La flore bacterienne dans un reseau de distribution: Bacteria in distribution systems

Abstract

Water quality upkeep in distribution systems is a major concern of drinking water suppliers. In the case of extended systems transporting water through several thousand kilometers of pipelines, the problem is not a simple one. This concern was, among others, at the root of the Water Authority for the Paris Suburbs' decision to modify treatment at the Méry-sur-Oise drinking water production plant (Damez, 1982). As can be seen in Fig. 1, the old line was upgraded by the addition of a raw water basin, two more ozonation stages, and activated carbon filtration. Furthermore, pre-chlorination is no longer used at the plant. A study to assess the impact of these changes on the quality of water in the system was launched prior to the switchover, in collaboration with researchers from the Pasteur Institute in Paris. It involved the enumeration and identification of the germs in analysed samples of river water, treated water and distributed water. Bacterial aftergrowth in the distribution system was significant with the old line, the mean value of the total germs at 20°C exceeding the limit value of European standards (100 germs ml −1). With the new line, bacterial aftergrowth has been limited to a great extent (Table 6). Removal of the biodegradable fraction of organics during treatment probably has a fundamental effect on this limitation of the aftergrowth (Brunet et al., 1981; Gaïd, 1981), which is achieved notwithstanding lower post-chlorination dosages. Moreover, chlorine is now in the form of free chlorine (Table 1). As regards organics, TOC removal has been only slightly increased by the new line (Table 7), but the permanganate consumption has been greatly reduced, from 1.5 to 0.5 mg −1 at the plant outlet. In river water, the germs are mainly Escherichía, Aeromonas hydrophila, Acinetobacter calcoaceticus, Alcaligenes faecalis, Yersinia enterocolitica were isolated in 6% of the samples. At the plant outlet, the samples are mostly uncontaminated in analysis conditions. In the distribution system, the species isolated are Aeromonas hydrophila and Alcaligenes faecalis. Enterobacteriaceae and Pseudomonodaceae also contaminated the samples, but with a very low frequency (5%) (Tables 3 and 4). As regards acid-fast bacteria, many species were isolated in river water ( M. fortuitum, M. chelonei, M. terrae and M. lactis). But no species were found in samples of 250 ml at the plant outlet. Still, 39 acid-fast bacteria were isolated from 126 samples of distributed water (Table 5). Most germs belong to Runyon's group 11 but were not identified (Runyon, 1959). The study is still under way. The onset of deterioration of bacteriological quality may be a criterion for selecting the regeneration cycle for activated carbon.