Human urine-based fertilizers: A review

Urine contains most of the nutrients excreted by humans. They are mainly released into the environment and contribute to the strong disruption of planetary biogeochemical cycles. These nutrients could be separated at source and used in agriculture. Once the urine is separated different treatments can be applied to stabilize nitrogen, reduce the volume, treat contaminants, or extract nutrients. These treatments and the resulting fertilizers have different characteristics and environmental impacts that must be assessed. We reviewed the characteristics of six treatments and the resulting fertilizers. The studied treatments were restricted to those that retain the most nutrients: storage, acidification, alkalinization, nitrification, and mixture with organic substrate. Phosphorus precipitation was also included, as it is one of the most developed treatments. Volume reduction posttreatments were also considered. We compared these treatments and the resulting fertilizers in terms of nutrient forms and concentrations, fertilizer efficiency, contaminant removal, energy, and chemical consumption. Although some parameters, such as fertilizer efficiency or pharmaceutical removal were in the same range, the nutrient concentrations and the energy and chemical consumption requested by fertilizer production varied widely. Holistic studies and pilot facilities implemented at a larger scale are needed in order to assess the whole value chain.