New Strategy to Maximize Phosphorus Utilization of Sewage Sludge Incineration Ash for Struvite Crystallization.

Struvite crystallization can recover nitrogen and phosphorus simultaneously from various kinds of wastewaters as a slow-release fertilizer. However, the enhancement of the removal efficiency of NH₄-N is challenging because the molar concentration of NH₄-N is higher than that of PO₄-P in many types of sewage including digested sludge filtrate. In this study, phosphorus eluate was recovered from sewage sludge incineration ash (SSA) and applied to the struvite crystallization process to increase the removal efficiency of NH₄-N for the digested sludge filtrate. Under acidic conditions, a maximum of 98.4% of phosphorus was eluted from SSA; in alkaline conditions, a maximum of 51.2% was eluted; and in sequential elution conditions with (NaOH+H₂SO₄), a maximum of 98.0% was eluted. Jar tests were performed by injecting three types of eluates (H₂SO₄ 1 N_elulate, NaOH 1 N_elulate, and (NaOH+H₂SO₄)_eluate), and PO₄-P was stably removed (>86%) under all tested conditions. When the NaOH 1 N_eluate was injected, the NH₄-N removal efficiency was highest at 84.4%, followed by 78.4% with the (NaOH+H₂SO₄)_eluate, and 58.7% with the H₂SO₄ 1 N_eluate at the molar ratio of Mg:P:N of 1.5:1.5:1. In addition, the sequential jar tests were conducted by injecting both the NaOH 1 N_eluate and (NaOH+H₂SO₄)_eluate. In the pH range of 8.5–9.5, the PO₄-P and NH₄-N removal efficiencies reached 92.3–94.5% and 97.9–99.1%, respectively. X-ray diffraction analyses confirmed that the majority of the crystal phases were struvite forms. Therefore, the combined application of both the NaOH 1 N_eluate and (NaOH+H₂SO₄)_eluate was adequate to enhance not only the phosphorus recovery but also the removal efficiencies of PO₄-P and NH₄-N. SSA recovering PO₄-P could be utilized as a new phosphorus source in the struvite crystallization process. [ABSTRACT FROM AUTHOR]