Organic matter as loose deposits in a drinking water distribution system

The distribution of treated water of good quality can generate in many cases the formation of unwanted loose deposits in the reservoirs and pipework of distribution systems. The approach used in this study seeks to characterize the organic component of deposits in a large distribution system, since organic matter is a key factor in controlling the biostability of water and has not previously been described with relation to these deposits. Deposits were found to contain a variable, but minority fraction of organic matter: less than 11% as organic carbon, 1.1% as nitrogen, or 28% as volatile solids, but the corresponding concentrations were much higher (g l−1) than those found in circulating water (mg l−1). The same ratio applies to bacterial counts. Proportions of Corganic and N were linked through a linear relationship, which results in a consistent C/N ratio close to 7 irrespective of the quantity of organic matter in the sediment. This low C/N value suggests that the in situ biological activity has a strong influence on the C and N fractions. This is confirmed by (i) the high heterotrophic plate counts (HPC) in the deposits (up to 5.8×108 CFU 14 d g−1 of dry matter); (ii) the observation of a mature trophic chain in most samples resulting in the presence of macro-organisms (Asellus, Gasteropodae), in some cases; and (iii) positive correlations between the organic matter fractions (as Corganic and N) and the HPC. No relationship was found between the organic characteristics of deposits and upstream/downstream locations in the system. The deposits appear to contain a reserve of organic matter capable of supporting the development of bacteria as well as bacterial predators, which is likely to provide favourable conditions for the contamination of water carried by the distribution system. Chlorine demand of loose deposits was high and would be predicted from nitrogen content, but inactivation of bacteria associated with resuspended deposits appears unfeasible with current disinfection practices due to oxidant reduction resulting from deposit resuspension.