Recycling nutrients: The promise and perils of wastewater use in global and Brazilian agriculture

As global water demand rises - driven by climate change, population growth, and agricultural expansion - treated wastewater irrigation (WWI) offers a promising strategy for water conservation and nutrient recycling. Agriculture consumes nearly 70 % of global freshwater, while only 50.8 % of wastewater is treated in Brazil, where WWI represents less than 0.1 % of total irrigation. This review critically assesses the potential and challenges of WWI in Brazilian agriculture by comparing global practices, regulatory frameworks, and treatment technologies. WWI can significantly reduce freshwater withdrawals and dependence on chemical fertilizers, enhancing soil fertility through the recycling of nitrogen, phosphorus, and potassium. However, persistent contaminants of emerging concern (CECs) - including antibiotic-resistant bacteria (ARB), resistance genes (ARGs), microplastics, and heavy metals - pose environmental and health risks, as conventional systems such as UASB reactors and stabilization ponds, which are vastly implemented in Brazil, were not designed to remove them efficiently. Despite successful examples in high-income countries, regulatory gaps persist in low- and middle-income countries like Brazil, where only 9 of 27 states have local guidelines for wastewater reuse. The adoption of advanced technologies (e.g., membrane filtration, ozonation, UV disinfection) and the development of risk-based regulatory approaches are essential to ensure safety and public acceptance. Educational initiatives and participatory governance can further promote informed decision-making. By investing in technological innovation, harmonized regulation, and interdisciplinary research, WWI could evolve from a niche practice to a mainstream solution for sustainable agriculture, food security, and water resource management in Brazil and globally.