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

## TL;DR

This study shows that flushing hot water systems can temporarily reduce bacteria like Legionella, but chloramines are more effective at long-term control.

## Contribution

The study reveals how chloramine disinfection prolongs Legionella suppression compared to temperature or flow interventions in premise plumbing.

## Key findings

- Flushing and showering temporarily reduce bacterial concentrations but lead to rapid rebound within 1–4 days.
- Chloramines suppress Legionella spp. for over 10 days even after they decay during water use.
- Flushing is more effective than showering at removing Legionella from biofilms.

## Abstract

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, Legionella spp., and Vermamoeba vermiformis 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, Legionella spp. ssrA gene concentrations increased over 3–10 days following flushing
or showering, whereas chloramines suppressed Legionella 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 Legionella spp. as compared
to cold-water flushing. V. vermiformis concentrations decreased temporarily following water use, independent
of disinfectant or temperature. Flushing was more effective than showering
at removing Legionella 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 Legionella in premise plumbing.