Most phosphorus fertilizer used in agriculture comes from phosphate rock, a non-renewable resource. Recovering phosphorus entering a wastewater treatment plant can be done via chemically acidified sludge or incinerated ash, but they're expensive processes if used only to produce fertilizer.

Together, CalPrex and AirPrex capture more than 50% of phosphorus entering treatment plants with minimal addition of chemicals. CalPrex harnesses acidogenic bacteria power to acidify bio-P sludge. Over 60% of phosphorus solubilizes in the acid phase digestion process. Dewatering acidogenically digested sludge isolates the dissolved phosphorus (orthophosphates) in the centrate. The centrate is sent to the CalPrex reactor, where orthophosphates are precipitated by calcium hydroxide and calcium chloride. By maintaining a pH solution of 6.5 or below, phosphorus is recovered as brushite (CaHPO4· 2H2O), a market-ready, high-quality fertilizer. AirPrex converts the remaining orthophosphates in digested sludge to struvite (MgNH4PO4· 6H2O), another market-ready fertilizer, by air-stripping carbon dioxide and adding magnesium chloride. AirPrex reduces phosphorus recirculation by 90%.

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