Domestic hot water (DHW) production at 60 (BE)/65°C (NL), with a temperature drop of no more than 5°C throughout the entire distribution system, is a well-established strategy for controlling Legionella growth in building installations, but is increasingly challenged by energy transition goals. Lower production temperatures can improve heat pump efficiency, reduce thermal distribution losses, and decrease overall energy consumption. However, ensuring Legionella safety at reduced temperatures remains a major barrier.
One potential approach is to combine DHW production at reduced temperatures (<60 °C) with additional physical control techniques. At Buildwise, the performance of ultrafiltration (UF) and UV-C disinfection was evaluated in collaboration with KWR Water Research Institute and the system manufacturers. The systems were installed in a domestic hot water test installation consisting of a 200 L electric storage tank, a 40 m circulation loop, and two draw-off points (kitchen and bathroom). A daily hot water consumption of 156 L was applied following a usage profile representative of a single family household (four persons). Every single system was installed in the circulation loop immediately after the storage water heater, so that the entire recirculating flow passes through the unit. The installation has been chronically contaminated with a Legionella pneumophila serogroup 1 wild type strain for years.
With the physical treatment not activated (UV-C) or in bypass (UF), Legionella concentrations were allowed to stabilise at 10⁶–10⁷ CFU/L at all sampling points during one week at a production temperature of 40 °C. Subsequently, the physical treatment was activated and the production temperature was successively set to 45, 50, and 55 °C, each for an evaluation period of six weeks. Legionella concentrations were monitored weekly at the inlet and outlet of the circulation loop, at both draw-off points and immediately upstream and downstream of the physical treatment unit.
Both physical treatment technologies consistently reduced Legionella concentrations immediately downstream of the treatment unit to the detection limit (< 50 CFU/L). Only in combination with a production temperature of 55 °C this also resulted in significantly reduced Legionella concentrations (around detection limit) throughout the circulation loop and at the bathroom tap, while concentrations at the kitchen tap remained around (UV-C) or well above (UF) 10³ CFU/L.