Abstract
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.
Highlights
The new generations of wastewater treatment plants need to target several developments including improving effluent quality, cutting greenhouse gas emissions and recovering energy and valuable resources
This confirms the potential of seawater for struvite production, slight reduction in phosphorus recovery was observed (z1-6%) when seawater was used compared to MgCl2 salt as the magnesium source
It was concluded that appropriate product properties by using seawater for struvite crystallization is achievable in addition to comparable P-recovery efficiency with respect to MgCl2
Summary
The new generations of wastewater treatment plants need to target several developments including improving effluent quality, cutting greenhouse gas emissions and recovering energy and valuable resources. The improvement of financial sustainability by reducing the operational costs is a strong financial driver for the nutrient recovery technologies. The preferred method for phosphorus recovery mainly depends on the infrastructure of the existing wastewater treatment plant. Struvite crystallization is one of the main methods used to recover phosphorous from wastewater due to the physical and chemical properties of struvite mineral (MgNH4PO4$6H2O)(Li et al, 2019). Struvite is comprised of primary (nitrogen and phosphorus) and secondary (magnesium) macronutrients and has low solubility in water, which results in reduced runoff and improved plant uptake. Using low-cost magnesium sources for struvite production is an effective approach to improve the economics of struvite crystallization while ensuring the quality of final product
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