Are alternative magnesium sources the key for a viable downstream transfer of struvite precipitation? Assessment of process feasibility and precipitate characteristics

Abstract

Phosphorus (P) recovery in source-separated decentralised wastewater treatment processes is an eco-efficient strategy in accordance with a circular economy perspective. Struvite (MgNH4PO4.6H2O) capture allows a reduction of P discharged to surface waters and generates a product known for its slow-release fertiliser properties. However, the cost and availability of current commercial Mg2+ sources hinder the downstream transfer of struvite precipitation. Although some alternative sources are available, there is a lack of information on how they compare to conventional sources; it is also not known how the Mg:PO4 ratio affects process performance or fertiliser product suitability. Therefore, this research evaluated the performance of two alternative sources, seasalt and seawater bittern, at different Mg:PO4 ratios. The results indicate that seasalt may be a problematic option owing to its high levels of NaCl, regardless of the Mg:PO4 level, which could affect soil fertility. Bittern, meanwhile, showed a high yield at pH 8.5 and Mg:PO4 1.2:1, achieving 99% recovery of PO4 and increasing also the recovery yield from 60% (using conventional sources) to 82% at Mg:PO4 1:2. In addition, bittern showed larger X-type crystals than commercial sources. The precipitate is composed mainly of struvite and Newberyite (MgHPO4.3H2O), with traces of K+, and no presence of NaCl at Mg:PO4 1.2:1; while at Mg:PO4 1:2 it forms a mixture of struvite with presence of Ca and K phosphates. In conclusion, bittern is an effective raw material to improve the downstream transfer of struvite precipitation into urine-diverting toilets in wastewater treatment systems.