Abteilung Siedlungswasserwirtschaft

Cities are increasingly adopting blue-green infrastructure (BGI) to address the dual challenges of extreme rainfall and rising temperatures driven by climate change. While the potential of BGI for urban stormwater management is well-studied, the cooling effect of stormwater-focused BGI remains underexplored. This study investigates the heat mitigation potential of three stormwater BGI elements, bioretention cells, porous pavements, and detention ponds, within three urban street canyons in a Swiss town near Zurich. The Urban Tethys-Chloris (UT&C) microclimate model was modified to explicitly represent stormwater BGI and assess their influence on the Universal Thermal Climate Index (UTCI) at 2 meters above the ground. Simulations were conducted under both historical climate and a future climate projection, including a sensitivity analysis of soil types. BGI can cool up to 2.7 °C, but their effectiveness depends on the type of BGI, the surface it replaces, the time of the day, and the availability of water. Soil properties were found to significantly influence the cooling effect of bioretention cells, with finer-textured soils achieving higher soil moisture levels and greater reductions in UTCI. A trade-off between cooling and stormwater infiltration also emerged. Sandy soils favor infiltration but dry out quickly, limiting cooling, while clay-rich soils limit infiltration but retain moisture and sustain evaporative cooling, even under future climate conditions with longer dry spells. These findings highlight the importance of integrating hydrological and thermal considerations into BGI design. Integrated approaches that balance both objectives are needed.

Urban climates typically exhibit high local variability and diverge markedly from mesoclimate conditions. Yet, urban microclimate data needed to assess the impacts of urban heat on biodiversity are lacking. We present a scalable modeling approach to generate high-resolution projections of urban bioclimatic conditions using Zurich, Switzerland, as a case study. We demonstrate that globally available land use and remote sensing data can effectively predict spatial patterns in urban bioclimatic conditions. Our findings show that mean annual temperatures can deviate by up to 2 °C from mesoclimate conditions, and that uncorrected mesoclimate data substantially underestimate risks to 182 species of amphibians, birds, butterflies, dragonflies, grasshoppers and trees. By the end of the century, up to half of these species are expected to exceed their climatic tolerance across all habitat patches. Our approach is transferable to cities globally using existing data, marking a major step forward for urban microclimate and biodiversity research.

Understanding how the performance of blue-green infrastructure (BGI), nature-based stormwater management systems such as bioretention cells and green roofs, evolves is essential to sustaining their hydrological function. However, traditional monitoring approaches typically compare BGI performance using rainfall events classified as similar based solely on total depth, which can obscure subtle differences in hydrological performance. This study presents a workflow that improves temporal assessment by (i) identifying statistically similar rainfall events across periods using rainfall characteristics such as intensity, duration, and peaking time, and (ii) analysing hydrological response using both volume- and time-based metrics.
We evaluated the workflow under controlled but synthetic conditions using simulations of bioretention systems with underdrains. Two Swiss case studies (Kloten and Bern) were used to represent rainfall-only and mixed inflow BGI configurations, respectively.
Results show that isolating temporally similar rainfall events reduces variability compared with volume-based grouping, enabling more reliable detection of performance change. Among the performance metrics tested, the volume-based indicator proved more robust, detecting temporal differences in 16–38 matched rainfall–response pairs for the two configurations. In the mixed inflow system, additional runoff increased variability, but the workflow still detected a consistent performance change over time. In contrast, time-based metrics such as P-UF lag and centroid shift required substantially larger samples to achieve comparable statistical power.
These findings demonstrate the workflow's value as a proof-of-concept for designing efficient BGI monitoring programmes. Applications to other BGI types, climatic regimes, and extended datasets remain future work.

1. Green roofs are increasingly acknowledged as novel habitats capable of sustaining diverse species assemblages. Due to their distinct abiotic conditions and stochastic colonization pathways, they are hypothesized to support ecological communities that differ from those of ground-level habitats, yet the capacity of these habitats to sustain coherent and robust food webs is hardly known.
2. Combining environmental DNA metabarcoding from soil samples with a metaweb approach, we compared the composition, structure and robustness of ground-level and green roof food webs across 52 paired green roof and ground-level sites along an urban densification gradient in Zurich, Switzerland.
3. Food webs on green roofs were compositionally and structurally distinct from those at ground level. These networks were denser (higher connectance), more diffuse (greater omnivory) and less structured (increased trophic incoherence) than ground-level webs. Together, this made the food webs on green roofs more prone to secondary extinctions. These patterns were largely driven by the predominance of predators and a lack of basal consumers, such as detritivores.
4. Variation in food web properties was mediated by green roof design. Specifically, older roofs with deeper substrates supported sparsely connected, vertically diverse food webs that exhibited greater robustness to secondary extinctions than more recently built green roofs.
5. Synthesis and applications. Green roofs operate as distinct habitats, offering novel environmental conditions, thus increasing urban habitat diversity and supporting denser but less robust food webs compared with ground-level habitats. Increasing substrate depth and allowing for long-term development can promote trophic diversity and robustness, informing strategies for urban biodiversity conservation and ecosystem management.

As municipal wastewater treatment regulations become more stringent, integrating source-separated urine treatment into centralized urban wastewater management offers a ‘hybrid’ solution. However, it is not clear how the environmental impacts of such hybrid systems compare to highly efficient centralized wastewater treatment plants (WWTPs) with low N₂O emissions and electricity use. In this study, a consequential life cycle assessment was used to compare the environmental impact of three urine separation hybrid wastewater treatment systems – which combine decentralized urine treatment with a highly efficient central WWTP– to a centralized WWTP treating mixed wastewater (baseline). The studied urine treatment systems include partial nitrification & distillation, struvite precipitation & stripping/scrubbing, and partial nitritation/anammox. Additionally, the environmental impact of urine pre-treatment by calcium hydroxide and electrochemical alkalinization methods on the partial nitrification & distillation system was evaluated. The results show that all hybrid scenarios have a lower environmental impact in the freshwater ecotoxicity, marine toxicity, freshwater eutrophication, and marine eutrophication categories compared to the baseline. However, all hybrid scenarios resulted in higher impacts on global warming compared to the baseline, with direct N₂O emissions being a key variable. Additionally, it was identified that urine alkalinization increased the environmental impact of the treatment system in 7 out of the 10 impact categories. A Pareto frontier analysis was developed to guide decision makers on where hybrid solutions could be used as a strategy to reduce the global warming impacts of conventional WWTPs. Using N₂O-N emission factors of 75 WWTPs, 87 % of centralized WWTPs had a lower global warming impact compared to partial nitrification & distillation, and 91 % compared to partial nitritation/anammox hybrid solutions. However, at electricity demands of 1 kWh/PE/day and 1.5 kWh/PE/day at the central WWTP, both hybrid solutions showed lower global warming impact than all the studied WWTPs. The study highlights the potential of hybrid wastewater treatment solutions as a strategy to reduce global warming impacts in WWTPs with high N₂O emissions and electricity use as well as a means to reduce marine eutrophication (i.e. nitrogen pollution).