Struvite pyrolysate recycling combined with dry pyrolysis for ammonium removal from wastewater

The dry pyrolysis of magnesium ammonium phosphate (MAP) with NaOH powder for ammonium release was investigated, as well as the utility of MAP pyrolysate recycling. The identities of the MAP pyrolysate and its derivatives were experimentally validated. The results showed that the pyrolysate was amorphous magnesium hydrogen phosphate (MgHPO4) and magnesium pyrophosphate (Mg2P2O7). The best molar ratio of sodium hydroxide (NaOH) powder to ammonium was 1:1, at 110 °C for 3 h. The optimum pH for pyrolysate recycling was 9.5. The ammonia removal ratio could be maintained above 80% with MAP pyrolysate recycling. Seed crystal inoculation increased the rate of MAP crystallization by 20.86%, as well as the MAP grain size (2.08 nm with seeding versus 1.72 nm without). MAP particle size with NaOH treatment decreased: d(0.5) = 19.34 μm versus d(0.5) = 30.35 μm for direct pyrolysis. The results demonstrated that crystal growth was controlled by adding NaOH during MAP pyrolysis.