ways to save costs for the wastewater treatment plant, while generating a valuable slow-release fertilizer product.

What are the benefits to implementing your technology?

  • 90+% orthophosphate P removal from sludge 
  • Mitigation  of uncontrolled  struvite precipitation in pipes and process equipment
  • Improved dewaterability of biosolids
  • Reduction of polymer consumption by sludge dewatering process
  • Harvesting of struvite which can be used as a slow releasing phosphorus fertilizer

Who can benefit from the AirPrex process? 
The best candidates for the AirPrex process would be plants with high orthophosphate levels in sludge prior to dewatering and already experiencing the problem of uncontrolled precipitation of struvite and poor dewatering performance. The typical candidate for AirPrex is a plant configured as a biological phosphorus removal (Bio-P) plant followed by anaerobic digestion or sludge storage without aeration, which triggers the release of orthophosphate. We also have observed the wastewater treatment plants accepting high strength industrial wastewater having similar problems without Bio-P processes. 

Has the technology been tested, demonstrated, or implemented to date?
AirPrex is installed at seven wastewater treatment plants (WWTP) in Europe, including Niersverband, MG Neuwerk WWTP since 2009; Berliner Wasserbetriebe, Wassmannsdorf WWTP since 2010; Waternet, Amsterdam WWTP since 2014 and Tianjin, China since November 2015. Plants range from 10 MGD to 170 MGD. AirPrex is a scalable technology, and even for smaller plant sizes, AirPrex® can provide favorable return of investment in ten years or less.

AirPrex® at Salzgitter WWTP, Germany, 20 MGD

AirPrex® at Amsterdam, West, Holland, 170 MGD

AirPrex® at Berlin Wassmannsdorf, Germany, 120 MGD 

AirPrex® Pilot Unit at Fox River WRD, Elgin, IL 

What are the next steps needed to advance your technology? 

1) Bringing the first full scale AirPrex installation to North America. CNP is currently conducting on-site pilot tests at several small (10 MGD) and large (>100 MGD) wastewater treatment plants in the US and hoping to bring the first full-scale installation in near future. A typical test lasts four weeks, including three weeks of testing, but could be shorter if the steady operation of AirPrex is achieved. If a plant experiences struvite related O&M issues and poor dewaterability, AirPrex can help to resolve those issues. As part of this test, CNP will collect data from these pilots trying to analyze the impact of orthophosphate concentrations and monovalent vs. divalent ions on the dewaterability of biosolids. If desired by the utility, CNP would be able to furnish a small skid mounted centrifuge for the duration of the test to compare the test results with their existing dewatering technology.

2) Assessing the combined effect of sludge pretreatment and phosphorus recovery. CNP would like to test the combination of its PONDUS Thermo-Chemical Hydrolysis Process (TCHP) with AirPrex. Addition of TCHP could allow installation of AirPrex prior to anaerobic digestion. More data needs to be collected evaluating the practicality and economic feasibility. We are looking for interested utilities to pilot test this concept.

3) Improving the marketability of harvested struvite. CNP will continue the research and development to improve the phosphorus recovery efficiency and quality of harvested struvite.  

How has LIFT helped, or how can LIFT help?
Via LIFT, CNP is looking forward to identifying potential projects and pilot opportunities for AirPrex in the US. We also hope to identify the technologies which could bring synergetic effects for the AirPrex process, such as Bio-P or thermal-chemical hydrolysis. Finally, CNP would like to learn from LIFT members about the challenges municipal wastewater treatment plants face when installing phosphorus removal and recovery technologies, to accelerate the process of solving phosphorus pollution.

For more information about the AirPrex process, please contact CNP at