Integrated Design of Centralized and Decentralized Reclaimed Water Networks

Urban water scarcity, intensified by climate change and seasonal variability, is increasingly driving cities toward the use of reclaimed water as a supplementary supply. Reclaimed water, derived from treated wastewater or greywater, can significantly reduce pressure on potable water resources, yet its large-scale integration into existing urban water infrastructure remains poorly optimized. In particular, most existing reclaimed water distribution networks (RWDNs) are designed either manually or using computationally intensive methods and rarely account for their functional interactions with potable water distribution networks (PWDNs). This is critical because reductions in potable water demand due to reclaimed water use can lead to adverse water-quality effects in PWDNs, especially in low-density urban areas.

This work presents a citywide, integrated, and computationally efficient framework for the optimal design of RWDNs that explicitly considers their interdependence with PWDNs and urban spatial structure. The framework is capable of generating and evaluating a wide spectrum of RWDN configurations, ranging from fully centralized systems to highly decentralized, multi-source networks. It relies on openly available spatial data, including street networks, land-use information, and digital elevation models, making it transferable to different cities.

The framework automatically generates initial RWDN layouts based on the correlation between street networks and water networks and refines them using information on land plots, topography, spatial demand patterns in the PWDN, and reclaimed water origin–destination relationships. Based on this integrated spatial analysis, a wide range of centralized and decentralized network configurations is produced, and optimal pipe diameters are determined using efficient graph-based optimization methods. Each candidate network is automatically evaluated with respect to its impact on the existing PWDN. This enables the identification of feasible designs that satisfy water reuse requirements while minimizing adverse water-quality effects in the potable water system. At the same time, it explores how many small installations can be meaningfully interconnected and optimally coordinated with the central supply network so that water availability and demand match in space and time.

The final centralized and decentralized RWDNs are compared in terms of cost, water savings, and overall system performance. Application of the framework to a large European city demonstrates that well-designed decentralized and hybrid reclaimed water systems can substantially reduce potable water demand while maintaining acceptable water quality in PWDNs, highlighting the importance of integrated planning for urban water reuse.

Funding: This research was funded by the Austrian Science Fund (FWF) [10.55776/P36737].