Recovery of phosphates as struvite from urine-diverting toilets: optimization of pH, Mg:PO₄ ratio and contact time to improve precipitation yield and crystal morphology

Phosphate (P) recovery from urban wastewaters is an effective strategy to address environmental protection and resource conservation, aiming at an effective circular economy. Off-grid wastewater treatment systems like urine-diverting toilets (UDT) can contribute to source separation towards nutrient recovery, namely phosphorus recovery. Effectiveness of P precipitation requires a process-based knowledge regarding pH, Mg:PO₄, contact time and their interactions in P recovery and crystal morphology. Several studies failed to see the process as a whole and how factors influence both morphology and P recovery for UDT hydrolysed urine. This study addressed the above-mentioned factors and their interactions, and results showed that pH and Mg:PO₄ ratio are the key factors for struvite precipitation, whereas contact time is relevant for crystal growth. The recommended set of factors proposed (pH 8.5, Mg:PO₄ ratio of 1.2:1 and 30 minutes contact time) not only promotes a high precipitation yield – 99% of P with co-precipitation of at least 21% of ammonium (NH₄⁺) – but also leads to larger crystals with lower water solubility (10% less crystals dissolved in water after 3 days). The obtained outcome facilitates the downstream process and leads to a more efficient slow-release fertiliser, as less P is wasted to receiving waters by leaching, minimising eutrophication processes.