Struvite Crystallisation of Synthetic Urine Using Magnesium Nitrate: Effect of Parameters on Crystal Size Distribution

Urine diversion toilets have become popular as a means of solving the challenges in sanitation. As a result, the source separated urine must be adequately treated so that it can be disposed of safely and valuable struvite can be extracted for use as fertiliser. Struvite crystallization has been investigated for this purpose, however, for the crystallization process to be viable and economical, a cost effective yet optimal magnesium source is required. Crystallisation must have optimal yield as well as crystal size and morphology (CSD). Crystal size is an important factor in crystallisation as it affects further processing steps such as washing and filtering, therefore CSD must be controlled in crystallisation processes. In this study, synthetic urine was prepared, and struvite crystallisation experiments carried out using Magnesium Nitrate (MgNO3). The effect of parameters on CSD were investigated. Residence time was found to be the parameter with the most effect on CSD. At residence time of 10, 30 and 60 minutes, mean particle sizes were 17, 34 and 53 µm showing that with higher residence times, larger crystal sizes can be achieved. SEM analysis of the crystal showed that the resultant crystals had the typical morphology of struvite crystals.