Human urine: A novel source of phosphorus for vivianite production

Abstract

Human urine contributes up to 50 % of the phosphorus load in domestic wastewater. Decentralized sanitation systems that separately collect urine provide an opportunity to recover this phosphorus. In this study, we leveraged the unique and complex chemistry of urine in favor of recovering phosphorus as vivianite. We found that the type of urine affected the yield and purity of vivianite, but the kind of iron salt used, and reaction temperature, did not affect the yield and purity. Ultimately, it was the urine pH that affected the solubility of vivianite and other co-precipitates, with the highest yield (93 ± 2 %) and purity (79 ± 3 %) of vivianite obtained at pH 6.0. Yield and purity of vivianite were both maximized when Fe:P molar ratio was >1.5:1, but <2.2:1. This molar ratio provided sufficient iron to react with all available phosphorus, while exerting a competitive effect that suppressed the precipitation of other precipitates. Vivianite produced from fresh urine was less pure than vivianite produced from synthetic urine, because of the presence of organics in real urine, but washing the solids with deionized water improved the purity by 15.5 % at pH 6.0. Overall, this novel work adds to the growing body of literature on phosphorus recovery as vivianite from wastewater.