Electrochemical Salinity Control and Nutrient Recovery in Off-Grid Eco-Flush Toilets

Water scarcity is a global challenge, and its impact on public health, the environment, and food production is particularly severe in areas with extreme water scarcity and ecologically sensitive regions. Without fresh water supply and drainage infrastructure, human excreta may enter the natural water cycle, disseminating pathogens to aquatic ecosystems and contributing to eutrophication. Biological treatment is currently one of the most cost-effective technologies for toilet wastewater treatment, but its application is affected by the continuous accumulation of salinity. In this work, we propose and investigate a hybrid human waste processing and recycling system called an eco-flush toilet framework that integrates biological treatment, electrochemical separation, membrane concentration, and evaporative crystallization to achieve sustainable and near-complete recycling of resources from human excreta. Specifically, electrodialysis was employed to regulate the salt concentration of the fecal flushing water. A novel pH-regulated electrodialysis process was employed to recover both nitrogen (N) and phosphorus (P) from fresh urine. The solutions with recovered nutrients were converted into high-quality solid fertilizers by membrane concentration and solar-driven evaporative crystallization. Mass flow analysis indicates that this eco-flush toilet design, integrating water–energy–nutrient management, has the potential to eliminate the need for external fresh water supply, achieve zero waste liquid discharge, and recover nutrients from human waste as fertilizer. In summary, the off-grid waterless and wasteless eco-flush toilet system represents a promising approach to addressing sanitation, environmental, and developmental challenges in severely water-scarce regions.