Phosphorus recovery from wastewater by struvite in response to initial nutrients concentration and nitrogen/phosphorus molar ratio

Abstract

Nutrients recovery from wastewater using struvite crystallization has received significant attention because the recovered nutrients can be used as a fertilizer in agriculture. However, the wastewater with different composition of nutrients might exert distinct struvite characteristics. The objective of this study is to explore the influence of nitrogen/phosphorus (N/P) molar ratio and initial nutrient concentrations on the phosphorus (P) recovery from wastewater and the crystallographic properties of struvite through batch experiments. Both the higher nutrients concentrations and N/P molar ratio in the wastewater resulted in higher recovery of P. However, initial nutrients concentration in the wastewater posed a larger effect on the formation of struvite than that of initial N/P ratio. The ratio of N to P consumed during the process varied with varying the N/P ratio or the initial nutrients concentration, indicating the existence of different crystallization mechanisms under different elemental compositions in the wastewater. The decline in solution pH was negatively correlated with the N/P ratio, whereas it was positively correlated with the nutrient concentration. The crystals produced were mainly rough (dendritic) under the N/P ratio of 2, while the morphology evolved from a regular coffin-like to irregular columnar with the increase in N/P ratio from 4 to 10. Meanwhile, the shape of the irregular cylinder gradually changed from elongated needle shape to the thick rods appearance with the increase in the nutrients concentration. Higher N/P ratio increased the nucleation rate, resulting in finer particles and an uneven crystal size distribution, but purer struvite crystals. The purity of struvite was also positively correlated with the initial nutrients concentration. The Visual MINTEQ modelling analysis demonstrated that the supersaturation ratio of struvite gradually increased with the increase in the N/P ratio or nutrients concentration, which led to a positive effect on struvite crystallization.