Advancing Urine Separation for Sustainable Food, Energy, and Water systems

Abstract

Inefficient nutrient management is an increasingly urgent challenge at the heart of the food-energy-water nexus, posing a threat to public and environmental health. Nitrogen (N) and phosphorus (P) emissions from agriculture and waste lead to detrimental disruptions in natural environments, such as harmful algal blooms. Additionally, fertilizer production and nutrient removal from waste are resource-intensive and contribute to greenhouse gas emissions and land degradation. Efficient nutrient management requires a circular approach that redirects nutrients in waste to beneficial uses, such as in agriculture. Urine separation promotes circular nutrient management by processing nutrients in urine into a fertilizer. However, a number of questions remain about the public and environmental health impacts of urine separation. The objective of this dissertation is to compare the public and environmental health impacts of urine separation to conventional toilets, fertilizers, and urban waste management. From a public health perspective, the general population interacts with urine separation at the toilet. Previous research has connected conventional toilet usage with virus exposure. To determine how urine-diverting toilets (UDTs) affect user exposure to viruses, virus emission levels from flushing were compared between a UDT, which has a urine-diverter, and a mix-flush toilet (MFT) that has one compartment. The results suggest that virus emissions from the MFT potentially exceed the minimum number of viruses that cause infection. In contrast, the UDT removes urine-associated viruses from the toilet, reducing their emission levels. The lower levels of urine-associated virus emissions from the UDT and high frequency of flushing associated with urination suggests that UDTs can significantly reduce exposure to urine-associated viruses. The results from this study can be used in future risk assessments of virus infection from toilets. Environmental health impacts of urine separation were assessed at the field and system scales. At the field scale, a greenhouse experiment was conducted to compare the impacts of urine-derived fertilizers (UDFs) to inorganic fertilizer and compost on soil health and N cycling. Due to their similarities in N availability, the effects on soil N cycling and plant yield were more similar between UDF and inorganic fertilizer than to compost. When compost was applied with UDF, there were higher N2O emissions than UDF alone, but the ratio of N loss to harvested significantly decreased. The similar behavior of the UDF and inorganic fertilizer suggests that UDFs can substitute inorganic fertilizers, significantly reducing resource consumption in agriculture. Additionally, there is an opportunity for UDFs to contribute to ecological nutrient management goals by combining their application with organic fertilizers. At the system scale, a nutrient flow analysis was used to quantify the potential benefits of nutrient recovery from food waste and wastewater for nutrient circularity and sustainable waste management in New York City (NYC). The analysis found that urine and food waste have the largest proportion of recoverable N and P, respectively. Almost half of the nutrient inputs used to produce food for NYC can be replaced by recovered nutrients. Additionally, a suitability analysis revealed specific sewersheds where NYC Department of Environmental Protection can implement urine separation to meet stringent nutrient discharge permits amid population increase. This dissertation advances circular nutrient management by quantifying the potential public and environmental health benefits of urine separation. Stakeholders such as toilet users, farmers, agricultural policy makers, and city agencies can use these results to make informed decisions about implementing urine separation.