The River Rhine is the main drinking water source of Duesseldorf, Germany. After river bank filtration the water is ozonated, decarbonated and finally filtered by a two-tiered activated carbon filter. The whole supply chain (catchment, treatment and distribution) is monitored continuously to reveal possible leakages in the multiple barrier system and to ensure distributing drinking water quality on a high level.

Despite this operating expense in one of the three waterworks of the Stadtwerke Duesseldorf coliforms were regularly found after river bank filtration. Their occurrence in the River Rhine was expected because of its usage as discharge system. But the appearance of coliforms in the river bank filtrate was unusual and leaded to a catchment survey and a risk assessment according to the requirements of the Water Safety Plan of the WHO.

A number of monitoring campaigns to identify the source of contamination were conducted in the River Rhine, the different types of wells (horizontal filter well, vertical filter well) and the several steps of the water treatment. An intensive discussion about the risk combining likelihood and impact of the presence of coliforms followed. Additional parameters like coliphages have been investigated in order to assess the information given by the presence of coliforms used as indicators for pathogene micro-organisms. The results showed, that there was not only one source of the coliforms in raw water, that the mode of operation just dislocated the contamination and that a continuously monitoring and a disinfection step during the water treatment are necessary.