Crystallization kinetics and growth of struvite crystals by seawater versus magnesium chloride as magnesium source: towards enhancing sustainability and economics of struvite crystallization.

The recycling of nutrients from wastewater and their recovery in the form of valuable products is an effective strategy to accelerate the circular economy concept. Phosphorus recovery from wastewater by struvite crystallization (MgNH4PO4·6H2O) is one of the most applied techniques to compensate for the increasing demand and to slow down the depletion rate of phosphate rocks. Using low-cost magnesium sources, such as seawater, improves the financial sustainability of struvite production. In this study, the potential of seawater for struvite crystallization versus the commonly used magnesium source, MgCl2, was tested by crystal growth and kinetic experiments. The impact of ammonium concentration, magnesium concentration and pH on the growth kinetics of struvite in synthetic and real reject water were studied. The results showed that simultaneous precipitation of calcium phosphate was insignificant when using seawater, while presence of struvite seeds diminished it further. Among the supersaturation regulators, pH had the most significant effect on the struvite growth with both MgCl2 and seawater, while high N:P molar ratios further improved the struvite crystal growth by seawater. The N:P molar ratios higher than 6 and Mg:P molar ratios higher than 0.2 are recommended to improve the crystal growth kinetics. It was concluded that seawater is a promising alternative magnesium source and the control of supersaturation regulators (i.e., Mg:P, N:P and pH) is an effective strategy to control the reaction kinetics and product properties.

Struvite crystallization by using raw seawater: Improving economics and environmental footprint while maintaining phosphorus recovery and product quality.

Seawater, as an alternative magnesium source, has the potential to improve the overall economics and environmental footprint of struvite production compared to the use of pure magnesium salts. However, the dilution effect and the presence of other ions in seawater can reduce the phosphorus recovery potential and the simultaneous precipitation of other compounds may reduce the quality of the produced struvite. This work presents a comparative study of seawater and MgCl2 by performing a series of thermodynamic equilibrium modeling and crystallization experiments. The results revealed that acceptable phosphorus recovery (80-90%) is achievable by using seawater as the magnesium source for struvite precipitation. Further, the simultaneous precipitation of calcium phosphates was successfully controlled and minimized by optimum selection of reaction pH and seawater volume (i.e. Mg:P and Mg:Ca molar ratios). The increase of temperature from 20 °C to 30 °C reduced the phosphorus recovery by 15-20% while it increased the particle size by 30-35%. The presence of suspended solids in reject water did not have significant effects on phosphorus recovery but it made the struvite separation difficult as the obtained struvite was mixed with suspended solids. The experimental results and economic evaluation showed that the use of seawater can reduce the chemical costs (30-50%) and the CO2-footprint (8-40%) of struvite production. It was concluded that seawater is a potential alternative to pure magnesium sources in struvite production, while studies in larger scale and continuous mode are needed for further verification before full-scale applications.

Enhancing efficiency and economics of phosphorus recovery process by customizing the product based on sidestream characteristics - an alternative phosphorus recovery strategy.

The enhanced biological phosphorus removal process makes the phosphorus recovery feasible from the dewatering streams of biological sludge. The physicochemical properties of these sidestreams, as an input to a crystallizer, are different before and after anaerobic digestion. In this study, phosphorus recovery by calcium phosphate is proposed for pre-digestion sidestreams and by struvite precipitation for post-digestion sidestreams. The thermodynamic modeling followed by experimental tests was performed to evaluate the recovery efficiency and product properties of struvite and calcium phosphates. The variations in phosphorus recovery potential, reaction kinetics and particle size distribution emphasize the importance of the adjustment of initial supersaturation and pH of the reaction. The optimum pH, considering the economics and recovery efficiency, for both calcium phosphate and struvite precipitation was found to be pH = 8.5, whereas further increase of pH will not improve the overall efficiency of the process. In the case of calcium phosphate precipitation, it was shown that possible phase transformations should be considered and controlled as they affect both process efficiency and product properties. The economic evaluation indicated that the optimized operational condition should be determined for the phosphorus recovery process and that chemical costs for the production of calcium phosphates is lower than for struvite.