Thermal decomposition of struvite pellet by microwave radiation and recycling of its product to remove ammonium and phosphate from urine

Abstract

The precipitation of struvite (MgNH4PO4·6H2O) consumes many chemicals to completely remove ammonium and phosphate from urine and has the difficulty in solid separation from solution. This study proposed an alternative approach for the complete nutrient removal through recycling use of microwave-induced decomposition product of struvite pellet with sizes of 2–4 mm. Results showed that microwave radiation effectively decomposed the struvite pellet in an alkaline solution within 8 min. An increase in microwave power and NaOH concentration enhanced the decomposition. The double-layer structure of the pellet led to multiple paths of struvite decomposition. Active components of the decomposition product were newberyite, brucite, and amorphous MgNaPO4 and MgHPO4. The removal efficiencies of ammonium and phosphate from urine both reached 93% using the decomposition product at optimized P/N ratio and pH. Maximum recycles of 4 were recommended because further decomposition of the regenerated struvite pellets induced high losses of magnesium and phosphate. Calculations showed that the total cost of chemical consumption of the proposed approach was reduced by 47% compared with that of a conventional chemical struvite precipitation. Moreover, the volume index of the regenerated struvite pellets was 15 mL/gP which was much lower than that of conventional struvite fines (116 mL/gP), thereby indicating a better solid-liquid separation ability. Therefore, recycling of struvite pellets combining with microwave decomposition was chemical saving and easily separating of solid from liquid for the complete removal of nutrients from urine.