Urban Water in Hannover for Recreational and Climate-Resilient Use

The increasing frequency of extreme weather events, water scarcity, and growing urban populations have placed significant pressure on cities to adopt more sustainable and resilient water management strategies. Among these, the use of roof-harvested rainwater (RHRW) in blue-green infrastructure for ornamental and recreational purposes has emerged as a promising solution. However, the integration of such systems in urban settings introduces new and often overlooked microbial exposure pathways that can pose potential risks to public health. This dissertation explores the microbial quality of alternative urban water sources and evaluates associated health risks, with a specific focus on informal water contact in nontraditional urban water bodies. The research is grounded in three peer-reviewed case studies conducted in the city of Hannover, Germany, each addressing a different type of infrastructure: public ornamental systems, private cistern system, and a public urban pond. Across these studies, a combination of field-based monitoring, microbial analysis, and Quantitative Microbial Risk Assessment (QMRA) was applied to assess contamination levels, pathogen presence, exposure conditions, and health risks, particularly for children, who represent a highly exposed and vulnerable population in these environments. The findings consistently revealed that the use of untreated or minimally treated RHRW may exceed the acceptable microbial risk thresholds defined by international guidelines. Pathogens such as Salmonella spp., Pseudomonas aeruginosa, and Enterococci were frequently detected, with risk levels increasing significantly during warmer months and following rainfall events. The studies also underscored the limitations of relying solely on fecal indicator bacteria (FIB) for health risk assessment, as these indicators do not necessarily reflect the presence or concentration of specific pathogens. Furthermore, user behavior including type, frequency, and duration of water contact - was shown to significantly influence the final risk estimation, yet it is often underrepresented in current QMRA models. Additional factors such as system design, infrastructure maintenance, and stagnation periods were found to directly impact microbial water quality and pathogen proliferation. The absence of first-flush diverters, irregular maintenance, and limited microbial monitoring contributed to elevated contamination levels. Despite the increasing prevalence of such systems in schoolyards, parks, and courtyards, current regulatory frameworks in Germany and the EU remain insufficient to address non-potable uses of RHRW, especially in contexts where informal contact and incidental exposure occur. These gaps present regulatory blind spots in the governance of water reuse in urban settings. This dissertation highlights the urgent need for multidisciplinary approaches that integrate environmental microbiology, urban planning, public health, and behavioral science to ensure the safe and sustainable use of alternative water systems. Recommendations include the development of pathogen-specific monitoring tools, the use of sensor-based early warning systems, implementation of microbial source tracking, and the co-creation of risk communication strategies that involve local communities. By addressing these critical aspects, the research contributes to ongoing discussions around urban sustainability and public health, offering practical insights for cities working to align with Sustainable Development Goals, particularly SDG 6 (Clean Water and Sanitation) and SDG 11 (Sustainable Cities and Communities).