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

Sina Shaddel,Tonje Grini,Seniz Ucar,Kamal Azrague,Jens-Petter Andreassen,Stein W Østerhus

doi:10.1016/j.watres.2020.115572

Sina Shaddel, Tonje Grini + Show 4 more

Open Access

https://doi.org/10.1016/j.watres.2020.115572

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Abstract

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.

Highlights

The transition towards modern generations of wastewater treatment is being shaped by several developments that aim at improving effluent quality, cutting greenhouse gas emissions and recovering energy and valuable resources
The minimal reduction of phosphorus recovery (z1e6%) by using seawater compared to MgCl2 indicates seawater can be an adequate magnesium source for struvite production
The particle size of the final crystals was highly dependent on supersaturation and pH has a prominent impact on supersaturation as the median particle size was reduced by 40e70% at the pH values higher than 8

Summary

Introduction

The transition towards modern generations of wastewater treatment is being shaped by several developments that aim at improving effluent quality, cutting greenhouse gas emissions and recovering energy and valuable resources. Among all available resources, is a prominent action as phosphorus (P) is a vital nutrient for all forms of life, while it is expected that the quality and quantity of the main source for phosphorus (i.e. mineral rocks) will be reduced in coming decades (Cordell et al, 2011). Regardless of an exact timetable of global phosphate rocks depletion, phosphorus is a limited and irreplaceable resource. The recovery of phosphorus from waste streams can be a promising approach to reduce the conventional mining and moderate the expected future shortage of phosphate (Shaddel et al, 2019b). A critical challenge for a sustainable phosphorus cycle is to improve the economics of struvite crystallization by using low-cost magnesium sources while ensuring the quality of the final product

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