Determination of struvite crystallization mechanisms in urine using turbidity measurement

Sanitation improvement in developing countries could be achieved through wastewater treatment processes. Nowadays alternative concepts such as urine separate collection are being developed. These processes would be an efficient way to reduce pollution of wastewater while recovering nutrients, especially phosphorus, which are lost in current wastewater treatment methods. The precipitation of struvite (MgNH4PO4∙6H2O) from urine is an efficient process yielding more than 98% phosphorus recovery with very high reaction rates. The work presented here aims to determine the kinetics and mechanisms of struvite precipitation in order to supply data for the design of efficient urine treatment processes. A methodology coupling the resolution of the population balance equation to turbidity measurement was developed, and batch experiments with synthetic and real urine were performed. The main mechanisms of struvite crystallization were identified as crystal growth and nucleation. A satisfactory approximation of the volumetric crystal size distribution was obtained. The study has shown the low influence on the crystallization process of natural organic matter contained in real urine. It has also highlighted the impact of operational parameters. Mixing conditions can create segregation and attrition which influence the nucleation rate, resulting in a change in crystals number, size, and thus final crystal size distribution (CSD). Moreover urine storage conditions can impact urea hydrolysis and lead to spontaneous struvite precipitation in the stock solution also influencing the final CSD. A few limits of the applied methodology and of the proposed modelling, due to these phenomena and to the turbidity measurement, are also discussed. Schematic representation of the methodology. k, n, A1 are the kinetics parameter of crystallization. τλexp% and τλtheo% are the experimental and theoretical turbidity (%) respectively. ΨV exp and ΨV theo are experimental and theoretical volumetric size distribution. ψN theo is the theoretical size distribution function in number. [Display omitted] ► Turbidity monitoring enables the struvite crystallization kinetics to be determined. ► Modelling of precipitation mechanisms give satisfactory particle size distributions. ► Organic matter has a low impact on struvite crystallization. ► Attrition, segregation and mixing influence struvite crystallization. ► Urine storage influences particle size distribution.