Modeling for Transition: Deploying rainwater harvesting systems in citywide water infrastructure systems

In modern cities, water flows are managed by centralized and mechanized infrastructures. Increasingly, this 20th-century water management paradigm is considered to be inadequate, vulnerable, inefficient and costly under both... [ view full abstract ]

In modern cities, water flows are managed by centralized and mechanized infrastructures. Increasingly, this 20th-century water management paradigm is considered to be inadequate, vulnerable, inefficient and costly under both current and future hydrological and socio-economic conditions. A transition towards a more sustainable urban water system is increasingly discussed but has been hindered by, apart from other factors, the lack of knowledge of practical paths. Rainwater collected through rainwater harvesting (RWH) systems can offset non-potable water demand in buildings while simultaneously serve for wastewater (e.g. combined sewer system (CSS)) and stormwater management. From the perspective of technology diffusion, RWH systems possess the advantages of low economic cost, easy installation, and well-understood technological performances. Despite the potential benefits, economic and environmental effects of citywide implementation of RWH systems, given the city-specific building portfolios and other physical-engineering-socioeconomic characteristics, are still unknown.

This research develops a novel analytical framework that systematically assesses and optimize the life-cycle environmental and economic effects of deploying RWH systems at the city scale. Various configurations of RWH systems are identified and modeled based on city-specific building portfolios. Depending on the designated usage (e.g. household, office, hospital, shopping mall, etc.) and structure (e.g. heights and roof areas) of buildings, the deployment of RWH systems are associated with a range of installation and maintenance costs, water services realized and fees avoided, and electricity demand. The environmental and economic effects of a citywide RWH deployment are then modeled based on the structures and capacities of existing water infrastructural systems (i.e. water supply, wastewater management, and stormwater drainage), utility fees for various water services and electricity, and the climate pattern of a city. Finally, the environmental and economic performances of the citywide deployment are optimized towards the multiple managerial goals of a city, such as mitigating marine eutrophication or reducing the financial gap of infrastructure upgrades needed for existing potable water supply system. Methodologically, the framework is an integration of the state-of-the-art hybrid life cycle assessment (LCA), systems dynamics modeling, and multi-objective optimization (MOO). 

The proposed framework is applied to case studies in two cities: New York City (NYC), NY and Austin, TX. For both cities, RWH has the potential to relieve the aggravating pressures on existing centralized water supply systems owing to infrastructure aging, the dilemma of increasingly expensive infrastructure upgrades and stringent environmental regulations, and expected demand growth. Given the different precipitation patterns and centralized drainage systems in New York City and the City of Austin, the case studies also offer complementary perspectives regarding RWH’s wastewater and stormwater management potentials, and the different opportunity costs of water conservation for cities with various water stress risks. Findings of this research elucidate the long-anticipated transition towards sustainable urban water infrastructure systems in the United States and many other matured economies.