The effects of water residence time on the biological quality in a distribution network

This study analyses the interactions of water residence time with heterotrophic plate count (HPC) bacterial density, biodegradable dissolved organic carbon (BDOC) and disinfectant residuals within a full size distribution network. These results have demonstrated the different influences of residence time in reservoirs and in pipes on bacterial water quality. In this context, travel of water in the pipes has no significant influence on HPC bacterial density, the supply water can be regarded as “biologically stable”. However, reservoirs, in which water has an elevated residence time, have a significant impact on HPC bacterial density. The importance of regular reservoir and network maintenance is stressed. In spite of the “biological stability” of supply water, BDOC decrease vs residence time was observed for high water temperatures. Thresholds of nutrients for which no bacterial regrowth is observed must always be associated with temperature. Disinfectant residual, which rapidly disappears in the network, was seen to have a very limited role in “biological stability”.