The perception that high water retention time (WRT) in buildings increases microbial and pathogen growth drives costly flushing interventions, which lack a scientific framework to guide effective implementation. Here, we evaluate the effect of WRT from 1 to 21 days in an at-scale building plumbing rig with influent chloramine residuals of <0.2–2.5 mg/L as Cl2 and water heater set points of 40 and 60 °C. We found that the classic microbial growth curve consisting of lag, exponential growth, stationary, and decay phases provided a robust explanation of trends in bulk water total cell counts (TCC) and Legionella pneumophila over a wide range of conditions. We extended this observation to develop a framework to understand how various controls and operating conditions act to stop (e.g., cold temperatures, disinfectant) or reset (e.g., pasteurization in water heaters killing microbes and recycling nutrients) the growth curve as a function of WRT. Bulk water TCC and L. pneumophila reached a consistent peak/plateau at a building WRT of ∼7 days before decaying up to 90% at higher WRT. These findings suggest that recent guidelines recommending weekly flushing of buildings may sometimes be counterproductive and that very high WRT does not necessarily indicate microbial risk.
Odimayomi et al. (Fri,) studied this question.