This study aimed to fully recover ammonia contained at a high concentration in anaerobic digestate of piggery wastewater (ADPW) by forming struvite. As magnesium and phosphorus sources, ferronickel slag (FNS) and K2HPO4 were used, respectively. By leaching 200 g L-1 of FNS with 3.0 M H2SO4, 10,309 mg L-1 of magnesium ions were extracted, and this acid-leachate of FNS (FNSL) also contained 5965 mg L-1 of total iron. In order to simultaneously remove both high concentrations of organic matters in ADPW and iron in FNSL which were known to hinder struvite formation, the mixture of ADPW and FNSL was added with H2O2 at the H2O2/Fe molar ratio of 0.75 and pH 4.0. After Fenton reaction, removal efficiencies of COD and total iron reached 77.36% and 99.89%, respectively. Then COD and an iron-reduced mixture of ADPW and FNSL were added with K2HPO4 satisfying Mg:N:P molar ratio of 1.2:1:1.15 at pH 9.5 to produce struvite for 1 h. From 1 L of ADPW (2.21 g NH3-N), 0.65 L of FNSL (4.65 g Mg2+), and 5.63 g of PO4 3-P, 46.7 g of precipitates were obtained. Overall removal efficiencies of magnesium, NH3-N, and phosphorus were 98.59%, 94.25%, and 99.97%, respectively. Obtained precipitates were analysed by using XRD, XRF, SEM-EDX and found to be struvite with impurities of potassium and metals. Additionally, the economic feasibility of FNS was assessed by estimating chemical costs of various magnesium sources.
Keywords: Acid extraction; Fenton reaction; anaerobic digestate of piggery wastewater; ferronickel slag; struvite.