Assessing Water Savings and Cost-Effectiveness of Non-Potable Water Systems in Brazilian Residential Buildings

This study evaluates the potential for reducing residential potable water consumption in the Federal District, Brazil, through the adoption of rainwater harvesting systems (RWHS) and greywater reuse systems (GWRS). The integrated analysis combined hydraulic and economic simulations, considering saturation volumes, economically feasible limits, levelized cost of water (AIC), and payback periods across four representative residential typologies. The results show that the feasibility of these solutions varies significantly across income groups and system types. RWHS proved economically viable only in high-income households, where larger roof areas allowed for optimal storage volumes of 20 m³ in scenarios including irrigation and floor washing, with or without toilet flushing, and 50 m³ in scenarios that also included laundry use. In these cases, AIC values remained positive (0.14–0.78 US$/m³), although payback periods were long (16–18 years). In the remaining typologies, low annual savings combined with installation costs made the system unfeasible. GWRS showed broader applicability. In high-income households, simple solutions such as bucket-based systems or subsurface diversion for irrigation outperformed RWHS, presenting very short payback periods (0.1–2.1 years) and positive AIC values (2.13–3.22 US$/m³). In upper-middle-income households, treated greywater systems were the most advantageous due to higher greywater generation and the dilution of fixed costs across multiple housing units (AIC of 1.43–1.57 US$/m³; payback of 1.2–2.1 years). In lower-middle- and low-income typologies, only low-complexity solutions proved economically viable.