Quantification of mains water savings from decentralised rainwater, greywater, and hybrid rainwater-greywater systems in tropical climatic conditions

Decentralised rainwater harvesting, greywater recycling, and hybrid rainwater-greywater systems mitigate water scarcity in urban areas. However, data on the mains water savings potential of these systems is not well documented in Malaysia, and real site characteristics are often neglected. The main objectives of this study were to (i) quantify mains water savings potential of six rainwater harvesting systems, two greywater recycling systems, and a hybrid rainwater-greywater system in Malaysia using real site characteristics as inputs to an in-house spreadsheet RainTANK water balance model; (ii) evaluate and recommend optimal connected roof area and rainwater tank volume combinations to maximise yields in rainwater systems; (iii) determine differences in mains water savings potential between domestic and commercial rainwater and greywater systems; and (iv) evaluate the possibility of upgrading an existing rainwater system to a hybrid rainwater-greywater system. Results showed that domestic rainwater systems supplied more than 90% of non-potable water demand for toilet flushing, laundry, and irrigation (i.e. with an overall reliability ranging between 35.5% and 52.5% for the modelled sites), whereas commercial rainwater system supplied less than 43% of non-potable water demand for toilet flushing and irrigation (i.e. with an overall reliability ranging between 11.2% and 22.1% for the modelled sites). Greywater recycling provided overall reliabilities of 21.1% and 41.0% for commercial and domestic systems, respectively, for toilet flushing and irrigation. Domestic rainwater systems had optimal roof areas and tank volumes, whereas commercial rainwater systems can improve rainwater yields by 41–67% by increasing either connected roof area or total rainwater tank volume. Upgrading to a hybrid system increases mains water savings for both domestic and commercial buildings, and maximum overall reliabilities of 32.3%, 57.1%, and 25.1% at Sites 1, 2, and SP, respectively, can be gained from reusing rainwater followed by greywater. A domestic hybrid system should primarily reuse rainwater and supply remaining demand with greywater, whereas a commercial hybrid system should reuse greywater and top-up with rainwater.