Life Cycle Assessment of Stormwater Management Systems - Quantification of environmental impacts for decision support

Abstract

Stormwater management (SWM) is necessary to control flooding and pollution, and to reduce the risk of resulting damage to humans, property and nature. Large investments have been made in Denmark to adapt existing SWM systems to expected climatic changes. Besides traditional subsurface systems, alternative solutions utilising green infrastructure (GI) are increasingly implemented. These alternatives can provide additional benefits for humans and the environment that are rarely quantified, especially with economic assessments remaining the focus of SWM planning. In this project, a life cycle assessment (LCA)-based framework, to systematically and holistically assess environmental impacts caused by SWM systems, was developed and tested. Boundary conditions for implementing the method in the existing planning process at water utilities were analysed to establish environmental sustainability as a parameter in decision-making. LCA is a standardised tool employed to evaluate cradle-to-grave environmental sustainability. Environmental impacts are quantified in different categories (e.g. climate change, resource scarcity) and in terms of resulting damage to areas of protection (e.g. ecosystems). It is applicable to a wide range of products, services and systems, which makes choices necessary. We developed a framework in which an LCA can be used systematically to quantify comprehensively the impacts of SWM systems at the catchment level. Two aspects were identified as especially important. First, we suggested defining the primary function, which has to be provided equally by all assessed alternatives, as the provision of flood safety targets for different rain domains. This approach ensures comparability while reflecting the essential characteristics of SWM systems. Second, impacts caused by the implementation, operation and decommissioning of the infrastructure, as well as those caused by point source emissions, should be considered in the assessment. Point source emissions of toxic substances and nutrients are caused by discharging polluted stormwater into the environment. Both infrastructure and point source emission impacts are highly site-dependent, and limited data are available during system planning. In this project, we therefore developed generic inventories for both impact sources. Infrastructure processes were assessed and quantified using documentation for implemented systems, planning documents, expert interviews, guidelines and other types of literature. Point source emissions were assessed by first defining average pollutant concentrations based on a literature review, and then assessing the flow of polluted water through the single elements of the systems with different removal efficiencies. The developed framework was used to assess different SWM systems in two Danish urban catchments (Nørrebro in Copenhagen, and Skibhus in Odense). As a novel approach, we quantified both infrastructure and point source emission impacts comprehensively and on the catchment scale. Across categories, we found the infrastructure impacts of subsurface systems to be higher than of GI systems, caused mainly by the production of materials and decommissioning processes. When assessing infrastructure processes and resulting damage to resource availability, GI systems were environmentally preferable due to limited material demands, avoided road renewal and recycling. Evaluating point source emissions and damage to ecosystems led to contrary conclusions: subsurface systems, where stormwater is treated in a central wastewater treatment plant, caused the lowest impacts, while GI was less efficient in removing pollutants, leading to higher ecotoxicity and eutrophication impacts. This highlighted the importance of including point source emissions, and quantified the trade-off between impacts caused by advanced treatment versus those avoided as a result of increased pollutant removal. Existing tools for evaluating environmental sustainability often require detailed inputs and expert knowledge, limiting their applicability in the planning context of SWM systems. An analysis of the existing planning processes revealed a highly iterative nature, with detailed system descriptions only being available right before the implementation of the systems. Based on these findings, we identified the need for a simplified, LCA-based tool to assess the environmental impacts of SWM systems at the conceptual planning stage. A generic process inventory was developed for 30 different SWM elements, which is required as background data for the tool. Necessary input parameters were developed to specify relevant characteristics of the single elements. Together with planners working in the Danish water sector, the format and types of output were defined, in order to support decision-making effectively. The suggested tool, based on the developed LCA framework, provides the basis for including environmental sustainability in the future planning of SWM systems.