Resilience to Future Changes: Assessing the Impact of Human Wastewater Emissions on Microbiological Water Safety Along the Danube

Transboundary rivers are crucial resources for drinking water, recreation, and irrigation. However, wastewater emissions and global environmental and demographic changes can impair raw water quality and pose risks to public health. This study aims to assess the impact of emissions from wastewater treatment plants, combined sewer overflows (CSOs), and inland waterway transport on microbiological water safety along the upper Danube River Basin (DRB).

To achieve this, we developed a stochastic mathematical model to trace pathogen load emissions throughout the river network. The model predicts concentrations of reference pathogens (Cryptosporidium, Giardia, Campylobacter, norovirus, enterovirus) in the river for current conditions and a future climate scenario represented by a selected CORDEX RCP 8.5 high emission scenario. The study estimates bathing water infection risks and determines the required pathogen logarithmic reduction in raw river water to ensure safe drinking water. The model accounts for exponential pathogen inactivation rates influenced by water temperature, solar ultraviolet radiation, and lake sedimentation (for protozoan cysts/oocysts). High-resolution navigational information based on automated identification system (AIS) data, detailing the number and location of ships, were used to model the impact of inland waterway transport. The data were aggregated into monthly average daily ship volumes across three ship types (cruise, passenger, and freight) along a section of the Danube River. Model validation was conducted using monthly data sets spanning 2–4 years, including cultivation-based standard fecal indicators, human-associated genetic fecal microbial source tracking markers (HF183/BacR287, BacHum), and reference pathogens (Cryptosporidium, Giardia). Additionally, the study investigates whether the crAssphage marker (CPQ_056) serves as a suitable proxy for human viral fecal contamination in water quality modeling, compared to standardized viral indicators such as somatic coliphages. To understand the effects of future changes on water safety, various scenarios and combinations up to the year 2100 are analyzed, including population growth (affecting wastewater emissions), climate change (impacting river discharge and microbial inactivation), advanced wastewater treatment, reduction of CSOs (in line with the recast of the European Urban Wastewater Treatment Directive), and changes in inland navigation and ship wastewater handling.

The findings indicate that tertiary treated municipal wastewater currently has the greatest impact on river water safety. However, if additional disinfection (quaternary treatment) is implemented, other pollution sources, such as ship navigation and CSOs, as well as climate change effects, will play an increasingly significant role in determining microbiological water safety.