Struvite recovery from swine wastewater using fluidized-bed homogeneous granulation process

Resource recovery in wastewater treatment is the major challenge for achieving sustainable development. Further than only treatment, the recovery and reuse of nitrogen (N) and phosphorus (P) as a slow release fertilizer from actual swine wastewater (SW) was performed in this study. The development of a novel technique of converting soft sludge into hard particles using homogeneous granulation process in a fluidized bed reactor helps address the recovery. Under conditions of pH 8.5 ± 0.2, an Mg:P ratio of 1.2, total suspended solids (TSS) < 100 mg/L, and a 25 m/h up-flow velocity, the removal of N and P reached 69.8% and 96.8%, respectively. XRD and SEM-EDS analysis confirmed that N and P were crystallized as magnesium ammonium phosphate hexahydrate (struvite, NH4MgPO4·6H2O) phase with a purity greater than 68.3% and an average particle size of 0.88 mm. The increase in total suspended solids resulted in not only decreases in the removal efficiency and struvite purity but also an irregular morphology in the struvite crystals. Furthermore, increasing the magnesium concentration significantly reduced the crystal size of the pellets. The optimal up-flow velocity should be 1.5-2.4 times the minimum fluidization velocity (12.75 m/h). A cost-benefit estimation of actual swine wastewater treatment indicated a profit of approximately 0.345 USD per m3-actual swine wastewater. Fluidized-bed homogeneous granulation was proven to be a promising technology for recovering N and P from swine wastewater.