COMPLEX LEGIONELLA PROBLEM SOLVED IN LARGE RESIDENTIAL PROPERTY
A Danish housing association has successfully eradicated Legionella and biofilm in the water pipes of a large property by installing and testing a DCW disinfection solution. Additionally, the solution has generated annual energy savings, ensuring a quick return on investment.
These results stem from a field test conducted as part of a national Legionella project in Denmark. The project has examined and concluded that harmful bacteria in hot water systems can be effectively controlled through biocide dosing with hypochlorous acid – importantly, even at lower temperatures.
The residential property, located in Herlev near Copenhagen, had previously faced issues with biofilm in its water pipes and high Legionella levels. The social housing administrator, KAB, initially attempted to address the problem by increasing water temperatures. However, this approach proved both costly and ineffective, making it a logical choice to participate in the Legionella project and trial an alternative solution.
A fast-acting solution to a complex problem
The property comprises seven buildings with a total of 193 apartments, supplied by three very long hot water circulation systems. A generator from Danish Clean Water (DCW) was installed to produce the disinfectant Neuthox via an electrolysis process using salt and water. The disinfectant contains a low concentration of hypochlorous acid and is automatically dosed into the water.
In all three water lines, Legionella levels dropped within weeks to below the recommended threshold of 1,000 CFU/litre and have since been maintained at very low levels.
During the initial phase, occasional fluctuations occurred as large amounts of biofilm containing bacteria were released and flushed out of the pipes. These results were achieved even though the water temperature in the pipes was only around 35-45°C at the taps during the test period – well below Denmark’s regulatory requirement of 50°C.
Significant cost savings
Following the test, the KAB housing association decided to keep the generator, which paid for itself after just a year and a half, as the association can run with lower hot water temperatures than before and avoid frequent flushes with very hot water. Furthermore, KAB can postpone investing a double-digit million-krone amount in a new water system capable of handling 60-70°C water, which would otherwise be required for effective Legionella control.
”The traditional way of combating Legionella has been ’temperature gymnastics’, where water systems are periodically flushed with very hot water. This is both practically challenging and highly energy intensive. In large housing associations, it’s virtually impossible to get all residents to open their taps simultaneously, as required for such an operation. For us, this project has been an eye-opener, demonstrating that centralised biocide dosing can be a highly effective and energy-saving alternative to temperature gymnastics,” says Nikas Arp-Wilhjelm, energy and heating consultant at KAB.
Going forward, KAB will include biocide solutions in its toolkit for combating Legionella in the 40 housing associations it manages in Greater Copenhagen.
About the Legionella project
The project was conducted from 2020 to 2023 as a multidisciplinary collaboration with, amongst others, the Danish Technological Institute and Statens Serum Institut. It resulted in three proposed methods to better address the growing Legionella problem while at the same time saving energy. The DCW solution with hypochlorous acid was one of these. The project was part of the Danish Energy Agency’s EUDP programme.
The DCW generator being installed in the property
The DCW generator was activated on 10 November 2022. Following this, a significant reduction in Legionella levels was observed and maintained. Occasional fluctuations during the initial phase were due to the release of biofilm in the pipes.
Approvals
