Miljøprojekt, 1162 – Vurdering af metodeskifte for coliforme bakterier i drikkevand

Vurdering af metodeskifte for coliforme bakterier i drikkevand

Summary and conclusions

The purpose of this report is to investigate the consequences of changing the method of analysis. In this report the different definitions of the coliform group in the literature as well as in the methods of analysis are discussed. Possible interpretations when coliform bacteria are found in drinking water are reviewed. The analytical principles in the methods of analysis are described and the previously used method, (DS 2255) is compared to the EU-reference method ISO 9308-1 as well as with the Colilert Quanti Tray method regarding detection limits and uncertainty.

Analytical results are compared before and after the change of method. Some of the coliform bacteria are identified and the impact of the bacteria identified on health is evaluated.

Coliform bacteria are natural habitants in the environment, in soil, in decaying plants and in surface water. Presence of coliform bacteria in untreated drinking water might indicate contamination with surface water, plants and/or soil, but not always fecal contamination.

Detection of coliform bacteria, without concurrent presence of E. coli, in drinking water is regarded as indication of present or former leaks in the water distribution system somewhere between the source and the sampling point, resulting in penetration of surface water, soil or plant material.

Coliform bacteria are not suitable as indication of fecal contamination, but can be used to evaluate the performance of water distribution systems. Coliform bacteria are, if drinking water is disinfected, suitable to evaluate the effectiveness of disinfection. If no disinfection is performed, which is the case in Denmark, the content of coliform bacteria can be used to evaluate if water distribution systems are working properly, if they are tight so no contamination are feed into the system.

The change in method means that there are more coliform positive samples.

The EU- reference method and the Colilert Quanti Tray method are better than the previous Danish method DS 2255 because they detect more positive samples. The Colilert Quanti Tray method is better than the previous Danish method (DS 2255) because the recovery of coliform bacteria is better and because it detects injured and stressed bacteria if they have enzymatic activity.

896 analytical results have been evaluated. In 222 cases coliform bacteria have been detected without concurrent detection of E. coli or high heterotrofe count. 91.3% of the samples contained ≤ 20 coliform bacteria per 100 mL. These results are in accordance with the Danish equivalence study which showed that higher numbers of coliform positive samples as well as higher counts of coliform bacteria could be expected with the Colilert method.

Detected coliform bacteria have been identified and it is concluded that the group of coliform bacteria (as detected in Danish drinking water), except from E. coli, do not indicate presence of bacteria harmful to health. Except from contamination per se coliform bacteria can be present in drinking water in low numbers without causing problems.