Abstract: Transforming nitrogen and phosphorus in sewage sludge from solid to dissolution is the key for the treatment or resource utilization of nitrogen and phosphorus. In this paper, the effects of pH value on the release of nitrogen and phosphorus from sewage sludge, and the effects of different crystal seeds (vermiculite, coconut-shell biochar and quartz sand) on the removal of nitrogen and phosphorus by struvite precipitation were studied. In addition, the effects of only crystal seeds and the co-treatment of crystal seeds and magnesium salts on removal efficiency of nitrogen and phosphorus in the supernatant of sewage sludge were also investigated. Results show that when the pH of the supernatant increased from 7.45 to 12, the concentration of phosphate (PO₄^3--P) in the supernatant increased from 15.83 to 188.20 mg·L^-1, and ammonia nitrogen (NH₄⁺-N) increased from 48.97 to 166.46 mg·L^-1. There is better removal efficiency of NH₄⁺-N and PO₄^3--P when n(Mg):n(P)=1.2, pH=10, followed by stirring and standing both for 30 min, then adding vermiculite, coconut shell biochar and quartz sand with 2, 2 and 0.5 g·(100 mL^-1), respectively. Among the three crystal seeds, the removal efficiency of NH₄⁺-N and PO₄^3--P was higher by coconut shell biochar than those by vermiculite and quartz sand based on struvite precipitation induction. Compared to non-addition of crystal seed, when adding coconut shell biochar with 2 g·(100 mL^-1), the removal rates of PO₄^3--P and NH₄⁺-N reached to 82.48% and 74.98% and increased by 9.14% and 13.38%, respectively. The results of X-ray diffraction and scanning electron microscopy (SEM) analysis show that the precipitates are mainly struvite which has high recovery value.

Key words: struvite, seed, sludge supernatant, nitrogen and phosphorus