The thermal heat treatment works best on systems with circulation, as the entire domestic water system, apart from taps, connecting pipes, and dead ends, is heated to 60°C. For systems without circulation, it is necessary to open the taps at the end of the lines to obtain this heating of the entire system. Continuing a series of laboratory tests (figure 3), a booster was demonstrated to a single-family house with district heating and a Legionella content significantly over 1,000 CFU/L (figure 4). The instalation was without circulation, and the heating of the domestic hot water took place in a plate heat exchanger having a standby function, which means that the water in the plate heat exchanger was kept at 44°C during standby, and only during tapping was the temperature raised to 53°C. Derived from the heat exchanger result, the booster conducted a 60°C heating of the domestic water producer every night. However, it was confirmed that this heating did not affect the infected installation. In fact, there was a sharp increase in the number of Legionella during the four-week summer holi- day without water consumption, and it was then decided to conduct a disinfection of the complete installation by open- ing the taps. According to the immediately achievable via the booster, the disinfection was carried out at 63°C. This resulted in a Legionella content far below the recommended max.
1,000 CFU/L, despite a lower disinfection temperature than the recommended min. 65 - 70°C. Further, the single demo test indicated that the electric booster can prevent Legionella for- mation in a new/cleaned system.
Innovative biocide solution for protection at lower temperatures
In this innovative solution from Danish Clean Water a generator produces and doses hypochlorous acid (HOCl) into the hot water systems that kill planktonic Legionella at lower temperatures and remove biofilm. HOCI is produced via salt, water, and electrolysis and has been demonstrated to be more reactive than chlorine in combating Legionella. In addition, it is a mild disinfectant that is environmentally friendly and safe for humans. The solution was tested at two residential sites with severe Legionella problems (up to 120,000 CFU/L) in hot domestic water (Figures 5 & 6). Field test 1 was a small property in Southern Jutland, and field test 2 was a large, completed property near Copenhagen. After the HOCl generator was switched on, viable Legionella was drastically reduced in the water, even though the temperature in both cases was lower than the 50°C required in Denmark. To ensure effective dosing, the project incorporated water flow measurement and strategically positioned sensors for robust control. The field tests underpin the effectiveness of hypochlorous acid not only for disinfection but also for maintaining low Legionella values in ongoing operations. However, a potential risk can be that the biocide does not get around in the entire system due to dead ends, which are more difficult to determin than at temperature treatment. This requires special attention and documentation. The field tests showed that the biocide solution can efficiently reduce Legionella counts and keep a low, tolerable level – at lower temperatures. This allows for saving energy and reducing climate footprints. Consequently, the project proposes that the solution be included formally in authority requirements, providing that it meets specific documentation requirements for its efficiency throughout the installation. The overall project is documented through several professional reports in Danish as well as through an English final summary report of 20 pages: https://www.teknologisk.dk/_/media/87227_EUDP%20 Jnr64020-1099%20Legionellasikring%20-%20Final%20 report%2020230228.pdf.
Figure 5: Generator for hypochlorous acid installed for demo (field test 2).
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