EFFECTS OF STORAGE ON ANALYSIS RESULTS FOR THE TOTAL AND FAECAL COLIFORM PARAMETERS Final Report to the Department of the Environment
Report No DWI0314

Oct 1994

EXECUTIVE SUMMARY

It is currently recommended that water samples for bacteriological analysis should be tested within six hours of being taken. However, this recommendation is based on the results of studies carried out by the Public Health Laboratory Service in the early 1950's. That work was based on analysis by the Most Probable Number technique, which has now been almost universally replaced by the membrane filtration method for water analysis. This project was therefore commissioned to investigate storage effects using the currently used methodology.

The objective set by the Department of the Environment for this study was to establish whether there was significant difference between the concentrations of faecal and total coliform bacteria in a water sample analysed after 6, 12 and 24 hours of refrigerated storage. A fixed sampling regime involving bacteriological tests and measurements of temperature, pH and conductivity was specified in the project schedule. It was also specified that three water types should be examined, namely a groundwater, a lowland surface water, and an upland surface water. Monitoring of each water type was carried out over a 12-month period to identify possible seasonal effects, and over 500 samples were included in the study.

Analysis of the results showed that overall there was no evidence of a significant change in either total coliform or faecal coliform counts for samples tested after 6 and 12 hours of refrigerated storage. For samples tested after 6 and 24 hours of refrigerated storage, a drop in counts of total and faecal coliforms was seen. This was of marginal statistical significance for total coliforms, but more significant for faecal coliforms. On average the counts of total coliforms were 5% lower, and those of faecal coliforms were 10% lower after 24 hours storage, compared with 6 hours.

On the basis of the results obtained in this study, extending the allowable period of storage of samples before analysis from 6 to 12 hours should not affect the overall results, provided the samples are refrigerated. However, extension of the allowable period of storage to 24 hours would result in lower overall results.

The samples tested in this study were representative of raw, untreated waters. It is not known how far the results could be extrapolated to other water types, such as disinfectant-treated water.

Copies of this report may be available as an Acrobat pdf download under the 'Pre 2000 Reports' heading on the DWI website.