In Germany, most distribution systems are run without disinfectant residuals. To allow an adequate judgement of coliform-positive samples, it was important to know if growth of coliform bacteria is to be expected also under the conditions present in German drinking water distribution systems. Therefore, the growth conditions for coliform bacteria in drinking water were characterized in a DVGW-funded project.

Pure cultures of coliform bacteria (e.g. Citrobacter freundii, Enterobacter cloacae) required mostly elevated temperatures of 20°C in order to show growth in the water. A coliform strain directly isolated from a drinking water sample however showed growth at lower temperatures (12°C). Thus, in principle coliform bacteria can grow even with the low nutrient content present in drinking water.

Yet, if bacteria of the natural biocenosis were present in the water they successfully outcompeted the coliform bacteria and no growth of the coliform strains could be detected. Thus, under the conditions usually present in German drinking water distribution systems (aerobic, low temperature, low nutrient content, presence of a natural biocenosis) growth of coliform bacteria within the water body is not to be expected.

However, under anaerobic conditions growth of coliform bacteria was detected even in the presence of a natural biocenosis. Such anaerobic conditions can be present in sediments of drinking water reservoirs or in the pipes. Hence, anoxic sediments within a drinking water distribution system can serve as potential habitat for coliform bacteria and therefore account for a high percentage of positive samples in sediments and flushing waters.