Recycling struvite pyrolysate obtained at negative pressure for ammonia nitrogen removal from landfill leachate

• A magnesium stabilizing agent served as the precursor of struvite precipitation.
• Ammonium was effectively released by pyrolyzing struvite under negative pressure.
• Recycling struvite pyrolysate could achieve approximately 82% of ammonia removal.
• Ammonia removal rate of >68% was maintained during eight recycle cycles.
• The recycling process was economically feasible for the ammonia pretreatment.

Struvite precipitation coupling low-cost materials and recycling was used to remove the total ammonia nitrogen (TAN) from landfill leachate in order to reduce the overall cost of precipitator. Struvite used for recycling was formed using a stabilizing agent (SA) as the precursor, which was formulated by using phosphorus acid and low-grade magnesium oxide (LG-MgO). Results showed that use of SA and LG-MgO for struvite precipitation can achieve TAN removal near that obtained by using pure chemicals. The results of struvite decomposition indicated that maintaining a negative pressure in struvite pyrogenation has a significant effect on the improvement of the ammonium release rate. When struvite pyrogenation was conducted at a vacuum degree of 0.04 MPa and at a temperature of 120 °C for 3 h, the ammonium release rate was >90%. The obtained pyrolysate was recycled for the treatment of landfill leachate without the addition of other chemicals, and the TAN removal efficiency was comparable to that by struvite precipitation using fresh chemicals. When pyrolysate was used repeatedly by supplementing LG-MgO at the MgO:MgHPO4 molar ratio of 0.5:1 and by prolonging the stirring time to 2 h, a TAN removal efficiency >68% could be maintained in the eight-cycle process. Economic analysis revealed that the proposed recycling process could reduce 86% of the cost of struvite precipitation using pure chemicals.