Predictive parameters of coliform occurrence in drinking water distribution systems
Coliform occurrences in drinking water in the absence of known breaches of treatment barriers are still a problem inthe drinking water industry. The purpose of this study was to define threshold values for different bacteriological, chemical and physical parameters, beyond which coliforms occurred more frequently along a full scale distribution system in a northern suburb of Paris. A predictive model of coliform occurrence at different sites of a distribution network was produced (AL COL model). This study consisted of two stages. The first one involved following coliform occurrences, epifluorescent counts, temperature, biodegradable dissolved organic carbon (BDOC), and residual chlorine level in the treatment plant effluent and at five distribution sampling sites. Each site was sampled once a week for one year (n=47 ; February 1991 - February 1992). This first stage of study showed that the occurrence of coliforms in the network could be associated with temperatures greater than a threshold value of 15°C, a BDOC consumption greater than 0.15 mg/l, a logarithm of the epifluorescent counts higher than 5.2 and a residual chlorine lower than 0.10 mg/l. Graduation of coliform occurrences was possible by the combination ofoverstepped thresholds for these four parameters. Three levels of risk associated with an increasing frequency of coliform occurrence were defined : site with no risk when no or only one threshold value was exceeded, slightly exposed site when 2 or 3 threshold values were exceeded and exposed site when 4 threshold values were exceeded. The second stage of the study concerned the application of this predictive model. It consisted of following weekly the same parameters as in the first stage at 7 and 10 distribution sites situated on two difrerent distribution systems (Nortlrern and Eastern suburbs of Paris) for 21 weeks (April - September 1992). The threshold values determined were similar for the two distribution systems. They were 15°C for temperature ; 0.10 - 0.15 mg/l for BDOC consumption, 5 - 5.2 for the logarithm of epifluorescent counts and 0.10 mg/l for residual chlorine. The frequency of coliform occurrences was about 60 % at sampling points assessed as exposed sites, 5 % at sampling points assessed as slightly exposed sites and 1 % at sites considered with no risk. Moreover, it was possible to define a model of mean risk of coliform ocurrences over all sites (corresponding to the mean of overstepped thresholds at the different studied sites for a given week). Four levels of mean risk (minimum, non negligible, critical, and maximum) were associatcd with increasing frequencies of coliform observations in the distribution network. The AL COL model could be an efficient and simple tool to control and limit occunences of coliforms in drinking water. When a site is assessed as exposed, disinfection treatments have to be performed in order to reduce this risk and to prevent coliform occurrences during the weeks following this risk. Moreover, it was also possible to define a chlorine level in the water to reduce the bacterial contamination and subsequently decrease the risk of coliform occurrences at the sites of the network.