Tank Water Heater Sediment as a Thermal Refuge for Legionella pneumophila

Abstract

It is often recommended to set water heaters ≥55 °C to reduce pathogen growth potential in premise plumbing, but Legionella pneumophila (Lp) can some times mysteriously survive under such conditions. We compared two full-scale recirculating electric water heaters, one maintained at 37 °C (Control) and the other elevated to ≥55 °C (Experimental) and examined potential reservoirs of Lp. After 4 weeks of operation at the elevated temperature, culturable Lp in bulk water declined to nondetect in the Experimental system, but Lp gene copy numbers remained high. It was hypothesized that sediment could be a reservoir and source of persistent release of Lp. Sediment collected from the bottom of the tank was found to contain ≈104 MPN/L Lp, while temperature in the sediment zone was as low as 49 °C. The sediment surface area was estimated to be ≈ 26,000 m2, 4 orders of magnitude higher than the plumbing system surface area available for biofilm formation. Microcosm studies with sediment collected from the Experimental heater demonstrated persistent release of Lp gene copies into bulk water, even at ≥ 49 °C, whereas culturability was lost within a few days. Follow-up experiments demonstrated how floor temperature, cold-water flushing, and insulation can further influence Legionella habitat within water heater sediment layers.