Water Age in Residential Premise Plumbing

Abstract

In most countries around the world, water is treated physically and chemically to a quality that is safe for human consumption. In spite of these efforts, every year people die as a consequence of drinking water-associated disease outbreaks. Legionella is arguably the deadliest pathogen in drinking water in the US and efforts are underway to reduce the likelihood of infecting potable water consumers. One of the primary factors to measure water quality degradation is water age. Water quality degrades with the time that the water sits in pipes. Over time, the residual disinfectant decays, disinfectant by products are created and the water becomes more susceptible to pathogen regrowth. This concern is not limited in the distribution systems but carries over to residential premise plumbing system. A key factor affecting water age in the premises is fixtures’ idle times. As a result, poorly designed plumbing layouts and intermittent usage patterns may lead to high residence times. In the present study, a methodology was developed to numerically quantify water age in residential premise plumbing systems. The scheme is composed of a hydraulic solver, EPANET with modifications, a demand stochastic simulator, SIMDEUM-UA, and a plumbing layout generator based on CAD models. This method was used to determine layout design practices that contribute to lower water ages. The layout is shown to have a significant impact on water age. Modified layouts reduced the water age metrics of absolute maximum age, mean maximum and mean water by up to 76%, 66% and 58%, respectively. A best practice is to connect the water closets at the end of the premise distribution branches. The effect of water heater types on residence times was also assessed. It was found that instant or on demand heater helps reduce water age across all layouts for all the metrics, at both the outlet and the point of connection of the fixture to the distribution system. To further decrease water age, auto-flushers were installed on certain nodes, as the USEPA (2016) recommends flushing the system at regular intervals, and further if combined with a flush of hot water at a temperature of at least 60 °C (140 °F), it would help sterilize the hot system between the heater and the flusher as recommended by the WHO (2007). Proposed methods to implement these so-called hot super-flushing were discussed for future research. However, none of the hydraulic approaches proposed here impact the “last foot” of pipe connecting plumbing fixtures with the premise distribution pipes. Lastly, when comparing the resulting pressures using the simulated demands against the peak demand estimates with flows from the plumbing code, code pressures are always lower than the simulated ones. This may indicate that the design method conservatively overestimates demands. Nonetheless, oversized pipes are detrimental for water age and should be avoided, as greater demands are required to flush the system.