Linkage Between Characteristics of Premise Plumbing Systems and Microbial Quality of Stagnant Drinking Water

The length of pipe between the water main and the point of use (POU), known as premise plumbing, has a direct impact on public health. However, relatively little is known about factors that affect the microbial quality in premise plumbing. Although microbial quality typically degrades during stagnation, little research is present linking factors such as building age and level of usage to the extent to which stagnation occurs.

To gain insight into the effect of premise plumbing service age on the level of microbial contamination in stagnant drinking water, water at the POU was analyzed. It was observed with statistical significance (p<0.05) that as service age of a premise plumbing system increased so did microbial concentration in stagnant drinking water. Results also indicated that partially repiping premise plumbing systems may greatly reduce the abundance of microorganisms, again demonstrating the impact of service age on microbial quality of stagnant drinking water in premise plumbing and suggesting re-piping as a potential risk-mitigating strategy.

Two rapidly measurable parameters (free chlorine and temperature) were analyzed for their suitability in use for indicating the extent of drinking water stagnation at the POU. Because of the seasonal variability of temperature in water supply, water temperature was not found to be a suitable indicator of stagnancy. Free chlorine, however, was found to be a suitable indicator of stagnancy. Free chlorine concentrations were also linked to heterotrophic plate counts. Additionally, the effect of pipe size on free chlorine concentration after overnight stagnation was analyzed. These results were used to show flushing tap water for 15s for every 100ft of 0.5in copper piping in order to reduce microbial contamination is a simple potential risk-mitigation strategy for premise plumbing users.

Further analyses focused on the chlorine decay rates in copper pipes. Chlorine was found to decay faster in newly built premise plumbing systems than old systems and fastest in unused copper pipes. The effect of increased usage was also analyzed; increasing the usage in older premise plumbing systems decreased the chlorine decay rate while the same increase in usage in newer systems increased the chlorine decay rate.