Evaluation of the risk of Legionella spp. development in sanitary installations (part 2).

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In order to determine whether it is possible to reduce energy use for domestic hot water (DHW) production and distribution, without increasing the risk of Legionella spp. development in sanitary installations, a full-scale test facility was built. On a daily basis, a consumption profile corresponding to the DHW use of a single family was applied separately using two tap pipes, one corresponding to a kitchen and the other to a bathroom. Legionella spp. was cultivated in a separate water tank and then transferred into the test facility. The DHW production temperature was kept at 45°C with a periodical heating to 60°C for different durations and different frequencies. Legionella spp. concentrations were then measured, both in the water and in the biofilm. Previously, we found that the thermal shocks at 60°C of the water storage tank for many hours without treatment of the bottom of the tank and the draw-offs pipes at the same time was not effective to control Legionalla spp. growth. The Legionella spp. concentrations reached 105 - 106 cfu/l within a couple of days after a thermal shock. During the second part of 2017, we have increased the thermal shocks to 65°C in the test facility for different duration, and with or without thermal disinfection of the draw-off pipes. The expansion vessel, installed on the cold water inlet of the DHW storage tank, seemed to be an important source of recontamination of the installation. After removing the vessel, and by applying a weekly thermal shock at 65°C in combination with a regular draw-off on each draw-off pipe during this thermal shock for at least 150 seconds, we were able to maintain the Legionella spp. concentration beneath the limit of 1000 cfu/l.