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.