Summary:
Water conservation is a critical aspect of sustainable building design, particularly in urban areas where potable water consumption remains high. This study examines the implementation of a zero-energy rainwater harvesting system on the rooftop of HEPIA in Geneva, designed to supply water for toilet flushing and irrigation. While rainwater harvesting is an established practice, its application in buildings—both in Geneva and elsewhere—remains limited. This study highlights the potential savings and feasibility of such systems, aiming to encourage wider adoption. The project was initiated to reduce potable water use at HEPIA, where toilet flushing alone accounts for 3,160 m³ per year (25% of the school’s annual water demand), while future irrigation needs are projected at 892 m³ annually. The approach involved assessing day-to-day rainfall availability, water demand, structural constraints, and water quality requirements. Historical precipitation data (1995–2024) from MeteoSwiss informed the evaluation of rainwater collection potential, while utility bills, occupancy data, and an assessment of planned external green spaces provided estimates of current and future water consumption.
Results based on the reference rainfall year for Geneva (30-year mean values for each calendar day) show that HEPIA’s 1,493 m²highest rooftop could collect 1,257 m³ of rainwater annually, enabling 10% savings in potable water, which corresponds to one-third of the water required for toilet flushing, and reducing costs by approximately 5,300 CHF per year. A 10–15 m³ storage capacitywould provide up to five consecutive days of autonomy, covering all sanitary needs for 131 days per year (36% of the time). However, results based on actual years (non-averaged data) indicate that while annual savings remain consistent, system parameters (storage capacity, autonomy, and potential coverage of irrigation and other needs) can vary significantly. These variations highlight the need for dynamic, real-time, multi-variable simulations to optimise storage and other system parameters. Furthermore, while the system operates purely by gravity, requiring no energy for distributing the collected rainwater, maintaining water quality requires disinfection. Low-energy, cost-effective options such as UV treatment, chlorination, or hydrogen peroxide injection are being considered.
By demonstrating the feasibility and potential savings of rainwater harvesting, this study provides a replicable model for sustainable water management in urban contexts and aims to inspire broader adoption in Geneva and beyond. The next steps involve final structural validation for the storage reservoirs, the potential integration of a modular treatment system, and the monitoring of the operational system to validate simulation results.
Keywords: Gravity-fed system, Rainwater harvesting, Sustainable buildings, Urban water management, Water reuse.
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