Organic matter as loose deposits in a drinking water distribution system

The distribution of treated water of good quality can generate in many case s the formation of unwanted loose deposits in the reservoirs and pipework o f distribution systems. The approach used in this study seeks to characteri ze 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. Deposit s 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 sol ids, but the corresponding concentrations were much higher (g l(-1)) than t hose found in circulating water (mg l(-1)). The same ratio applies to bacte rial counts. Proportions of C-organic and N were linked through a linear re lationship, which results in a consistent C/N ratio close to 7 irrespective of the quantity of or ganic matter in the sediment. This low C/N value sug gests 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 coun ts (HPC) in the deposits (up to 5.8 x 10(8) 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, an d (iii) positive correlations between the organic matter fractions (as C-or ganic and N) and the HPC. No relationship was found between the organic cha racteristics of deposits and upstream/downstream locations in the system. T he deposits appear to contain a reserve of organic matter capable of suppor ting the development of bacteria as well as bacterial predators, which is l ikely to provide favourable conditions for the contamination of water carri ed by the distribution system. Chlorine demand of loose deposits was high a nd would be predicted from nitrogen content, but inactivation of bacteria a ssociated with resuspended deposits appears unfeasible with current disinfe ction practices due to oxidant reduction resulting from deposit resuspensio n. (C) 1999 Elsevier Science Ltd. All rights reserved.