Characteristics of particles contributing to turbidity in potable water distribution networks

Melbourne (Australia) is one of the few major metropolitan cities which have a water catchment area sufficiently pristine that it does not require filtration, and is only treated with minimal chemical disinfection, before supply to consumers. The absence of filtration provides an obvious source of turbidity in the system through particulates in the source water, however the quantitative extent and health risk of such turbidity is not well understood. The present work addresses the characterisation of turbidity causing particulate material found within the Wantirna Water Quality Zone (WWAZ) as a typical example of a reticulated water distribution system in Melbourne. Several origins of turbidity were investigated within this system including: Natural turbid spikes - events of high turbidity sometimes found to occur within the system and the subject of continued investigation to determine their origin; Artificial turbid spikes - events of high turbidity deliberately brought about by opening a hydrant valve; Particulate material found to occur naturally throughout the distribution system; Particulate material found to occur naturally within the source material. The particle size distribution and particle composition was found to be similar for all samples, including those sampled as a function of distance from the source. The possible exception to this was the measurement of zeta potential where particles found close to the source had a pH independent negative zeta potential whilst those found further downstream had a pH dependent zeta potential which exhibited an isoelectric point of approx pH 8. This is consistent with a humic acid covering (which is removed with time spent in the distribution system) over otherwise identical particles. All results are consistent with a common, external (source) origin for particles found within the distribution system.