Flushing as a Control Measure for <i>Legionella</i> spp.: Impacts of Water Age, Chloramine Disinfection, and Elevated Temperature

Pilot-scale experiments were used to examine how hot-water temperature, chloramine disinfection, and pipe material (copper or PE-Xa) influence the effectiveness of two flow-based interventions─high-flow flushing vs showering─in controlling total bacteria, <i>Legionella</i> spp., and <i>Vermamoeba vermiformis</i> in premise plumbing. Flushing (ca. 16 L/min for 5 min) and showering (ca. 7 L/min for 8 min) both temporarily reduced bacterial concentrations by removing accumulated planktonic biomass. However, total bacteria rebounded within 1-4 days after a flush or shower, regardless of the presence of chloramines (1 mg/L Cl₂). In the absence of chloramines, <i>Legionella</i> spp. <i>ssrA</i> gene concentrations increased over 3-10 days following flushing or showering, whereas chloramines suppressed <i>Legionella</i> spp. for >10 days despite completely decaying within hours of water use. Without chloramines, hot-water flushing at 49 or 60 °C provided little additional control of <i>Legionella</i> spp. as compared to cold-water flushing. <i>V. vermiformis</i> concentrations decreased temporarily following water use, independent of disinfectant or temperature. Flushing was more effective than showering at removing <i>Legionella</i> spp. from biofilms, while pipe material had limited influence in this pilot-scale system. Overall, the results demonstrate organism-specific recovery dynamics and highlight the role of chloramines in prolonging the suppression of <i>Legionella</i> in premise plumbing.