A Multi-Stage Passive Solar Distillation System with Radiative Cooling and Zero-Liquid Discharge for Sustainable Water Security in Arid and Coastal Regions

Global freshwater scarcity is intensifying due to climate change, population growth, aquifer depletion, and industrial expansion. Conventional desalination technologies such as reverse osmosis require substantial electrical energy, high operational costs, and complex maintenance infrastructure, making them unsuitable for decentralized deployment in vulnerable regions. This study proposes a fully passive, electricity-free, multi-stage solar distillation system optimized for arid coastal and saline inland environments. The system integrates:(1) shallow multi-basin cascading evaporation trays,(2) radiative cooling condenser panels,(3) natural draft solar chimney ventilation, and(4) zero-liquid discharge salt recovery channels. By thermally cascading latent heat across multiple evaporation stages and enhancing night-time radiative cooling, the proposed model significantly improves water productivity compared to traditional single-basin solar stills. The design enables modular expansion from community-scale (1,000 m²) to semi-urban scale (10,000 m² and above) without grid electricity. The study outlines engineering design parameters, thermodynamic basis, material selection, salt management strategy, and feasibility for desert regions such as coastal Middle East and future water-stressed South Asian zones.