Combining Hydrologic Analysis and Life Cycle Assessment Approaches to Evaluate Sustainability of Water Infrastructure

Designing urban water infrastructure systems for environmental sustainability requires consideration of hydrologic performance design criteria and results from life cycle assessment (LCA). The objective of this paper is to present a study of merging hydrologic and LCA criteria into the evaluation of the environmental sustainability of rainwater harvesting (RWH) to control combined sewer overflows (CSOs). A case study of the City of Toledo, Ohio combined sewer system serves as the platform to investigate the two approaches and to compare RWH with centralized gray infrastructure strategies for controlling CSOs. Results show that the four RWH scenarios studied (two RWH-only: one to supply toilet flushing demand and the other to serve as extended detention; and two hybrids based on combining the two previous with gray infrastructure) delivered higher combined sewage volumes to wastewater treatment facilities compared to a gray infrastructure-only scenario. This resulted in elevated life cycle global warming potential (GWP) impacts (1.10kg CO2e per 1m3 of reduced CSOs) on average for the four RWH scenarios. The gray infrastructure-only scenario reduced GWP impacts because it included sewer separation leading to lowered amounts of combined sewage volume treated (-1.89kgCO2e per 1m3 of reduced CSOs). But, due to the untreated stormwater discharges to receiving waters, the gray infrastructure-only scenario led to a higher ecotoxicity water (ETW) impact [-12.11 comparative toxic unit for ecotoxicity (CTU eco) per 1m3 of reduced CSOs] compared to the RWH scenarios (-19.62 CTU eco per 1m3 of reduced CSOs on average). In conclusion, the new watershed-scale LCA framework led to more information on the CSO control strategies compared to hydrologic-only analysis, but it created a more complicated decision. Information from Toledo water stakeholders must be taken into account before nominating a scenario as the one that globally outperforms the others according the multi-hydrologic-LCA criteria.