STATISTICAL ASSESSMENT OF ASSOCIATION OF EXTERNAL PARAMETERS WITH
COLIFORM FAILURES IN DISTRIBUTION SYSTEMS
Report No FR0326

Dec 1992

SUMMARY

I BENEFITS

• The customer will benefit from using fluctuations in external parameters to predict an increased probability of coliforms in the distribution.

• Identifying associations of the presence of coliforms with external parameters may provide information on the root causes and the origin such that steps may be taken to eliminate the problem.

II OBJECTIVES

To assess the significance of bacteriological failures in water supply and to evaluate available information on associated factors.

III REASONS

Bacteriological failures in samples of water collected from distribution systems can occur for a variety of reasons such as problems at the treatment works, infiltration and regrowth of organisms normally considered to be of some sanitary significance. Current monitoring procedures are unable to discriminate between these causes of failure and as a consequence remedial action has to be taken on the assumption that the failure indicates a risk to public health.

Better information on the source and type of organisms associated with bacteriological failures would assist in deciding on the correct remedial action.

IV CONCLUSIONS

Associations of higher or lower values of certain external parameters with water samples registering a coliform have been identified. In particular, for data from Water Companies A and B, the arithmetic mean temperature for samples registering the presence of total coliforms is 2 - 3°C higher than for samples without coliforms. This difference is statistically significant for the Water Company A data. Further temperature data from Water Company B are to be analysed. In addition, statistically significant associations have been found between the plate count (22° C, 72 h) and the presence of coliforms, with the geometric means for samples with coliforms being 3 - 11 fold higher than for samples without coliforms. There is some evidence for an association with lower total chlorine concentrations and the presence of coliforms on the basis of data from Water Company A.

V RECOMMENDATIONS

On the basis of results analysed so far it is recommended that other water companies in addition to those represented by steering group members are approached to obtain more data. It is suggested that companies with the largest numbers of zones failing requirements are targeted.

In addition it would be of interest to test for associations between the presence of coliforms and factors such as the type of water source (e.g. river or bore hole), the type of treatment and levels of total organic carbon.

VI RESUME OF CONTENTS

This Progress Report details statistical analyses performed to date to identify associations of external parameters with water samples registering positive for total coliform bacteria. Section 2 describes the strategy of data collection and the appropriate statistical treatment, emphasizing the importance of determining the statistical distribution for each external parameter. In Section 3, statistical analyses of associations between the presence of coliforms and external parameters including temperature, conductivity, total chlorine concentration, plate counts (72 h, 22°C), pH and concentrations of metal ions (Al₃⁺ Mn⁺⁺ and Fe₃⁺) and nitrite are reported. The statistical analyses are not yet completed in that not all the data obtained have been fully analysed, more data are expected and further statistical tests are to be implemented. Some parameters analysed to date require additional or modified statistical analysis.

Copies of the Report are available from FWR, price £15.00 less 20% to FWR Members