*2.2. Rainwater Harvesting Systems*

Rainwater harvesting systems can be installed at the household level to capture roof runoff, providing an alternative source to meet non-potable water demands [29]. Harvesting rainwater is a millennia-old agricultural practice, with increasing implementation in modern cities with large population demands, including in Adelaide and Addis Ababa [29,30]. Rainwater harvesting is used widely because it provides easy collection with low cost, treatment, and maintenance requirements [31]. Captured rainwater can be applied on-site or on a larger scale for community purposes, and communal rainwater tanks may be economically feasible [32]. Rainwater harvesting may be feasible for individual users, subject to the specific water demand and roof area [31,33], but its economic feasibility may be limited for some households because it does not provide a continuous supply of water and needs to be supplemented with other sources [34]. As an alternative source, however, rainwater can provide significant volumes of water, and it is estimated that up to 80% of rainfall could be harvested from urban rooftops in the U.S. [35]. For example, a study of California water indicates that recycling irrigation runoff water (priced at (\$0.43–1.21 per 1000 gallons) was a cost-effective alternative to using the region's municipal water (\$2.39–2.91 per 1000 gallons) [36]. Rainwater harvesting has other benefits, and it can reduce stormwater infrastructure costs by reducing peak flows. Liang et al. [37] showed that implementing smart rainwater harvesting systems reduces peak system flows by 35% to 85%.

There are practical limitations and costs that prevent the transition to the wide-scale use of rainwater infrastructure. Rainwater harvesting is shown to have long payback periods before benefits outweigh costs, with economic returns that are very sensitive to local policy, water quality concerns, and government rebates [38,39]. Grants from local initiatives and environmental agencies can reduce capital costs. For example, rebates of \$0.50 per gallon of installed tank capacity were used to incentivize rainwater harvesting in Barbados [38]. The costs and benefits indicate that green infrastructure solutions have market value and should be strongly considered, though the economics of purchasing tanks and pumps, as required in the smart water grid, may need economic incentives to encourage wide-spread adoption. The system that is proposed in this research creates a new efficiency in the use of rainwater by providing the means to share rainwater within a community and creating a reliable source of non-potable flows by integrating rainwater harvesting within a reclaimed water reticulation system.
