Discoloration Material Accumulation in Water Distribution Systems

Despite a significant ongoing investment in asset renewal and rehabilitation and high compliance rates for drinking water quality standards, water companies continue to receive customer contacts relating to water quality, dominated by discoloration. This paper investigates the accumulation of material responsible for causing such discoloration in drinking water distribution systems from new field data. Engineering experience suggests that factors influencing this accumulation might include local pipe properties such as age, material, or diameter. On the basis of recent research, it also seems reasonable to expect hydraulic conditions and bulk water quality to be important. Results of extensive repeated flushing field trials in two representative distribution management areas (DMAs) over a two and a half year period are presented. Initial flushing suggested that the maximum thickness of discoloration material accumulated within the pipes was tentatively inversely proportional to the daily conditioning shear stress, in agreement with previous research, but independent of pipe material and/or diameter. An analysis of discoloration material from different strength layers, facilitated through stepped flushing, showed that the metal composition was uniform, and the process of accumulation simultaneously occurs across all strength characteristics. In plastic pipes, a limiting layer strength of 0.7 N/m(2) was observed, above which significant additional discoloration material was not mobilized. Repeat flushing at different return intervals showed the accumulation rate of discoloration material in both DMAs to be consistent, within field-based experimental error, with an average of 0.0057 mm/month (95% confidence level of 0.0015). This rate was consistent across the different pipe materials, diameter, age, and hydraulic conditions within each DMA and between the two DMAs. Results from repeat flushing at increasing time intervals show that accumulation is a linear function of time; although, because of the uncontrolled nature of field work, this assertion is only definitive for five pipe lengths. From this, it is suggested that accumulation is primarily dependent on factors external to the DMA, and it is likely that this is dominated by the supplied water quality. This accumulation rate might seem small; however, after only 43 days of accumulation, the mobilization of the total accumulated volume of material into one pipe volume of a 140 mm pipe would be sufficient to exceed UK regulatory limits of 4 NTU for turbidity. (C) 2011 American Society of Civil Engineers.