Life-cycle assessment and life-cycle cost analysis of decentralised rainwater harvesting, greywater recycling and hybrid rainwater-greywater systems

Decentralised rainwater harvesting (RWH), greywater recycling (GWR), and hybrid rainwater-greywater systems (HRG) mitigate urban water scarcity at both domestic residential dwelling and commercial building scales. However, few studies have been conducted on HRG in mixed urban water provision schemes under tropical climatic conditions. Thus, this study evaluates and compares the environmental and economic impacts of a centralised mains water system (MWS) against a decentralised RWH, GWR and HRG at a domestic and commercial building using life-cycle assessment (LCA) and life-cycle cost (LCC) analysis under tropical climatic conditions. A functional unit (FU) of 1 m3 of non-potable toilet flushing and irrigation water for a project lifespan of 50 years was used. Results indicate that CML 2001 and TRACI 2.1 methods produce similar results. LCA and LCC indicate that the optimal systems are the commercial HRG and domestic RWH. The commercial HRG has the highest mains water savings (55.3%), lowest environmental impact scores for seven categories, and is the second fastest system to become financially attractive at USD5.20/m3. Similarly, the domestic RWH has the second highest mains water savings (95.3%), lowest environmental impact scores relative to a MWS for seven impact categories, and is the first system to become financially attractive at USD2.00/m3. Sensitivity analysis revealed that global warming, water stress index, and eutrophication are most sensitive to a ∓20% variation in GW and RW energy intensity values at commercial and domestic buildings, respectively. The financial viability of decentralised RWH, GWR, and HRG systems increase with increasing discount rate and mains water tariff, as well as decreasing electricity tariff and installation factor. Financial incentives and subsidy schemes from the Malaysian government may promote uptake of decentralised water systems for mixed urban water provision, as none of the systems were financially attractive otherwise at a discount rate of 6% for 50 years.