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.

Ana Paula Carvalho,António Guerreiro De Brito,Denis Manuel Rodrigues,Rita Do Amaral Fragoso,Thomas Hein

doi:10.2166/wst.2019.371

Abstract

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.

Highlights

60–65% of the phosphorus and 90% of nitrogen entering human systems is excreted in urine and discharged into sewage systems (Simha & Ganesapillai )
100 mg of precipitate was dispersed in 100 cm3 of distilled water and kept under stirring at room temperature for 24, 48 and 72 h, and the solid was recovered by filtration and dried until constant weight
While an increase in pH and Mg:PO4 leads to a higher recovery of P, increasing the contact time from 20 to 30 minutes leads to almost the same P recovery, not justifying higher contact time if more than 98% is already being recovered, as seen in Table A1, which summarises the data obtained in this study

Summary

Introduction

60–65% of the phosphorus and 90% of nitrogen entering human systems is excreted in urine and discharged into sewage systems (Simha & Ganesapillai ). Due to the lack of efficiency of most nutrient removal processes applied in wastewater treatment plants, most of the nutrients end up accumulating in water bodies, triggering eutrophication phenomena in rivers, lakes and coastal zones (Martins et al ). In this framework, conventional wastewater management concepts are being highly challenged because urine, despite representing only 1% of municipal wastewaters, contributes ∼75% and ∼45% of nitrogen and total phosphorus loads, respectively. In applying source separation of urine and faeces under proper process conditions, phosphorus compounds can be extracted from urine as struvite (NH4MgPO4·6H2O) (Simha & Ganesapillai ). The driver behind phosphorus recovery efforts is the asymmetrical geological distribution of phosphate rock, namely in Europe, where it is almost non-existent, as well as the negative environmental impacts from mining operations (Edahwati et al )

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Conclusion