Comparative study of environmental impacts related to wheat production with human-urine based fertilizers versus mineral fertilizers

Abstract

Most of the fertilizers used in conventional agriculture are derived from nonrenewable resources: natural gas for nitrogen fertilizers and fossil mines for phosphorus and potassium fertilizers. On the other hand, the nutrients from human urine are poorly recycled with regard to agricultural repurposing, despite the fact that urine can be a nutrient resource for agriculture. Human urine may be collected through source separation and treated to produce urine-based fertilizers, resulting in various value chains. However, the environmental impacts and benefits of this fertilization method compared to current fertilization practices are not well known.Using a life cycle assessment, we assessed the environmental impacts of three scenarios of wheat production fertilized with three different urine-based fertilizers: stored urine, nitrified concentrated urine and alkalinized dehydrated urine. We compared them to a reference scenario in which mineral fertilizers are used for wheat production and urine is treated in a wastewater treatment plant.The environmental impacts for the scenarios using the urine-based fertilizer value chains were lower than those for the reference scenario for climate change, eutrophication, fossil resource and water consumption impact categories. These lower impacts were partly related to the avoided impacts from mineral fertilizer production. Moreover, the avoided impacts from the sanitation system (water consumption, greenhouse gas emissions, and nutrient discharge to the river) were significant and allowed for further and greater reduction of the environmental impacts for the urine-based fertilizer scenarios compared to those for the reference scenario. Nevertheless, stored and alkalinized dehydrated urine can induce higher ammonia volatilization compared to mineral fertilizers, resulting in a higher acidification potential and particulate matter emissions. The electricity consumption values for the nitrified concentrated urine and alkalinized dehydrated urine fertilizers are also higher than that of the reference system. Thus, ammonia volatilization and energy consumption of the urine treatments have been identified as the main environmental hotspots of urine-based fertilizer value chains. In conclusion, the use of urine-based fertilizers completes biogeochemical cycles for the sustainable management of nutrients. It also enables a global reduction of the environmental impacts of fertilization in agriculture and of wastewater treatment.