Nitrogen and phosphorus recovery from sludge treatment by supercritical water gasification coupled with struvite crystallization

To recover nitrogen and phosphorus in the process water from the treatment of sewage sludge using supercritical water gasification (SCWG), struvite crystallization was coupled into the process. K₂CO₃ was added in SCWG to increase syngas production, and improve the migration of N and P into the aqueous phase, which was favorable for the subsequent struvite crystallization. The effect of MgCl₂ and pH on the removal of N, P, and COD as well as the purity of struvite was studied during the struvite crystallization process. The results showed that the syngas yield significantly increased from 9.1 mol/kg to 14.2 mol/kg with K₂CO₃ addition. In the struvite crystallization process, pH had significant impact on the crystallization and the purity of struvite, while MgCl₂ had only a slight effect. The highest N and P removal of 94.8 % and 99.5 % from the aqueous phase was achieved by struvite crystallization at pH 8. The obtained struvite had a high purity of 93.3 %, and the heavy metal content in struvite was lower than the value of standard, so that the obtained struvite sample can be used as fertilizer. This research could be a guideline to utilize the process water with high nutrients from sludge treatment.