Solar-driven rooftop interfacial evaporation system for decentralized rainwater purification.

A global increase in freshwater scarcity coupled with a heterogeneous distribution of water treatment infrastructure highlights an urgent need for decentralized, low-energy water treatment solutions, particularly in remote off-grid regions. Here, a rooftop-based, solar-driven in situ rainwater evaporation and purification system (IREPS) was developed to address these issues. The system was enabled using a dual-network hydrogel (CSA-Fe), formed from chitosan and sodium alginate networks embedded with Fe₃O₄ nanoparticles. The coupling of rainwater harvesting during wet periods with solar-driven evaporation on sunny days, enabled the IREPS system to sustain an average water production flux of 4.1 kg m⁻² d⁻¹ and a peak flux of over 6.6 kg m⁻² d⁻¹ during a 70-day continuous outdoor trial. The collected water was suitable for drinking having consistently met the World Health Organization (WHO) drinking-water standards. The system was also relatively cost effective with a techno-economic analysis revealing a treatment cost of only USD 3.85 per cubic meter, significantly lower than other reported rainwater purification approaches. To assess global potential, the Solar Rainwater Purification Suitability Index (SRPSI) was calculated using multi-source geographic and socio-economic datasets coupled with machine learning, which identified Central Africa, South Asia and Southeast Asia as priority deployment regions. This work successfully demonstrated a practical, scalable strategy for decentralized rainwater purification that combined material design, system integration and global assessment, offering a feasible pathway for clean water access in remote, arid and resource-constrained settings.