*2.1. Dual Reticulation Networks*

Only a fraction of water used for urban purposes needs to be potable quality, and reclaimed water can serve as an alternative water source for non-potable applications [18]. Reclaimed water is wastewater that has been treated to levels lower than potable water quality and can be used for nonpotable applications, including washing, cooling, gardening, toilet flushing, and lawn irrigation [19]. Reclaimed water can be provided to a large group of consumers through a dual reticulation system, which includes two parallel pipe networks: the primary water network distributes potable water, and the secondary network conveys reclaimed water [20]. A centralized utility typically manages water reclamation programs to ensure that treatment standards are met and to distribute reclaimed water. Dual reticulation programs have been implemented in cities in the United States, Japan, and Australia [21]. Reclaimed water products can help conserve high-quality water produced by utilities for essential purposes.

Dual reticulation systems impose high capital costs, but costs can be offset by a reduction in demands that are exerted on aquifers and surface water sources, leading to improved ecosystem health and drought resilience [22]. While it is difficult to quantify the externalities in a cost-benefit analysis of non-potable water systems [23,24], dual reticulation systems can offset the use of potable water, creating savings in utility energy costs for water treatment [25]. Energy savings can be sufficient to offset the capital costs required for building a dual pipe infrastructure [26]. Dual reticulation systems may also create benefits by reducing the need for infrastructure investment for the main potable system [27] and systematically encouraging conservation by adding new value to water. The cost-benefit analysis of dual reticulation systems, however, can vary widely based on the characteristics of a location, such as infrastructure design, topography, energy sources, quality of source water, and existing infrastructure [28]. The distance between users and a water reclamation plant and the amount of

uphill pumping can limit the feasibility of dual systems. A study of four U.S. cities that recycle water found that there were significant economic barriers to implementing dual-reticulation systems. Challenges were cited, including diminishing returns, due to the lack of additional large consumers of nonpotable water near the treatment plant; commitments to return treated effluent for instream flows; more efficient options for selling recycled water for cooling and industrial processes; lack of clear and convergent regulations around water reuse programs [22].
