Improving the microbiological quality of the Ruhr River near Essen: comparing costs and effects for the reduction of Escherichia coli and intestinal enterococci.

After rain events in densely populated areas, combined sewer overflows (CSOs) can have severe health-related effects upon surface water quality, as well as diffuse overland runoff and wastewater treatment plants (WWTPs). All of these sources emit pathogens and fecal indicator bacteria into the surface water, which the EU Bathing Water Directive addresses by giving threshold values for the indicators Escherichia coli and intestinal enterococci. This study presents a comparison between 21 scenarios of costs for processes that reduce the load of Escherichia coli and intestinal enterococci into the Ruhr River during and shortly after rain events. The methods examined include UV irradiation for WWTPs, integrated sewer management and treatment of CSOs with vertical flow constructed wetlands or performic acid. For pollution by diffuse overland flow, we evaluated organizational measures. The treatment of only diffuse pollution shows merely a slight effect on intestinal enterococci, but none on Escherichia coli and hence, was not considered further. Combining all three CSO reduction methods with the irradiation of WWTP effluent provides the best simulation results in terms of reducing both indicator bacteria.

Developing an easy-to-apply model for identifying relevant pathogen pathways into surface waters used for recreational purposes.

Swimming in inner-city surface waters is popular in the warm season, but can have negative consequences such as gastro-intestinal, ear and skin infections. The pathogens causing these infections commonly enter surface waters via several point source discharges such as the effluents from wastewater treatment plants and sewer overflows, as well as through diffuse non-point sources such as surface runoff. Nonetheless, the recreational use of surface waters is attractive for residents. In order to save financial and organizational resources, local authorities need to estimate the most relevant pathways of pathogens into surface waters. In particular, when detailed data on a local scale are missing, this is quite difficult to achieve. For this reason, we have developed an easy-to-apply model using the example of Escherichia coli and intestinal enterococci as a first approach to the local situation, where missing data can be replaced by data from literature. The model was developed based on a case study of a river arm monitored in western Germany and will be generalized for future applications. Although the limits of the EU Bathing Water Directive are already fulfilled during dry weather days, we showed that the effluent of wastewater treatment plants and overland flow had the most relevant impact on the microbial surface water quality. On rainy weather days, combined sewer overflows are responsible for the highest microbial pollution loads. The results obtained in this study can help decision makers to focus on reducing the relevant pathogen sources within a catchment area.