AWWA WQTC65980

The objectives of this paper were to: identify the factors explaining the spatiotemporaldistribution of heterotrophic plate count (HPC) in the distribution system (DS); and, develop models which permit the prediction of HPC.The case under study is the main DS of Quebec City, Canada, which serves around 240,000people. For the routine surveillance of water quality, the utility managers monitor about 50sampling points (daily, twice a week and weekly, according to the location). For this study, a robust database built using results from the routine monitoring of water quality wasused (about 3600 measurements of HPC and other parameters). The database containsinformation about various physical and chemical parameters, such as residual free and totalchlorine, temperature, absorbance, pH, turbidity, true and visible color, conductivity, hardness,fluorides, metals, organic compounds among which haloacetic acids and trihalomethanes. Itcontains information about the following microbiological parameters: coliforms; atypical bacteria;and, HPC.For analysis purposes, the distribution of HPC was grouped into 4 categories: (HPC = 0, HPCfrom 1 to 100; from 101 to 500, and HPC higher than 500 cfu/ml). It is interesting to note that when the levels offree residual chlorine are below 0.3 mg/l, which is a usual threshold at the exit of the watertreatment plant (USEPA 1989), HPC sometimes exceed the threshold of 500 cfu/ml. Theseresults confirm the observations made by other authors concerning the importance of maintainingminimum levels of residual disinfectant (Zhang and DiGiano 2002; Huck and Gagnon 2004;Ndiongue et al. 2005). Includes 22 references, tables, figures.



Keywords: Bacteria; Water Quality; Modeling; Distribution Systems; Databases; Heterotrophic Plate Count