Dead-end flushing of a distribution system: Short and long-term effects on water quality

The effectiveness of routine spot flushing in two dead-end locations of the Montreal distribution system (DS) was assessed. The two 203 mm (8 in) pipes were roughly 500 m long. Two successive annual spot flushings, 25 minutes each, were performed and the impact on water quality was assessed during the first 24 hours, 2 weeks later, and the following year. The flushing water was also analysed in order to evaluate the quantity and nature of loose deposits that were drained. High numbers of atypical coliforms were removed during the first annual flushing procedure. No atypical coliforms were recovered from the flushing water during the second annual procedure, suggesting that the first procedure was effective in eliminating these organisms. During the first 24 h after flushing, chlorine decreased rapidly at both dead-end locations while heterotrophic plate counts (HPC) counts increased during the same period. Therefore, only minor improvements in water quality (mainly turbidity and total iron) were measured during the two weeks after the flushing procedure. With respect to spot flushing dead-end locations on a routine basis, the principal benefits observed in this specific DS were related to short-term improvements in the aesthetic characteristics of the distributed water.