COMPARISON OF THE PROPORTION OF COLIFORM POSITIVE DRINKING WATER SAMPLES WITH STATISTICAL PARAMETERS FOR LOGNORMAL DISTRIBUTION DESCRIBING COLIFORM CONCENTRATIONS WITHIN THE SUPPLY
Report No FR0462
Describing coliform concentrations within water supply networks as parametric statistical distributions would provide a more informed assessment of water quality than currently allowed by the proportion of failures;
allow more appropriate comparisons of water qualities in different companies;
define the increased risk of detecting coliforms in 100 ml statutory water monitoring samples after failure of treatment processes.
The objectives are to compare the statistical parameters (geometric mean and logarithmic standard deviation) for lognormal distributions of coliforms with the proportion of coliform-positive samples. In addition, the reliability of the current statutory water quality sampling scheme would be assessed.
Coliform organisms are intermittently detected in 100 ml volume samples from consumer premises in some water supply zones, although the major proportion of samples register 0 per 100 ml. It is probable that coliforms are present in regions of the water supply network where 100 ml samples register zero coliforms. At present, however, little is known about the coliform concentrations in such regions. Furthermore, the water companies have no information on the probability of detecting coliforms on further sampling from those regions. Currently the microbiological qualities of water supply networks are quantified in terms of the proportion of 100 ml volume water samples registering coliforms. Defining coliform concentrations as parametric statistical distributions would allow water qualities to be assessed in terms of a central tendency (geometric mean) and a dispersion (logarithmic standard deviation). At present it is not known whether some water companies report higher coliform compliances because of differences in the geometric mean coliform concentration or the logarithmic standard deviation. Furthermore, the impact on coliform detections of failures in particular treatment processes which increase overall numbers of coliforms could be predicted on the basis of changes in the statistical distributions of coliform concentrations.
The interpretations of the statistical models in this report are theoretical.
Coliform concentration data from statutory water quality monitoring are used to support the models. The main conclusions are:
It is recommended that water companies undertake a limited 10 litre sampling programme to assess the statistical distribution of coliform concentrations at this critical concentration region within their water supply networks.
This would provide companies with better information on the effect of treatment failure on coliform compliance and allow more accurate definition of statistical parameters for coliform concentrations. In addition, 10 l volume sampling data may be sufficient to develop predictive models for coliform concentrations from fluctuations in associated factors. Useful information on the impact of failure of a particular treatment process on the proportion of coliform-positive samples and compliance with The Water Supply Regulations (1989) would also be obtained.
VI RESUME OF CONTENTS
Section 1 outlines the need for investigation of statistical distributions of total coliforms within the water supply network. Two parametric statistical distributions, namely the Poisson and Normal distributions, are introduced. In Section 2, a statistical model is developed for coliform concentrations. Methods for improving the model are discussed. In Section 3, the theoretical implications of the model are described in relation to assessing water quality in terms of the proportion of coliform-positive 100 ml volume samples. The practical realities are then assessed in Section 4- using coliform water quality data from the water companies A to I. Conclusions are made in Section 5.
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