Cost-effective urban farming with alternative water sources treated by nature-based solutions: A critical review of global practices

Urban agriculture supports food security and circular resource use in cities, yet expansion is hindered by limited clean irrigation water. This review examines five alternative water sources available in urban environments (i.e., greywater, blackwater, groundwater, rainwater, and stormwater) for urban crop irrigation, assessing their agronomic suitability and contaminant profiles through systematic global literature synthesis. Four nature-based solution (NBS) technologies (i.e., constructed wetlands, biofilters, green roofs, and green walls) were evaluated as pre-treatment systems, alongside international regulatory frameworks and an economic model applied to tomatoes, faba beans, and sweet peppers. Each water source presents a distinct risk-benefit profile, with contaminants spanning nutrients, pathogens, heavy metals, and emerging pollutants. NBS technologies effectively reduce these risks while retaining crop-beneficial nutrients, with hybrid systems such as wetland-biofilter combinations achieved removal efficiencies exceeding 97% for TSS, BOD, and COD. Crop selection was decisive for economic viability, with high-value crops such as tomatoes and sweet peppers irrigated with treated groundwater yielded the greatest profitability, while faba beans remained unviable regardless of treatment approach. Policy analysis identified substantial governance gaps, including inconsistent enforcement, limited public acceptance, and minimal regulatory guidance specific to urban agricultural water reuse. By integrating treatment technology, crop suitability, economic viability, and policy considerations into a unified framework, this review demonstrates that alternative water sources can meaningfully advance food security and support multiple Sustainable Development Goals when appropriate safeguards are in place.