When Are Decentralised Non-Potable Water Systems Environmentally and Financially Viable? Evidence from a Water–Energy–GHG Evaluation of a Healthcare Facility in an Arid City

Rapid urbanisation in arid regions has increased reliance on energy-intensive desalinated water, intensifying environmental and financial pressures on the built environment. Although non-potable water (NPW) reuse is promoted within regional water strategies, empirical validation of decentralised systems at asset scale remains limited. This study applies a greenhouse gas (GHG) intensity metric (kgCO 2 e/m 3 ) to multi-year operational data from a large healthcare facility in Abu Dhabi. The analysis integrates calibrated water balance records, onsite pumping energy (Scope 2), embedded desalination emissions (Scope 3), and a 20-year discounted cash flow framework. Three configurations are evaluated: a fully desalinated baseline, the observed mixed-supply system, and an optimised NPW configuration. The baseline exhibits an emission intensity of 19.53 kgCO 2 e/m 3 . The observed configuration reduces desalinated supply but achieves only marginal decarbonisation (0.40 kgCO 2 e/m 3 ) due to continued dependence on desalinated make-up water. The optimised configuration reduces outdoor water demand by 36.7% and achieves 10.94 kgCO 2 e/m 3 net decarbonisation while improving life-cycle cost (LCC) performance. The results show that GHG intensity is primarily driven by water source substitution and system configuration rather than volumetric reuse alone, providing asset-level evidence for evaluating decentralised NPW systems in arid-climate buildings.