The early warning system implements the requirements of the North Rhine-Westphalia Bathing Water Ordinance and ensures a warning of short-term hygienically relevant pollution of the Ruhr. It has enabled swimming in Lake Baldeney at the Seaside Beach bathing area in Essen since 2017.

The early warning system is based on the observation that increased concentrations of E. coli and intestinal enterococci occur after rain events at the bathing water Seaside Beach. The concentrations can exceed values of 1,800 CFU/100 ml for E. coli, which, according to the NRW Bathing Water Ordinance, must result in the issuing of a temporary bathing ban. The early warning system is based on a logistic evaluation of the daily precipitation totals from precipitation measuring stations in the vicinity of the bathing water and the measured concentrations of E. coli and intestinal enterococci, which provide information on whether or not the maximum permissible concentrations according to the NRW Bathing Water Ordinance are exceeded.

Based on historical measurement data from selected precipitation measuring stations, the maximum permissible daily precipitation sum and the period before the assessment day (including the assessment day) within which this daily precipitation sum must not be exceeded so that the maximum permissible E. coli concentration of 1,800 CFU/100 ml is not exceeded at the measuring point of the bathing water Seaside Beach were determined. In the past, exceedances of the maximum allowable concentration of intestinal enterococci had only occurred in conjunction with exceedances of the maximum allowable concentration of E. coli, so the early warning system is limited to predicting exceedances of E. coli.

The early warning system was implemented as a web application by Essener Systemhaus on behalf of the city of Essen and has been in operation since 2017. Since then, it has been further optimized by IWW and Ruhrverband.

The principle of the early warning system is transferable to other bathing waters with short-term pollution due to combined sewer overflows.