OPTIMIZATION OF ALKALINE–THERMAL HYDROLYSIS TO OBTAIN STRUVITE FROM DIGESTED SLUDGE USING A BOX–BEHNKEN DESIGN: SOLUBILIZATION OF NUTRIENTS AND METALS

In this study, we investigated ways by which to optimize metals and nutrients solubilization from sewage sludge using alkaline–thermal hydrolysis and the Box–Behnken design. We also examined through struvite crystallization the recovery of solubilized nutrients from hydrolyzed liquid and determined the effect of NaOH concentration, the liquid/solid ratio, and temperature on the hydrolysis process. Nutrients solubilization was positively affected by decreasing liquid/sludge ratio and increasing NaOH concentration. Ca, Al, and Zn solubilization increased with increasing temperature. The optimum condition for solubilization of nutrients and metals was 0.7 M NaOH and a 5/1 mL/g liquid/solid ratio at 35 °C. EDS analyses of hydrolyzed sludge obtained under optimum conditions showed that the mass percentage of C, P, Fe, Al, and K decreased compared to that of the digested sludge. Under optimum conditions, the removal efficiencies of NH4+ and PO43- from hydrolyzed liquid by struvite precipitation were 57.43 and 79.22% at a N:Mg:P molar ratio of 1:1:1, and 73.31 and 99.02% at a N:Mg:P molar ratio of 1:1.5:1, respectively. XRD analyses of the dry precipitate showed hazenite in addition to struvite formation at a molar ratio of N:Mg:P of 1:1:1.