Climate change and industrialization necessitate a reassessment of water management strategies, particularly in agriculture, where reclaimed water supply often fails to meet irrigation needs. Storage can bridge supply gaps but raises concerns about water quality deterioration due to microbial changes and pathogen regrowth. This study examined microbial dynamics and regrowth during reclaimed water storage from a municipal wastewater treatment plant in Germany. The treatment train included ozonation, filtration and UV disinfection, and samples were analyzed using traditional culture methods for indicator organisms (e.g., Escherichia coli,  Clostridium perfringens spores, and somatic coliphages) and 16S rRNA gene amplicon sequencing. Samples were collected throughout the treatment train and stored at 22 °C in the dark for up to 15 days. Results showed effective microbial reduction by treatment, with storage alone achieving similar declines in many cases. While treatment reduced bacterial diversity, storage gradually restored it, forming distinct microbial profiles from the original water quality. Bacterial communities converged during storage, suggesting a succession-like stabilization process. The findings highlight the dynamic nature of reclaimed water microbiomes and the importance of stimulating stable microbial communities to preserve water quality during storage. Advanced treatment should remove contaminants while supporting microbiomes that protect public health and the environment.

Agricultural water reuse is one approach to mitigate water stress. In addition to the minimum requirements, the European water-reuse regulation 2020/741 mandates a risk management approach for agricultural water reuse. In contrast to the microbiological monitoring, the extent of the chemical risk assessment and monitoring is not clearly defined. The resulting complexity of a typical agricultural water-reuse scheme was analyzed. Potentially relevant parameters were identified based on European and German regulatory frameworks, concerning key subjects of protection. An interdisciplinary assessment of efforts and challenges, regarding required analyses, was accomplished, using expertise from recent research investigating agricultural water reuse in Germany. Suggestions were provided for disinfection validation, microbiological monitoring parameters and analytical methods. Additionally, chemical indicator parameters were suggested to address relevant processes during monitoring. Both microbiological and chemical parameters presented analytical challenges, which were described with future needs to support water-reuse implementation. Costs for analyses were estimated using available price information, highlighting the high costs of certain analyses, especially for organic micropollutants. Therefore, analyses need to be further facilitated by the application of process indicators and the implementation of cost-effective, multi-target methods tailored to the requirements of risk management for agricultural water reuse.

Die Wiederverwendung von gereinigtem Abwasser zur landwirtschaftlichen Bewässerung ist eine vielversprechende Lösung, um die landwirtschaftliche Produktivität in Zeiten des Klimawandels zu erhalten. Allerdings sind mit dieser Praxis auch Umweltrisiken verbunden, da gereinigtes Abwasser, Rückstände von Krankheitserregern und Schadstoffen, darunter vor allem Spurenstoffen enthalten kann. Eine weitergehende Abwasserreinigung ist daher entscheidend, um potentielle Risiken zu minimieren. Forschungsprojekte wie FlexTreat arbeiten an technischen Lösungen zur sicheren Wasserwiederverwendung. Diese Szenarienanalyse zeigt, dass die Bewässerungsmenge und die Art der Abwasserbehandlung wesentliche Faktoren für den Eintrag von Spurenstoffen ins Grundwasser sind. Eine bedarfsgerechte Bewässerung während der Vegetationsperiode kann die benötigte Wassermenge reduzieren und den Spurenstoffeintrag in das Grundwasser verringern. Zusätzlich kann eine höhere Bewässerungsfrequenz den Spurenstoffeintrag weiter reduzieren, wobei sich die Summe der verwendeten Bewässerungsmenge nicht erhöht. Diese Erkenntnisse unterstreichen die Bedeutung von Managementansätzen, die sowohl technische Lösungen als auch angepasste Bewässerungspraktiken umfassen, um eine sichere Wasserwiederverwendung zu gewährleisten und Grundwasserbelastungen zu minimieren.