Alternative magnesium source for phosphorous recovery – a feasibility and economic analysis

Conventional reservoirs of phosphorous are in high risk of depletion in near future, thus nontraditional and sustainable recovery-practices are essential to ensure its adequate supply in future. Today phosphorous is being recovered from wastewater at industrial scale by addition of MgCl2. However, the addition of magnesium is costly and not sustainable. In this logic membrane technology can play an important role due to the close alignment with sustainable development and process intensification strategy [1]. Current study analyzes the feasibility of integrated membrane processes for magnesium recovery from seawater for utilization in phosphorous recovery process. Seawater cannot be used directly since it contains high amount of chloride ions, which affect the following removal of NH4, and therefore, treatment is needed.

The integrated membrane systems consist of nanofiltration (NF), membrane distillation (MD) and membrane crystallization (MCr). The first scheme 1 considers stand-alone NF, whereas scheme 2 and scheme 3 involve NF-MD and NF-MD-MCr, respectively. Magnesium in the form of MgCl2 has an estimated treatment cost of 0.43 $/kg of struvite (MgNH4PO4·6H2O), whereas membrane treatment ranges from 0.14 – 0.36 $/kg of struvite depending on the considered flow sheet and the NF membrane utilized. The lowest associated cost is found for stand-alone NF treatment with a rough pretreatment. However, it has to be taken into account that an additional treatment with MD and MCr produces fresh water and other salts like NaCl, thus increasing the overall profit. In perspective, integrated membrane systems aiming at minerals’ recovery from seawater can be of great interest for cost reduction and sustainability of phosphorous recovery from nontraditional resources.

[1] E. Drioli, A. Brunetti, G. Di Profio, and G. Barbieri, “Process intensification strategies and membrane engineering,” Green Chem., vol. 14, p. 1561, 2012.